Need a bus driver solution

[Fred Bloggs]

Ferrites applied to ribbon cables with random
signal/ground conductor topologies are not guaranteed to preserve rise times.

Anyone using that kind of arrangement is not trying to preserve rise
times the ferrite is capable of attenuating.

 

[Fred Bloggs]

John Larkin wrote:

How about 20 5-volt cmos-driven lines and a couple of commons (ground)
from the transmit end. At the receive end, use RS-485 line receivers,
all sharing the low-side common lines. That should zap any common-mode
noise. Hooks for some modest value caps across the line receiver
outputs would be insurance against picking up fast normal-mode noise
or crosstalk glitches (or RC-schmitt things, to be more politically
correct.)

That sounds like a suggestion from the olden days with slew rate limited
transitions and nice long mucho microsecond settling time waits before
receiver latches data off the unterminated lines- well the whole idea
was to make the line into a simple wire and circumvent xmission line
effects.

 

[neil-at-giganews]

Neil, you should study a bit the theory, there are a lot of mistakes and
guesswork in your reply.
1.) the ferrite beads or clamps around the whole cable do *not* have any
effect on rise times of the signals. The signal currents cancel with the
return currents and so the clamp has *no* effect. It will have a high
impedance for common mode disturbances, that is what it is used for. If
you want it to have an effect on the signal, you will need to put a
separate bead in all of the lines.
2.) transmission line theory should be a must if you work in this area.
This would explain the "wobbling" and give you a good idea what to do to
avoid it.
As to the aircraft, I hope there is at least a good hardware engineer in
the company to superwise.
--
ciao Ban
Bordighera, Italy
well sorry i spoke.

All I suggested was what I'd seen on a scope monitoring a few signals in the
lab, and empirically tried a few things, more from curiosity than anything
else, I was just trying to give someone the benefit of my personal
experience - which is mostly not transmission line related.
Theory is great, but I've seen too many circuits which were theoretically
perfect (according to the calculations done by the designer) that didn't
work in practice.
As to the aircraft, yes there are many much more competent designers than me
working on the flight circuits, and all hardware goes through rigorous
enviromental and stress tests before going anywhere near an aircraft, then
goes through ground tests, then flight tests ... so I'm quite happy to fly
in them.
Perhaps one day I'll know enough to write a reply to a question which
doesn't attract so much criticism.
And please excuse any spelling or grammar mistakes.

 

[neil-at-giganews]

"Fred Bloggs" <nospam@nospam.com> wrote in message
news:42EE0E11.6010603@nospam.com...

neil-at-giganews wrote:
"Fred Bloggs" <nospam@nospam.com> wrote in message
news:42ECED56.2060008@nospam.com...

- include a 33R series resistor at the driver end of each hi/lo to limit
instantaneous current output

Whew- a lot of signal loss into 100 ohm load there- better to put a
ferrite clamp on cable.

That's why I suggested 500 ohm terminators. No doubt it doesn't match
the line impedance properly, but all I was thinking is that limiting the
max current that is available to change the line level will reduce the
noise radiated to other lines.

It will do that but only by a relatively small fraction compared to other
methods. There are two aspects to the twisted pair that eliminate B-field
interference: 1) the current imbalance is minuscule so that net B-field
around the pair is too; and 2) the alternating orientation of a victim
pair relative to the net B-field from another pair results in alternating
field induced voltages in series which cancel. This usually reduces the
differential effects of pair-to-pair coupling to negligible levels. It is
the common mode effect that causes problems, and this occurs only when the
differential driver for a pair changes state, introducing a transient
common mode due to slight differences in the propagation delays of the
OUT-/OUT pair. The resulting dVcm/dt injects currents through all
available coupling capacitances into victim circuits, and this current
will be a common mode current, which means each line of a victim pair
receives equal amounts of interference current. The common mode injection
current does not cancel in the victim circuit and develops a common mode
voltage at the receiver.

I've noticed that some "bus driver" devices (no doubt out of date now) had

series resistors on the outputs, and that some of our newer circuits which
have small surface mount devices also have such resistors (10R I think).
When I asked about them, I was told "to stop ringing and stuff coz the
devices are so small there is no parasitic lead inductance any more" so I
assumed they were a good idea.
Does the ground plane on the pcb make these device outputs act like a 'pair'
or transmission line with the ground plane?
'scuse my ignorance, I'm just curious and know not a lot.
Neil

 

[Tony Williams]

In article <42ED03E1.4050706@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:

Nah- the major idea missed here is that "serialization" does not
mandate bit serial. He could always do byte serial with latching
strobe or some other equally simple approach to fit this on a 25
DSUB, while retaining the best driver technology for the job ,
RS-422.

IEEE-488 does bytewide data exchange, or look at the
LTC design note DN168.... RS485 operating at 52Mps
over 100ft of shielded pair... Cat5 cable basically.

--
Tony Williams.