ribbon cable TDR test

[John Larkin]

I'm going to have one board in the front of a rackmount box, the
controller, with an FPGA. In the back of the box will be an output
board with an ADUM7703 isolated delta-sigma converter measuring
current. The boards will be connected by an 18" ribbon cable. The
signals are a 20 MHz clock to the ADUM and 20 mpbs delta-sigma data
coming back to the FPGA. Both sigs are source terminated.

I was worried about signal integrity and didn't find much useful stuff
online, so I tested a ribbon cable .

https://www.dropbox.com/sh/9l0l34qqyyyg2nh/AABtNoqvuOct1kQ74a3lRUrHa?dl=0

The waveform and edge rate look fine, clean 390 ps rise at the end.
The FPGA will have to deal with the prop delay.

I was going to derive a lossy-line Spice model of the cable based on
these measurements (still might some day) but it looks like this cable
will be plenty good enough.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Bill Sloman]

On Friday, October 25, 2019 at 9:57:24 AM UTC+11, John Larkin wrote:

I'm going to have one board in the front of a rackmount box, the
controller, with an FPGA. In the back of the box will be an output
board with an ADUM7703 isolated delta-sigma converter measuring
current. The boards will be connected by an 18" ribbon cable. The
signals are a 20 MHz clock to the ADUM and 20 mpbs delta-sigma data
coming back to the FPGA. Both sigs are source terminated.

I was worried about signal integrity and didn't find much useful stuff
online, so I tested a ribbon cable .

https://www.dropbox.com/sh/9l0l34qqyyyg2nh/AABtNoqvuOct1kQ74a3lRUrHa?dl=0

The waveform and edge rate look fine, clean 390 ps rise at the end.
The FPGA will have to deal with the prop delay.

I was going to derive a lossy-line Spice model of the cable based on
these measurements (still might some day) but it looks like this cable
will be plenty good enough.

Ribbon cable with every second wire grounded is pretty good. The next stage up is a balanced pair of signals on adjacent wires with grounded wires on either side of the pair.

The dielectric/insulator isn't chosen to give good very high frequency characteristics, but eighteen inches (half a metre) isn't far.

--
Bill Sloman, Sydney

 

[Lasse Langwadt Christense]

fredag den 25. oktober 2019 kl. 00.57.24 UTC+2 skrev John Larkin:

I'm going to have one board in the front of a rackmount box, the
controller, with an FPGA. In the back of the box will be an output
board with an ADUM7703 isolated delta-sigma converter measuring
current. The boards will be connected by an 18" ribbon cable. The
signals are a 20 MHz clock to the ADUM and 20 mpbs delta-sigma data
coming back to the FPGA. Both sigs are source terminated.

I was worried about signal integrity and didn't find much useful stuff
online, so I tested a ribbon cable .

https://www.dropbox.com/sh/9l0l34qqyyyg2nh/AABtNoqvuOct1kQ74a3lRUrHa?dl=0

The waveform and edge rate look fine, clean 390 ps rise at the end.
The FPGA will have to deal with the prop delay.

I was going to derive a lossy-line Spice model of the cable based on
these measurements (still might some day) but it looks like this cable
will be plenty good enough.

IDE Harddrives managed 33MHz or something like that on ribbon cables,
thought at the end they used the special cable with twice the conductors
every other one ground

if two pairs and three grounds will do you could use SATA cables

 

[Michael Kellett]

On 25/10/2019 03:39, Lasse Langwadt Christensen wrote:

fredag den 25. oktober 2019 kl. 00.57.24 UTC+2 skrev John Larkin:
I'm going to have one board in the front of a rackmount box, the
controller, with an FPGA. In the back of the box will be an output
board with an ADUM7703 isolated delta-sigma converter measuring
current. The boards will be connected by an 18" ribbon cable. The
signals are a 20 MHz clock to the ADUM and 20 mpbs delta-sigma data
coming back to the FPGA. Both sigs are source terminated.

I was worried about signal integrity and didn't find much useful stuff
online, so I tested a ribbon cable .

https://www.dropbox.com/sh/9l0l34qqyyyg2nh/AABtNoqvuOct1kQ74a3lRUrHa?dl=0

The waveform and edge rate look fine, clean 390 ps rise at the end.
The FPGA will have to deal with the prop delay.

I was going to derive a lossy-line Spice model of the cable based on
these measurements (still might some day) but it looks like this cable
will be plenty good enough.


IDE Harddrives managed 33MHz or something like that on ribbon cables,
thought at the end they used the special cable with twice the conductors
every other one ground

if two pairs and three grounds will do you could use SATA cables

Any one care to comment on this related problem.
8 digital outputs (3.3V CMOS from FPGA (guess)) and other things on 37
way D connector, one ground pin for the digital outputs, at one end of
the connector.
I have to connect this, via about 30cm of cable, to my box and buffer
the signals.
The goal is to get the best result (for speed and decent pulses) that I can.

Plan A is to try with IDC D plugs on the cable and use ribbon cable with
T networks of 0603 parts to be optimised in my box at the other end.

Plan B is to use tiny resistors in the cable D connector at the source
end and T networks of 0603 parts to be optimised in my box at the other
end. Cable to be whatever it takes.

Plan C is to have an active board at the source plug.

Plan D (doing it right with LVDS drivers in the source box) is too late
because the source box has been bought and paid for already.

A good result would be decent 20ns pulses at 25MHz rate.

MK

--
This email has been checked for viruses by AVG.
https://www.avg.com

 

[Bill Sloman]

On Friday, October 25, 2019 at 8:35:05 PM UTC+11, Michael Kellett wrote:

On 25/10/2019 03:39, Lasse Langwadt Christensen wrote:
fredag den 25. oktober 2019 kl. 00.57.24 UTC+2 skrev John Larkin:
I'm going to have one board in the front of a rackmount box, the
controller, with an FPGA. In the back of the box will be an output
board with an ADUM7703 isolated delta-sigma converter measuring
current. The boards will be connected by an 18" ribbon cable. The
signals are a 20 MHz clock to the ADUM and 20 mpbs delta-sigma data
coming back to the FPGA. Both sigs are source terminated.

I was worried about signal integrity and didn't find much useful stuff
online, so I tested a ribbon cable .

https://www.dropbox.com/sh/9l0l34qqyyyg2nh/AABtNoqvuOct1kQ74a3lRUrHa?dl=0

The waveform and edge rate look fine, clean 390 ps rise at the end.
The FPGA will have to deal with the prop delay.

I was going to derive a lossy-line Spice model of the cable based on
these measurements (still might some day) but it looks like this cable
will be plenty good enough.


