I2C cable

[Winfield Hill]

Jan Panteltje wrote...

Reflections from inductive and/or delay-line
loads, i.e. not properly terminated?

Unless those take round the world trips those
will be sufficiently attenuated after some short time.
How fast can bees fly? Why be in a hurry?
A fly is different those can be really fast ;-)

But seriously, just use cable with individually
screened pairs, like I used audio cable, if you
want to cover a few meters.

That's a thought, any with good pre-made connectors
and four separate conductors (maybe two grounds)?

For more use special drivers if unidirectional
(no checking of SDA pulldown). Else go optical.
Optical fiber does not take that much space, is
'trickety proof, a PIC as i2c to optical interface,
whatever.

Yessir, can optical carry 3.3V power?

With wasps it would be easy, you give those
individual stripes for different messages,
and use a barcode scanner, training is needed.

Other ideas ?
?
ants?

Check my other thread, has schematic:
Five beehive sensor ICs (I2C and 1.8V max)

And my new thread,
RJ45, RJ12, RJ14 cables straight-through or crossover?


--
Thanks,
- Win

 

[Jan Panteltje]

On a sunny day (12 May 2019 06:20:05 -0700) it happened Winfield Hill
<hill@rowland.harvard.edu> wrote in <qb96i5010p2@drn.newsguy.com>:

But seriously, just use cable with individually
screened pairs, like I used audio cable, if you
want to cover a few meters.

That's a thought, any with good pre-made connectors
and four separate conductors (maybe two grounds)?

I used DIN 180 degree audio connectors on a 8052 based box in ancient times,
still have that box :
http://panteltje.com/pub/8052AH_BASIC_computer/8052AH_BASIC_computer_inside_img_1727.jpg
remote connected PCF8591 ADC and PCF8574 IO/O expander..
temperature sensor, light control, some other transmitter related stuff.

For more use special drivers if unidirectional
(no checking of SDA pulldown). Else go optical.
Optical fiber does not take that much space, is
'trickety proof, a PIC as i2c to optical interface,
whatever.

Yessir, can optical carry 3.3V power?

How much power? 70 Watt has been done:
http://optics.org/news/4/5/1

 

[Rick C]

On Sunday, May 12, 2019 at 9:12:10 AM UTC-4, Winfield Hill wrote:

Martin Riddle wrote...

On 11 May 2019, Winfield Hill wrote:

Mounted RJ45 connector, but too big, cable too
stiff, unsure of how the shielded variant works.
Went to smaller, simpler RJ12 instead.

There is plenum rated cable, it does not outgas
when burning eg; low smoke.
PVC is not too good for outdoor use.
Might want to check for RoHS/REACH compliance,
that way you will not get the phthalates.

Interesting points. The little RJ12 cable we're
discussing is outside, totally outside. Outside
of the hive, and outside of my beehiove monitor.
If my beehive monitor goes up in flame, I'm sure
it'll outgas! It probably makes phthalates too.
Little RJ14 phone cables may not be happy with
an extended season of sunlight, but the Amazon
RJ12 data cables look like they're more robust.

Look? They need to be rated for outside use. They need to be weather tight or the cable ends need to be behind glands to seal out moisture as well as the plastic being resistant to UV light unless you are putting them in a race. You seem to be greatly over designing the noise issue and under designing the environmental issues.

Maybe they make glands for flat cable, but I haven't seen them. The ones I've seen are for round cable.

--

Rick C.

++ Get a 1,000 miles of free Supercharging
++ Tesla referral code - https://ts.la/richard11209

 

crosstalk from a DATA edge into the clock line can cause a false clock edge.

and therefore an incorrect read of the data.

m

 

[Tim Williams]

"Rick C" <gnuarm.deletethisbit@gmail.com> wrote in message
news:522ae226-1c75-4405-9315-f8d34b16345d@googlegroups.com...

Maybe they make glands for flat cable, but I haven't seen them. The ones
I've seen are for round cable.

I've seen seals that are a couple blocks of firm rubber clamped together
then clamped again by a frame. For example a "stocks" shape (two blocks
with facing semicircles cut out) that grips the cable. Two blank blocks
would do for flat flex, if maybe not ribbon cable (because of its ribbed
profile?).