IDE Harddrives managed 33MHz or something like that on ribbon cables,
thought at the end they used the special cable with twice the conductors
every other one ground

if two pairs and three grounds will do you could use SATA cables

Any one care to comment on this related problem.
8 digital outputs (3.3V CMOS from FPGA (guess)) and other things on 37
way D connector, one ground pin for the digital outputs, at one end of
the connector.
I have to connect this, via about 30cm of cable, to my box and buffer
the signals.
The goal is to get the best result (for speed and decent pulses) that I can.

It's a nice short cable - about 1.5nsec of propagation delay - so it shouldn't be too difficult.

You should have enough connections to let you put a ground wire between each of the eight signal wires. This gives each line a characteristic impedance of between 110R and 130R (depending on the ribbon - measure it if you have to, but the manufacturer should be able to tell you if it isn't in the data sheet).

If you put a big enough damping resistor on each driver to boost the output impedance to this level, the signal will travel along the cable as half voltage step, but if your receiver has a fairly high input impedance, the reflection at the receiving end will mean that you will see a full step at the receiver, and the should be very little reflection when this reflected step gets back to the driver.

Plan A is to try with IDC D plugs on the cable and use ribbon cable with
T networks of 0603 parts to be optimised in my box at the other end.

Eight resistors at the driving end should be all that you need.

They only dissipated heat whole the cable capacitance is charging up (1.5 nsec in each 40nsec for a 25MHz signal rate) so 0603 should be fine (but check).

Plan B is to use tiny resistors in the cable D connector at the source
end and T networks of 0603 parts to be optimised in my box at the other
end. Cable to be whatever it takes.

Okay. So what I am saying is that plan B should be fine, and the T network shouldn't be necessary.

> Plan C is to have an active board at the source plug.

If you need a high current drive to cope with getting your voltage swing into a 110R t0 130R transient load (for 1.5 nsec in 40nec whenever you are sending data at 25MHz, this might be necessary.

Plan D (doing it right with LVDS drivers in the source box) is too late
because the source box has been bought and paid for already.

LVDS sounds like a bit of an overkill for a 25MHz data rate and a 30cm cable.

> A good result would be decent 20ns pulses at 25MHz rate.

At 25MHz you should see steps no more frequently than once every 40nsec.

You want them to be reliably high or low when the receiving circuit looks at them, which means getting a 25MHz clock - high for 20nsec, low for 20nsec - down the ribbon cable, and using balance clock signal minimises the amount of noise (cross-talk) injected into adjacent wires.

Not doing that certainly screwed up one system that I worked on, which had been fine when originally designed for a 5MHz data rate over a few metres of ribbon cable, but got very cranky when the data rate got pushed up to 10MHz and the cable stretched to 18 metres, when the receiving circuits started getting moved into clean areas.

Throwing in opto-isolators made life even more complicated. It wasn't hard to fix, but getting it right took a bit of care.

--
Bill Sloman, Sydney

 

On Fri, 25 Oct 2019 10:34:50 +0100, Michael Kellett <mk@mkesc.co.uk>
wrote:

On 25/10/2019 03:39, Lasse Langwadt Christensen wrote:
fredag den 25. oktober 2019 kl. 00.57.24 UTC+2 skrev John Larkin:
I'm going to have one board in the front of a rackmount box, the
controller, with an FPGA. In the back of the box will be an output
board with an ADUM7703 isolated delta-sigma converter measuring
current. The boards will be connected by an 18" ribbon cable. The
signals are a 20 MHz clock to the ADUM and 20 mpbs delta-sigma data
coming back to the FPGA. Both sigs are source terminated.

I was worried about signal integrity and didn't find much useful stuff
online, so I tested a ribbon cable .

https://www.dropbox.com/sh/9l0l34qqyyyg2nh/AABtNoqvuOct1kQ74a3lRUrHa?dl=0

The waveform and edge rate look fine, clean 390 ps rise at the end.
The FPGA will have to deal with the prop delay.

I was going to derive a lossy-line Spice model of the cable based on
these measurements (still might some day) but it looks like this cable
will be plenty good enough.


IDE Harddrives managed 33MHz or something like that on ribbon cables,
thought at the end they used the special cable with twice the conductors
every other one ground

if two pairs and three grounds will do you could use SATA cables




Any one care to comment on this related problem.
8 digital outputs (3.3V CMOS from FPGA (guess)) and other things on 37
way D connector, one ground pin for the digital outputs, at one end of
the connector.
I have to connect this, via about 30cm of cable, to my box and buffer
the signals.
The goal is to get the best result (for speed and decent pulses) that I can.

Plan A is to try with IDC D plugs on the cable and use ribbon cable with
T networks of 0603 parts to be optimised in my box at the other end.

Plan B is to use tiny resistors in the cable D connector at the source
end and T networks of 0603 parts to be optimised in my box at the other
end. Cable to be whatever it takes.

Plan C is to have an active board at the source plug.

Plan D (doing it right with LVDS drivers in the source box) is too late
because the source box has been bought and paid for already.

A good result would be decent 20ns pulses at 25MHz rate.

MK

Schmitt triggers are always good. You could leave the transmit end
alone and lowpass+Schmitt each line on your end.

First guess, 100 ohms and 50 pF then a Schmitt, 5 ns time constant and
a partial termination to damp any ringing.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Jon Elson]

On Thu, 24 Oct 2019 15:57:15 -0700, John Larkin wrote:

I'm going to have one board in the front of a rackmount box, the
controller, with an FPGA. In the back of the box will be an output board
with an ADUM7703 isolated delta-sigma converter measuring current. The
boards will be connected by an 18" ribbon cable. The signals are a 20
MHz clock to the ADUM and 20 mpbs delta-sigma data coming back to the
FPGA. Both sigs are source terminated.

I was worried about signal integrity and didn't find much useful stuff
online, so I tested a ribbon cable .

https://www.dropbox.com/sh/9l0l34qqyyyg2nh/AABtNoqvuOct1kQ74a3lRUrHa?

dl=0
You've basically reinvented IBM tri-lead, which they devised for the IBM
370 series. They used smaller wires, and got a 91 Ohm single-ended
single-signal cable.

Jon

 

[John Larkin]

On Fri, 25 Oct 2019 15:43:58 -0500, Jon Elson <elson@pico-systems.com>
wrote:

On Thu, 24 Oct 2019 15:57:15 -0700, John Larkin wrote:

I'm going to have one board in the front of a rackmount box, the
controller, with an FPGA. In the back of the box will be an output board
with an ADUM7703 isolated delta-sigma converter measuring current. The
boards will be connected by an 18" ribbon cable. The signals are a 20
MHz clock to the ADUM and 20 mpbs delta-sigma data coming back to the
FPGA. Both sigs are source terminated.

I was worried about signal integrity and didn't find much useful stuff
online, so I tested a ribbon cable .

https://www.dropbox.com/sh/9l0l34qqyyyg2nh/AABtNoqvuOct1kQ74a3lRUrHa?
dl=0
You've basically reinvented IBM tri-lead, which they devised for the IBM
370 series. They used smaller wires, and got a 91 Ohm single-ended
single-signal cable.

Jon

It's standard 3M ribbon cable. Wiki says "The ribbon cable was
invented in 1956 by Cicoil Corporation."

I just couldn't find any data on pulse behavior.

I should test a much longer chunk, to see some serious drool, then
hack a Spice lossy transmission line model to match.

I was happy with the rise time over a 2' length.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Bill Sloman]

On Saturday, October 26, 2019 at 2:47:37 AM UTC+11, jla...@highlandsniptechnology.com wrote:

On Fri, 25 Oct 2019 10:34:50 +0100, Michael Kellett <mk@mkesc.co.uk
wrote:

On 25/10/2019 03:39, Lasse Langwadt Christensen wrote:
fredag den 25. oktober 2019 kl. 00.57.24 UTC+2 skrev John Larkin:

<snip>

Any one care to comment on this related problem.
8 digital outputs (3.3V CMOS from FPGA (guess)) and other things on 37
way D connector, one ground pin for the digital outputs, at one end of
the connector.
I have to connect this, via about 30cm of cable, to my box and buffer
the signals.
The goal is to get the best result (for speed and decent pulses) that I can.

Plan A is to try with IDC D plugs on the cable and use ribbon cable with
T networks of 0603 parts to be optimised in my box at the other end.

Plan B is to use tiny resistors in the cable D connector at the source
end and T networks of 0603 parts to be optimised in my box at the other
end. Cable to be whatever it takes.

Plan C is to have an active board at the source plug.

Plan D (doing it right with LVDS drivers in the source box) is too late
because the source box has been bought and paid for already.

A good result would be decent 20ns pulses at 25MHz rate.

Schmitt triggers are always good. You could leave the transmit end
alone and lowpass+Schmitt each line on your end.

This is remarkably bad advice. The problem with fast signals on ribbon cables (and every other kind of transmission line) is reflections.

If you drive a 1.5nec long transmission line with a device that has a lower output impedance than the transmission line (about 120R in this instance) the amplitude of the signal is doubled when it hits far end (potentially damaging the receiver, or at least putting it into a state where it can behave very strangely). Using a Schmitt trigger receiver won't help.

If the driver is fast enough to produce a 390psec rise-time, you will see this problem.

At the driven end, the reflection will come back as the same double height signal after 3nsec, and wont do the driver any good either.

First guess, 100 ohms and 50 pF then a Schmitt, 5 ns time constant and
a partial termination to damp any ringing.

A 100R resistor as source terminator (at the driving end) should do all that is necessary.

Futzing around at the receiver end is the sort mindless twiddling that ignorant amateurs go in for.

--
Bill Sloman, Sydney

 

[Bill Sloman]

On Saturday, October 26, 2019 at 8:44:24 AM UTC+11, John Larkin wrote:

On Fri, 25 Oct 2019 15:43:58 -0500, Jon Elson <elson@pico-systems.com
wrote:

On Thu, 24 Oct 2019 15:57:15 -0700, John Larkin wrote:

I'm going to have one board in the front of a rackmount box, the
controller, with an FPGA. In the back of the box will be an output board
with an ADUM7703 isolated delta-sigma converter measuring current. The
boards will be connected by an 18" ribbon cable. The signals are a 20
MHz clock to the ADUM and 20 mpbs delta-sigma data coming back to the
FPGA. Both sigs are source terminated.

I was worried about signal integrity and didn't find much useful stuff
online, so I tested a ribbon cable .

https://www.dropbox.com/sh/9l0l34qqyyyg2nh/AABtNoqvuOct1kQ74a3lRUrHa?
dl=0
You've basically reinvented IBM tri-lead, which they devised for the IBM
370 series. They used smaller wires, and got a 91 Ohm single-ended
single-signal cable.

Jon

It's standard 3M ribbon cable. Wiki says "The ribbon cable was
invented in 1956 by Cicoil Corporation."

I just couldn't find any data on pulse behavior.

Some data sheets list the characteristic impedance. The trick of using a grounded wire between each signal wire makes the signal wires look like tolerable transmission lines.

I should test a much longer chunk, to see some serious drool, then
hack a Spice lossy transmission line model to match.

You should think about what's actually going on - reflections and some attenuation of the higher frequency components in the current that charges up the transmission line and gets reflected at discontinuities in the transmission line - the ends, mostly.

Even Howard Johnson could do better than that.

> I was happy with the rise time over a 2' length.

When you should have been worrying about the reflections, if the rise-time were shorter than the roughly 3nsec propagation delay that you'd expect with two feet (61cm) of ribbon cable.

--
Bill Sloman, Sydney

 

[Jan Panteltje]

On a sunny day (Fri, 25 Oct 2019 14:44:14 -0700) it happened John Larkin
<jlarkin@highland_atwork_technology.com> wrote in
<liq6repurv3ga15m4tintrc8hq61g1q1em@4ax.com>:

It's standard 3M ribbon cable. Wiki says "The ribbon cable was
invented in 1956 by Cicoil Corporation."

I just couldn't find any data on pulse behavior.

I should test a much longer chunk, to see some serious drool, then
hack a Spice lossy transmission line model to match.

I was happy with the rise time over a 2' length.

There are many qualities of ribbon cable,
the cheap stuff I got from China has less copper[1], weaker plastic
than the flat cables to my old harddisks.
Never measured those.

[1] it is not copper I think...

It all depends, I have cheap ethernet cable from China
that does not even have twisted pairs.

 

[Michael Kellett]

On 26/10/2019 02:05, Bill Sloman wrote:

On Saturday, October 26, 2019 at 2:47:37 AM UTC+11, jla...@highlandsniptechnology.com wrote:
On Fri, 25 Oct 2019 10:34:50 +0100, Michael Kellett <mk@mkesc.co.uk
wrote:

On 25/10/2019 03:39, Lasse Langwadt Christensen wrote:
fredag den 25. oktober 2019 kl. 00.57.24 UTC+2 skrev John Larkin:

snip

Any one care to comment on this related problem.
8 digital outputs (3.3V CMOS from FPGA (guess)) and other things on 37
way D connector, one ground pin for the digital outputs, at one end of
the connector.
I have to connect this, via about 30cm of cable, to my box and buffer
the signals.
The goal is to get the best result (for speed and decent pulses) that I can.