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

 

[Winfield Hill]

Tim Williams wrote...

"Rick C" wrote ...

Maybe they make glands for flat cable, but I haven't seen them.
The ones I've seen are for round cable.

I've seen seals that are a couple blocks of firm rubber
clamped together then clamped again by a frame. For example
a "stocks" shape (two blocks with facing semicircles cut out)
that grips the cable. Two blank blocks would do for flat
flex, if maybe not ribbon cable (because of its ribbed
profile?).

We're still debating the best way to get cables in/out of the
beehive monitor case. The project P.I. wants to try individual
glands, on the removable top. Six cable connectors would be
assembled after running their wires through glands mounted on
the lid, with a painful PCB mating and cable-pulling step, to
mount the lid and dress wires, before tightening six glands.

I'm in favor of the mate-two-blocks approach, because the cable
connectors can be plugged in, strain-relieved with cable ties,
and remain as a single piece with the electronics. The lid,
with cable cutouts is mounted over. I did this last year,
using a mass of RTV for glands, but squeezable blocks would be
better. Our Raspberry Pi server boxes use two blocks, they're
fast and convenient to open and close, and they never leaked.
Their rubber pieces have considerable give, to accommodate any
wire type, or multiple wires.


--
Thanks,
- Win

 

[Simon S Aysdie]

On Sunday, May 12, 2019 at 12:34:23 AM UTC-7, upsid...@downunder.com wrote:

On 11 May 2019 15:18:46 -0700, Winfield Hill
hill@rowland.harvard.edu> wrote:

Rick C wrote...

Winfield Hill wrote:
Winfield Hill wrote...
Lasse Langwadt Christensen wrote...

the STM have programmable slewrate on the IOs

Aha! I'll see if our programmer can activate that.

I miss-spoke, we're using an Atmel ARM SAM D21.
Not sure if they have programmable slew rate,
but datasheet says the I2C outputs have reduced
slew-rates. Hmm, the IO spec says 20-50ns with
a 10 to 400pF load. Well, anyway, I've added a
resistor (100-ohm) in series with the I2C lines.

A twisted pair typically has an impedance of 100-120 ohs and since the
transmitter has some output impedance, a smaller resistor should be
sufficient to match to the line impedance.

The I2C transmitter has, in a sense, an output impedance, but it is a time dependent one. It is Low-Z when at logic low and Hi-Z when at logic high. That is (for any common I2C scheme), it is impossible to "match" I2C TXers and RXers to some fixed line impedance. (The line won't be properly terminated anyway. Slow slew can save I2C at the obvious cost of data transmission speed.

I2C signal integrity is like saying "Jumbo Shrimp." There are things that can be done, but it is (from the get-go) a crap system from the perspective of signal integrity.

A 100 ohm resistor in the transmitter line will also reduce the low
state noise margin. If the current through the open drain pull-up
resistot(s) is more than 4 mA, this will reduce the low state noise
margin by 400 mV, so if a conventional TTL receiver is used, this will
kill the noise margin completely. Using 47 ohm series resistor and
limiting the drain resistor current to 2 mA and the loss of noise
margin would be only 100 mV.


That seems excessive. Even with a 20 ns rise time
the round trip on a 1 meter cable is around half
the rise time. I think the magic number to ignore
reflections is for the rise time to be 6 times the
round trip delay. The capacitance you can expect
on this run would very likely put the rise time
higher than this threshold so no further actions
would be needed to prevent reflections.

Yes, I agree, but my worry isn't so much about
reflections, as about crosstalk. If a fast SCL
falltime induces an SDA signal, apparently this
can cause trouble. Fix with slow falltimes.

I do not understand this crosstalk objections if CAT5/6 cables are
used, with one pair for SCL and other pair for SDA. After all in
100/1000BasetT Ethernet, the whole symbol time is 10 or 8 ns (not just
rise/fall time) and these cables are specified for up to 100 m.

Of course, i you have split pairs, such as running SDA and SCL in the
same twisted pair, there are going to be problems.


Do you think your rise time will be less than
60 ns?

Yes, according to the spec, without the resistor.
Well, OK, I do have an 74HC4052 channel switch
in series; it may be about 100 ohms at 3.3 volts.