Plan A is to try with IDC D plugs on the cable and use ribbon cable with
T networks of 0603 parts to be optimised in my box at the other end.

Plan B is to use tiny resistors in the cable D connector at the source
end and T networks of 0603 parts to be optimised in my box at the other
end. Cable to be whatever it takes.

Plan C is to have an active board at the source plug.

Plan D (doing it right with LVDS drivers in the source box) is too late
because the source box has been bought and paid for already.

A good result would be decent 20ns pulses at 25MHz rate.

Schmitt triggers are always good. You could leave the transmit end
alone and lowpass+Schmitt each line on your end.

This is remarkably bad advice. The problem with fast signals on ribbon cables (and every other kind of transmission line) is reflections.

If you drive a 1.5nec long transmission line with a device that has a lower output impedance than the transmission line (about 120R in this instance) the amplitude of the signal is doubled when it hits far end (potentially damaging the receiver, or at least putting it into a state where it can behave very strangely). Using a Schmitt trigger receiver won't help.

If the driver is fast enough to produce a 390psec rise-time, you will see this problem.

At the driven end, the reflection will come back as the same double height signal after 3nsec, and wont do the driver any good either.

First guess, 100 ohms and 50 pF then a Schmitt, 5 ns time constant and
a partial termination to damp any ringing.

A 100R resistor as source terminator (at the driving end) should do all that is necessary.

Futzing around at the receiver end is the sort mindless twiddling that ignorant amateurs go in for.

Thanks (JL and BS) for all the suggestions.

I'll try " Futzing around at the receiver end" first - not because it's
good, but because it would be so much nicer to be able to use a ribbon
cable with IDC connectors plugged straight into the source connector.

One of my (many) concerns re. the source box is that it has just one
ground pin for many fast digital outputs. Unless I do lots of fancy work
inside the connector most of my signals are adjacent to two other
independent signals.

I'm hoping that the source box has some kind of series termination
between connector and drivers but I don't know yet.

If I get the job I'll let you know how it goes.

MK

--
This email has been checked for viruses by AVG.
https://www.avg.com

 

[Tom Gardner]

On 26/10/19 09:17, Michael Kellett wrote:

On 26/10/2019 02:05, Bill Sloman wrote:
On Saturday, October 26, 2019 at 2:47:37 AM UTC+11,
jla...@highlandsniptechnology.com wrote:
On Fri, 25 Oct 2019 10:34:50 +0100, Michael Kellett <mk@mkesc.co.uk
wrote:

On 25/10/2019 03:39, Lasse Langwadt Christensen wrote:
fredag den 25. oktober 2019 kl. 00.57.24 UTC+2 skrev John Larkin:

snip
Any one care to comment on this related problem.
8 digital outputs (3.3V CMOS from FPGA (guess)) and other things on 37
way D connector, one ground pin for the digital outputs, at one end of
the connector.
I have to connect this, via about 30cm of cable, to my box and buffer
the signals.
The goal is to get the best result (for speed and decent pulses) that I can.

Plan A is to try with IDC D plugs on the cable and use ribbon cable with
T networks of 0603 parts to be optimised in my box at the other end.

Plan B is to use tiny resistors in the cable D connector at the source
end and T networks of 0603 parts to be optimised in my box at the other
end. Cable to be whatever it takes.

Plan C is to have an active board at the source plug.

Plan D  (doing it right with LVDS drivers in the source box) is too late
because the source  box has been bought and paid for already.

A good result would be decent 20ns pulses at 25MHz rate.

Schmitt triggers are always good. You could leave the transmit end
alone and lowpass+Schmitt each line on your end.

This is remarkably bad advice. The problem with fast signals on ribbon cables
(and every other kind of transmission line) is reflections.

If you drive a 1.5nec long transmission line with a device that has a lower
output impedance than the transmission line (about 120R in this instance) the
amplitude of the signal is doubled when it hits far end (potentially damaging
the receiver, or at least putting it into a state where it can behave very
strangely). Using a Schmitt trigger receiver won't help.

If the driver is fast enough to produce a 390psec rise-time, you will see this
problem.

At the driven end, the reflection will come back as the same double height
signal after 3nsec, and wont do the driver any good either.
First guess, 100 ohms and 50 pF then a Schmitt, 5 ns time constant and
a partial termination to damp any ringing.

A 100R resistor as source terminator (at the driving end) should do all that
is necessary.

Futzing around at the receiver end is the sort mindless twiddling that
ignorant amateurs go in  for.

Thanks (JL and BS) for all the suggestions.

I'll try " Futzing around at the receiver end" first - not because it's good,
but because it would be so much nicer to be able to use a ribbon cable with IDC
connectors plugged straight into the source connector.

One of my (many) concerns re. the source box is that it has just one ground pin
for many fast digital outputs. Unless I do lots of fancy work inside the
connector most of my signals are adjacent to two other independent signals.

I'm hoping that the source box has some kind of series termination between
connector and drivers but I don't know yet.

If I get the job I'll let you know how it goes.

The traditional techniques are source termination or
receiver termination, but not both.

Source termination inserts a series resistor R at the
driver so that the driver's output resistance plus
R equals the transmission line impedance, Z. Driver
supplies V/2Z current.

Receiver termination has several options, each with benefits:
1 single resistor to ground Z=R
2 resistor R1 to Vcc, R2 to gnd, such that R1//R2=Z and
the potential divider voltage is the receiver's
threshold voltage
3 AC only version 1 inserts a capacitor in series

All those require the driver can supply double
the current, V/Z.
1 asymmetrically loads the driver. 2 has a constant
current through R1+R2. 3 is suboptimum except in special
circumstances.

A schmitt trigger does no harm in properly terminated
transmission lines. Your situation (1 gnd in a 37-way
cable) is not going to have a well-defined impedance,
and there will be more crosstalk, so a schmitt /might/
make the difference between it working or failing.

You might also like to consider the effects of static
hitting either the driver or the receiver. That can
result in catastrophic damage or more subtle parametric
shifts.

 

[Bill Sloman]

On Saturday, October 26, 2019 at 7:45:57 PM UTC+11, Tom Gardner wrote:

On 26/10/19 09:17, Michael Kellett wrote:
On 26/10/2019 02:05, Bill Sloman wrote:
On Saturday, October 26, 2019 at 2:47:37 AM UTC+11,
jla...@highlandsniptechnology.com wrote:
On Fri, 25 Oct 2019 10:34:50 +0100, Michael Kellett <mk@mkesc.co.uk
wrote:

On 25/10/2019 03:39, Lasse Langwadt Christensen wrote:
fredag den 25. oktober 2019 kl. 00.57.24 UTC+2 skrev John Larkin:

snip
Any one care to comment on this related problem.
8 digital outputs (3.3V CMOS from FPGA (guess)) and other things on 37
way D connector, one ground pin for the digital outputs, at one end of
the connector.
I have to connect this, via about 30cm of cable, to my box and buffer
the signals.
The goal is to get the best result (for speed and decent pulses) that I can.

Plan A is to try with IDC D plugs on the cable and use ribbon cable with
T networks of 0603 parts to be optimised in my box at the other end.

Plan B is to use tiny resistors in the cable D connector at the source
end and T networks of 0603 parts to be optimised in my box at the other
end. Cable to be whatever it takes.

Plan C is to have an active board at the source plug.

Plan D  (doing it right with LVDS drivers in the source box) is too late
because the source  box has been bought and paid for already.

A good result would be decent 20ns pulses at 25MHz rate.

Schmitt triggers are always good. You could leave the transmit end
alone and lowpass+Schmitt each line on your end.

This is remarkably bad advice. The problem with fast signals on ribbon cables (and every other kind of transmission line) is reflections.

If you drive a 1.5nec long transmission line with a device that has a lower
output impedance than the transmission line (about 120R in this instance) the amplitude of the signal is doubled when it hits far end (potentially damaging the receiver, or at least putting it into a state where it can behave very strangely). Using a Schmitt trigger receiver won't help.

If the driver is fast enough to produce a 390psec rise-time, you will see this problem.

At the driven end, the reflection will come back as the same double height
signal after 3nsec, and won't do the driver any good either.

First guess, 100 ohms and 50 pF then a Schmitt, 5 ns time constant and
a partial termination to damp any ringing.

A 100R resistor as source terminator (at the driving end) should do all that is necessary.

Futzing around at the receiver end is the sort mindless twiddling that
ignorant amateurs go in  for.

Thanks (JL and BS) for all the suggestions.

I'll try " Futzing around at the receiver end" first - not because it's good, but because it would be so much nicer to be able to use a ribbon cable with IDC connectors plugged straight into the source connector.

It's not nice to run the risk of blowing up the receiver.

It's not wise to risk feeding current into the catching diodes on the receiver - the current going into them can have unexpected effects. Even if they works as advertised, not blowing up the receiver is not the same as having it work properly while the inputs go outside the supply rails.

A T-network can stop that happening, but it messes up the waveform.

Source termination is a lot neater, and only needs one damping resistor per lead.

One of my (many) concerns re. the source box is that it has just one ground pin for many fast digital outputs. Unless I do lots of fancy work inside the
connector most of my signals are adjacent to two other independent signals.

The paths inside the connectors are pretty short, and there are going to be lots of grounded pins in the immediate vicinity too.

I'm hoping that the source box has some kind of series termination between
connector and drivers but I don't know yet.

The source box should have a source termination resistor between each driver pin and the corresponding pin on the connector.

The question is whether it's going to have the right value for your particular ribbon cable - it should be pretty much okay since most ribbon cables are pretty similar but you really need to at the driver and the cable before you can be sure.

If I get the job I'll let you know how it goes.

The traditional techniques are source termination or
receiver termination, but not both.

You can do both. It halves your signal level, but really cuts down reflection at both ends of the cable.

Source termination inserts a series resistor R at the
driver so that the driver's output resistance plus
R equals the transmission line impedance, Z. Driver
supplies V/2Z current.

But only while the cable is getting charged up (or down).

Receiver termination has several options, each with benefits:
1 single resistor to ground Z=R
2 resistor R1 to Vcc, R2 to gnd, such that R1//R2=Z and
the potential divider voltage is the receiver's
threshold voltage
3 AC only version 1 inserts a capacitor in series

All those require the driver can supply double
the current, V/Z.

All the time.

> 1 asymmetrically loads the driver.

ECL is designed to drive precisely such terminating resistors - signal swing is from -0.6V (one) to -1.2V (zero), with the terminating resistor returned to -2V

2 has a constant
current through R1+R2.

3 is suboptimum except in special circumstances,

But it dissipates less heat.

A schmitt trigger does no harm in properly terminated
transmission lines. Your situation (1 gnd in a 37-way
cable) is not going to have a well-defined impedance,
and there will be more crosstalk, so a schmitt /might/
make the difference between it working or failing.

If you link every second cable in the ribbon to that single ground you kill almost all the cross-talk.

You might also like to consider the effects of static
hitting either the driver or the receiver. That can
result in catastrophic damage or more subtle parametric
shifts.

30 cm of cable is likely to be well enough grounded at both ends to make this an unlikely problem.

The electron microscopes at Cambridge Instruments had a 30kV supply for the electron gun, and when that flashed over there were unfortunate side effects.

Most of them went away when the gun got a proper 30kV coaxial connector. The flash-over currents stayed inside the coax cable, and didn't show up as brief hundred ampere currents in inconvenient places.

--
Bill Sloman, Sydney

 

On Sat, 26 Oct 2019 09:17:35 +0100, Michael Kellett <mk@mkesc.co.uk>
wrote:

On 26/10/2019 02:05, Bill Sloman wrote:
On Saturday, October 26, 2019 at 2:47:37 AM UTC+11, jla...@highlandsniptechnology.com wrote:
On Fri, 25 Oct 2019 10:34:50 +0100, Michael Kellett <mk@mkesc.co.uk
wrote:

On 25/10/2019 03:39, Lasse Langwadt Christensen wrote:
fredag den 25. oktober 2019 kl. 00.57.24 UTC+2 skrev John Larkin:

snip

Any one care to comment on this related problem.
8 digital outputs (3.3V CMOS from FPGA (guess)) and other things on 37
way D connector, one ground pin for the digital outputs, at one end of
the connector.
I have to connect this, via about 30cm of cable, to my box and buffer
the signals.
The goal is to get the best result (for speed and decent pulses) that I can.

Plan A is to try with IDC D plugs on the cable and use ribbon cable with
T networks of 0603 parts to be optimised in my box at the other end.

Plan B is to use tiny resistors in the cable D connector at the source
end and T networks of 0603 parts to be optimised in my box at the other
end. Cable to be whatever it takes.

Plan C is to have an active board at the source plug.

Plan D (doing it right with LVDS drivers in the source box) is too late
because the source box has been bought and paid for already.

A good result would be decent 20ns pulses at 25MHz rate.

Schmitt triggers are always good. You could leave the transmit end
alone and lowpass+Schmitt each line on your end.

This is remarkably bad advice. The problem with fast signals on ribbon cables (and every other kind of transmission line) is reflections.

If you drive a 1.5nec long transmission line with a device that has a lower output impedance than the transmission line (about 120R in this instance) the amplitude of the signal is doubled when it hits far end (potentially damaging the receiver, or at least putting it into a state where it can behave very strangely). Using a Schmitt trigger receiver won't help.

If the driver is fast enough to produce a 390psec rise-time, you will see this problem.

At the driven end, the reflection will come back as the same double height signal after 3nsec, and wont do the driver any good either.

First guess, 100 ohms and 50 pF then a Schmitt, 5 ns time constant and
a partial termination to damp any ringing.

A 100R resistor as source terminator (at the driving end) should do all that is necessary.

Futzing around at the receiver end is the sort mindless twiddling that ignorant amateurs go in for.

Thanks (JL and BS) for all the suggestions.

I'll try " Futzing around at the receiver end" first - not because it's
good, but because it would be so much nicer to be able to use a ribbon
cable with IDC connectors plugged straight into the source connector.

Source termination would be great. It reduces the drive currents,
which may be a benefit with your grounding. That would involve adding,
say, 100 ohms in series with each line at the source, and maybe adding
a cap to ground and a Schmitt at the receive end.

But you don't know if the source connector is actually close to the
source! A series termination midway in a transmission line can get
messy.

You should probably experiment with the real source and an
oscilloscope and some parts. Or find out what's in the source box.

LT Spice transmission line models work pretty well.

One of my (many) concerns re. the source box is that it has just one
ground pin for many fast digital outputs. Unless I do lots of fancy work
inside the connector most of my signals are adjacent to two other
independent signals.

One shared ground is scary. Can you use a cable shield as another
ground?

I'm hoping that the source box has some kind of series termination
between connector and drivers but I don't know yet.

Ask! It's a very reasonable question.

If I get the job I'll let you know how it goes.

MK

If it's a big job, adding a little driver board inside a connector
shell wouldn't be a big deal.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Tom Gardner]

On 26/10/19 15:36, Bill Sloman wrote:

On Saturday, October 26, 2019 at 7:45:57 PM UTC+11, Tom Gardner wrote:
On 26/10/19 09:17, Michael Kellett wrote:
On 26/10/2019 02:05, Bill Sloman wrote:
On Saturday, October 26, 2019 at 2:47:37 AM UTC+11,
jla...@highlandsniptechnology.com wrote:
On Fri, 25 Oct 2019 10:34:50 +0100, Michael Kellett <mk@mkesc.co.uk
wrote:

On 25/10/2019 03:39, Lasse Langwadt Christensen wrote:
fredag den 25. oktober 2019 kl. 00.57.24 UTC+2 skrev John
Larkin:

snip
Any one care to comment on this related problem. 8 digital outputs
(3.3V CMOS from FPGA (guess)) and other things on 37 way D
connector, one ground pin for the digital outputs, at one end of
the connector. I have to connect this, via about 30cm of cable, to
my box and buffer the signals. The goal is to get the best result
(for speed and decent pulses) that I can.

Plan A is to try with IDC D plugs on the cable and use ribbon cable
with T networks of 0603 parts to be optimised in my box at the
other end.

Plan B is to use tiny resistors in the cable D connector at the
source end and T networks of 0603 parts to be optimised in my box
at the other end. Cable to be whatever it takes.

Plan C is to have an active board at the source plug.

Plan D (doing it right with LVDS drivers in the source box) is too
late because the source box has been bought and paid for already.

A good result would be decent 20ns pulses at 25MHz rate.

Schmitt triggers are always good. You could leave the transmit end
alone and lowpass+Schmitt each line on your end.

This is remarkably bad advice. The problem with fast signals on ribbon
cables (and every other kind of transmission line) is reflections.

If you drive a 1.5nec long transmission line with a device that has a
lower output impedance than the transmission line (about 120R in this
instance) the amplitude of the signal is doubled when it hits far end
(potentially damaging the receiver, or at least putting it into a state
where it can behave very strangely). Using a Schmitt trigger receiver
won't help.

If the driver is fast enough to produce a 390psec rise-time, you will
see this problem.

At the driven end, the reflection will come back as the same double
height signal after 3nsec, and won't do the driver any good either.

First guess, 100 ohms and 50 pF then a Schmitt, 5 ns time constant
and a partial termination to damp any ringing.

A 100R resistor as source terminator (at the driving end) should do all
that is necessary.

Futzing around at the receiver end is the sort mindless twiddling that
ignorant amateurs go in for.

Thanks (JL and BS) for all the suggestions.

I'll try " Futzing around at the receiver end" first - not because it's
good, but because it would be so much nicer to be able to use a ribbon
cable with IDC connectors plugged straight into the source connector.

It's not nice to run the risk of blowing up the receiver.

It's not wise to risk feeding current into the catching diodes on the
receiver - the current going into them can have unexpected effects. Even if
they works as advertised, not blowing up the receiver is not the same as
having it work properly while the inputs go outside the supply rails.

A T-network can stop that happening, but it messes up the waveform.

Source termination is a lot neater, and only needs one damping resistor per
lead.

One of my (many) concerns re. the source box is that it has just one
ground pin for many fast digital outputs. Unless I do lots of fancy work
inside the
connector most of my signals are adjacent to two other independent signals.

The paths inside the connectors are pretty short, and there are going to be
lots of grounded pins in the immediate vicinity too.

I'm hoping that the source box has some kind of series termination
between connector and drivers but I don't know yet.

The source box should have a source termination resistor between each driver
pin and the corresponding pin on the connector.

The question is whether it's going to have the right value for your
particular ribbon cable - it should be pretty much okay since most ribbon
cables are pretty similar but you really need to at the driver and the cable
before you can be sure.

If I get the job I'll let you know how it goes.

The traditional techniques are source termination or receiver termination,
but not both.

You can do both. It halves your signal level, but really cuts down reflection
at both ends of the cable.

In most cases you "throwing away" voltage swing is suboptimum.

In special, well-designed, cases it might be beneficial, but
clearly the OP's case doesn't fall into that category.

Source termination inserts a series resistor R at the driver so that the
driver's output resistance plus R equals the transmission line impedance,
Z. Driver supplies V/2Z current.

But only while the cable is getting charged up (or down).

Yes, but that's long enough for problems w.r.t. ground bounce
and crosstalk.

Receiver termination has several options, each with benefits: 1 single
resistor to ground Z=R 2 resistor R1 to Vcc, R2 to gnd, such that R1//R2=Z
and the potential divider voltage is the receiver's threshold voltage 3 AC
only version 1 inserts a capacitor in series

All those require the driver can supply double the current, V/Z.

All the time.

Of course.

1 asymmetrically loads the driver.

ECL is designed to drive precisely such terminating resistors - signal swing
is from -0.6V (one) to -1.2V (zero), with the terminating resistor returned
to -2V

ECL and "derivatives" are nice to work with, but I doubt
they are relevant in this case.

2 has a constant current through R1+R2.

3 is suboptimum except in special circumstances,

But it dissipates less heat.

That's about the only benefit.

A schmitt trigger does no harm in properly terminated transmission lines.
Your situation (1 gnd in a 37-way cable) is not going to have a
well-defined impedance, and there will be more crosstalk, so a schmitt
/might/ make the difference between it working or failing.

If you link every second cable in the ribbon to that single ground you kill
almost all the cross-talk.

Yup, but that isn't the case here

You might also like to consider the effects of static hitting either the
driver or the receiver. That can result in catastrophic damage or more
subtle parametric shifts.

30 cm of cable is likely to be well enough grounded at both ends to make this
an unlikely problem.

There's a minor industry devoted to just that problem!

If the cable is entirely within a cabinet, and basic anti-static
precautions are taken when the cable isn't connected, then
I wouldn't be too concerned. I don't know whether that is the
case here.

The electron microscopes at Cambridge Instruments had a 30kV supply for the
electron gun, and when that flashed over there were unfortunate side
effects.

Most of them went away when the gun got a proper 30kV coaxial connector. The
flash-over currents stayed inside the coax cable, and didn't show up as brief
hundred ampere currents in inconvenient places.

Yes.

 

[Bill Sloman]

On Sunday, October 27, 2019 at 6:02:27 AM UTC+11, Tom Gardner wrote:

On 26/10/19 15:36, Bill Sloman wrote:
On Saturday, October 26, 2019 at 7:45:57 PM UTC+11, Tom Gardner wrote:
On 26/10/19 09:17, Michael Kellett wrote:
On 26/10/2019 02:05, Bill Sloman wrote:
On Saturday, October 26, 2019 at 2:47:37 AM UTC+11,
jla...@highlandsniptechnology.com wrote:
On Fri, 25 Oct 2019 10:34:50 +0100, Michael Kellett <mk@mkesc.co.uk
wrote:

On 25/10/2019 03:39, Lasse Langwadt Christensen wrote:
fredag den 25. oktober 2019 kl. 00.57.24 UTC+2 skrev John
Larkin:

<snip>

The traditional techniques are source termination or receiver termination,
but not both.

You can do both. It halves your signal level, but really cuts down reflection at both ends of the cable.

In most cases you "throwing away" voltage swing is suboptimum.

Obviously, but there are situations where minimising reflections is essential, and trumps every other consideration.

In special, well-designed, cases it might be beneficial, but
clearly the OP's case doesn't fall into that category.

Whoever said it did? You were talking about "traditional techniques", not the OP's situation, and termination at both ends of cable is a traditional technique in certain particularly demanding situations. You may not have access to a complete set of traditional techniques ...

Source termination inserts a series resistor R at the driver so that the
driver's output resistance plus R equals the transmission line impedance,
Z. Driver supplies V/2Z current.

But only while the cable is getting charged up (or down).

Yes, but that's long enough for problems w.r.t. ground bounce
and crosstalk.

But can reduce power supply load and heat dissipation. Coping with V/Z all the time isn't usually necessary.

Receiver termination has several options, each with benefits: 1 single
resistor to ground Z=R 2 resistor R1 to Vcc, R2 to gnd, such that R1//R2=Z
and the potential divider voltage is the receiver's threshold voltage 3 AC
only version 1 inserts a capacitor in series

All those require the driver can supply double the current, V/Z.

All the time.

Of course.

1 asymmetrically loads the driver.

ECL is designed to drive precisely such terminating resistors - signal swing
is from -0.6V (one) to -1.2V (zero), with the terminating resistor returned
to -2V

ECL and "derivatives" are nice to work with, but I doubt
they are relevant in this case.

Probably not, but they do show up frequently in the kinds of high-speed circuits that need this sort of attention.

2 has a constant current through R1+R2.

3 is suboptimum except in special circumstances,

But it dissipates less heat.

That's about the only benefit.

But it can be a useful - sometimes vital - benefit.

A schmitt trigger does no harm in properly terminated transmission lines.
Your situation (1 gnd in a 37-way cable) is not going to have a
well-defined impedance, and there will be more crosstalk, so a schmitt
/might/ make the difference between it working or failing.

If you link every second cable in the ribbon to that single ground you kill
almost all the cross-talk.

Yup, but that isn't the case here

Michael Kellett just said that he had a 37-way ribbon cable. That should be enough ways to let him devote 15 ways to burying each of the eight data lines between grounded wires.

He'd be mad not to.

You might also like to consider the effects of static hitting either the
driver or the receiver. That can result in catastrophic damage or more
subtle parametric shifts.

30 cm of cable is likely to be well enough grounded at both ends to make this an unlikely problem.

There's a minor industry devoted to just that problem!

Cleaning up after people who didn't think about it in advance.

If the cable is entirely within a cabinet, and basic anti-static
precautions are taken when the cable isn't connected, then
I wouldn't be too concerned. I don't know whether that is the
case here.

It ought to be. Somebody who thinks hard enough to post questions here can probably be relied on to have done their homework.

The electron microscopes at Cambridge Instruments had a 30kV supply for the
electron gun, and when that flashed over there were unfortunate side
effects.

Most of them went away when the gun got a proper 30kV coaxial connector.. The flash-over currents stayed inside the coax cable, and didn't show up as brief hundred ampere currents in inconvenient places.

Yes.

The odd part about the story was that the proper 30kV coaxial connector got bought and fitted as a sort of make-the-product-look-neater exercise.

The improvement in flashover performance came as a surprise.

The people surprised were remarkably competent, which is why I mention it here from time to time. As patent lawyers say, everything is obvious to the Supreme Court, but lesser mortals have blind spots.

--
Bill Sloman, Sydney

 

[Tom Gardner]

On 27/10/19 02:08, Bill Sloman wrote:

On Sunday, October 27, 2019 at 6:02:27 AM UTC+11, Tom Gardner wrote:
On 26/10/19 15:36, Bill Sloman wrote:
On Saturday, October 26, 2019 at 7:45:57 PM UTC+11, Tom Gardner wrote:
On 26/10/19 09:17, Michael Kellett wrote:
On 26/10/2019 02:05, Bill Sloman wrote:
On Saturday, October 26, 2019 at 2:47:37 AM UTC+11,
jla...@highlandsniptechnology.com wrote:
On Fri, 25 Oct 2019 10:34:50 +0100, Michael Kellett
mk@mkesc.co.uk> wrote:

On 25/10/2019 03:39, Lasse Langwadt Christensen wrote:
fredag den 25. oktober 2019 kl. 00.57.24 UTC+2 skrev John
Larkin:

snip


The traditional techniques are source termination or receiver
termination, but not both.

You can do both. It halves your signal level, but really cuts down
reflection at both ends of the cable.

In most cases you "throwing away" voltage swing is suboptimum.

Obviously, but there are situations where minimising reflections is
essential, and trumps every other consideration.

In special, well-designed, cases it might be beneficial, but clearly the
OP's case doesn't fall into that category.

Whoever said it did? You were talking about "traditional techniques", not the
OP's situation, and termination at both ends of cable is a traditional
technique in certain particularly demanding situations. You may not have
access to a complete set of traditional techniques ...

Source termination inserts a series resistor R at the driver so that
the driver's output resistance plus R equals the transmission line
impedance, Z. Driver supplies V/2Z current.

But only while the cable is getting charged up (or down).

Yes, but that's long enough for problems w.r.t. ground bounce and
crosstalk.

But can reduce power supply load and heat dissipation. Coping with V/Z all
the time isn't usually necessary.

Receiver termination has several options, each with benefits: 1 single
resistor to ground Z=R 2 resistor R1 to Vcc, R2 to gnd, such that
R1//R2=Z and the potential divider voltage is the receiver's threshold
voltage 3 AC only version 1 inserts a capacitor in series

All those require the driver can supply double the current, V/Z.

All the time.

Of course.

1 asymmetrically loads the driver.

ECL is designed to drive precisely such terminating resistors - signal
swing is from -0.6V (one) to -1.2V (zero), with the terminating resistor
returned to -2V

ECL and "derivatives" are nice to work with, but I doubt they are relevant
in this case.

Probably not, but they do show up frequently in the kinds of high-speed
circuits that need this sort of attention.

2 has a constant current through R1+R2.

3 is suboptimum except in special circumstances,

But it dissipates less heat.

That's about the only benefit.

But it can be a useful - sometimes vital - benefit.

A schmitt trigger does no harm in properly terminated transmission
lines. Your situation (1 gnd in a 37-way cable) is not going to have a
well-defined impedance, and there will be more crosstalk, so a schmitt
/might/ make the difference between it working or failing.

If you link every second cable in the ribbon to that single ground you
kill almost all the cross-talk.

Yup, but that isn't the case here

Michael Kellett just said that he had a 37-way ribbon cable. That should be
enough ways to let him devote 15 ways to burying each of the eight data lines
between grounded wires.

He'd be mad not to.

Well yes. I presume there aren't sufficient
spare connections, or that the pinout is
pre-ordained.

If the latter, I'd try to find a way of
mutating the pinout.

You might also like to consider the effects of static hitting either
the driver or the receiver. That can result in catastrophic damage or
more subtle parametric shifts.

30 cm of cable is likely to be well enough grounded at both ends to make
this an unlikely problem.

There's a minor industry devoted to just that problem!

Cleaning up after people who didn't think about it in advance.

Or where cables will be external and inserted/removed
by the ignorant in uncontrolled conditions. USB is the
obvious example, but there are many others.

If the cable is entirely within a cabinet, and basic anti-static
precautions are taken when the cable isn't connected, then I wouldn't be
too concerned. I don't know whether that is the case here.

It ought to be. Somebody who thinks hard enough to post questions here can
probably be relied on to have done their homework.

Indeed, but there may be other "users" later on.

The electron microscopes at Cambridge Instruments had a 30kV supply for
the electron gun, and when that flashed over there were unfortunate side
effects.

Most of them went away when the gun got a proper 30kV coaxial connector.
The flash-over currents stayed inside the coax cable, and didn't show up
as brief hundred ampere currents in inconvenient places.

Yes.

The odd part about the story was that the proper 30kV coaxial connector got
bought and fitted as a sort of make-the-product-look-neater exercise.

The improvement in flashover performance came as a surprise.

The people surprised were remarkably competent, which is why I mention it
here from time to time. As patent lawyers say, everything is obvious to the
Supreme Court, but lesser mortals have blind spots.

Well, everyone except me. I'm perfect.

My daughter believed that for a while; it was a
useful "life lesson" for her.

 

[Michael Kellett]

Michael Kellett just said that he had a 37-way ribbon cable. That
should be
enough ways to let him devote 15 ways to burying each of the eight
data lines
between grounded wires.

He'd be mad not to.

Well yes. I presume there aren't sufficient
spare connections, or that the pinout is
pre-ordained.

If the latter, I'd try to find a way of
mutating the pinout.

Thanks for further suggestions.
Seems I didn't make the initial situation quite clear, the source box is
a standard thing already bought by my customer. The 37 way D connector
has all 37 pins committed. There are 3 gnd pins, all at one end of the D
connector.
The pins are obviously driven by octal bus drivers (spec says max 35mA
per pin and max 70mA per group of 8.)
So I have to take the signals from where they tell me (opposite end of D
conn from the gnd pins !).

MK

 

[Bill Sloman]

On Sunday, October 27, 2019 at 9:11:20 PM UTC+11, Michael Kellett wrote:

Michael Kellett just said that he had a 37-way ribbon cable. That
should be
enough ways to let him devote 15 ways to burying each of the eight
data lines
between grounded wires.

He'd be mad not to.

Well yes. I presume there aren't sufficient
spare connections, or that the pinout is
pre-ordained.

If the latter, I'd try to find a way of
mutating the pinout.


Thanks for further suggestions.
Seems I didn't make the initial situation quite clear, the source box is
a standard thing already bought by my customer. The 37 way D connector
has all 37 pins committed. There are 3 gnd pins, all at one end of the D
connector.
The pins are obviously driven by octal bus drivers (spec says max 35mA
per pin and max 70mA per group of 8.)
So I have to take the signals from where they tell me (opposite end of D
connector from the ground pins !).

Oops. You may have to find out if you can make it work, and if not, reconcile your customer to a little board to swap the connections around, possibly onto a wider cable.

It takes a while to get naive customers to appreciate how expensive it can be to spend money before they know exactly what they need.

Something that can't be made to do what they need done is entirely worthless, and the time spent working why it can't be made to work isn't cheap either.

--
Bill Sloman, Sydney