Extreme cable length signaling - Page 1
Forum section: electronics / projects

author: tatteredsocks
author_rank: Newbie
timestamp: February 06, 2025, 04:52:03 pm
content: Hello everyone,Looking for advice on how to improve basic single ended cable signaling (RS322 / AMI / manchester / etc) on extreme cable lengths >10k' with a DC offset of around 200Vdc.Current design we use is basic push pull where duty cycle is at 50%. Scope showing 50kbaud (20us) with 10us pulse width to allow cable to breath and reduce ISI. We are trying to increase buad rate to ~200k.

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 06, 2025, 06:02:36 pm
content: A few questions:1. Are the Pulse+ and Pulse- inputs differential, or does each one pulse independently (one for positive pulses, one for negative)?2. Are these waveforms taken with the long cable attached? How is the other end terminated?Overall I wouldn't expect to see such a lowpass filtering effect on the channel 1 trace, even if the long cable is attached, and regardless of its termination (10kft length means a delay of 7-10us, so the termination of the cable wouldn't affect the waveform until at least ~15us after the leading edge). It looks like there's a big fat shunt capacitor sitting right on the output of the driver.

author: tatteredsocks
author_rank: Newbie
timestamp: February 06, 2025, 06:22:31 pm
content: 1) Pulse independently. Because cable is mono-conductor, bipolar pulses can be utilized to maintain dc balance (AMI). I am interested in switching to different modulation like RS232 or PAM3.2) Yes this scope was with cable attached. Other end is decoupled then buffered. (see attached)

author: tggzzz
author_rank: Super Contributor
timestamp: February 06, 2025, 06:34:40 pm
content: I suggest treating the cable, tx and rx as a radio system. Then transmit two frequencies or phases in the form of BFSK or BPSK modulation.Understanding modulation schemes and the channel characteristics is the key to understanding noise budget and how to increase the bit rate, and why there is an important distinction between bit rate and baud rate.For details see communications theory textbooks. I've used Sklar’s Digital Communications: Fundamentals and Applications, but there are many others.

author: voltsandjolts
author_rank: Unknown
timestamp: February 06, 2025, 06:38:52 pm
content: Is this for a Camesa or Rochester type cable?Sour service high nickel content armour?

author: tatteredsocks
author_rank: Newbie
timestamp: February 06, 2025, 06:55:14 pm
content: Quote from: voltsandjolts on February 06, 2025, 06:38:52 pm

author: voltsandjolts
author_rank: Unknown
timestamp: February 06, 2025, 07:03:20 pm
content: In that case, yes, good old manchester has been the robust method used since the 70's. You say 180VDC, so at a guess WA or BA for cased. While robust, manchester is not bandwidth efficient. I'll send you a PM, but not sure if you can reply yet as a new member... I think you have to make 5 posts here or something.

author: tatteredsocks
author_rank: Newbie
timestamp: February 06, 2025, 07:51:35 pm
content: Quote from: voltsandjolts on February 06, 2025, 07:03:20 pmCorrect, I have not seen manchester used above 25kbaud in this application.

author: Marco
author_rank: Super Contributor
timestamp: February 06, 2025, 08:09:22 pm
content: Just use a carrier.

author: tggzzz
author_rank: Super Contributor
timestamp: February 06, 2025, 08:29:40 pm
content: Another point of information... Back in the late 70s 2Mb/s was being transmitted along twisted pairs between telephone exchanges, i.e.>>10kft.Use basic communications theory and practice, to see how close.you can get to the Shannon limit. You have a long way to go

author: moffy
author_rank: Super Contributor
timestamp: February 06, 2025, 10:15:52 pm
content: Since the cable has a simple RC rise and fall time you can compensate for that by changing the shape of the drive waverform i.e. high initial peak that tapers down.

author: jbb
author_rank: Super Contributor
timestamp: February 06, 2025, 10:30:45 pm
content: I think that’s called ‘pre-emphasis’ right?

author: tggzzz
author_rank: Super Contributor
timestamp: February 06, 2025, 10:35:12 pm
content: Quote from: moffy on February 06, 2025, 10:15:52 pmPre distortion is a valid technique, but it isn't a simple RC risetime, and isn't necessarily constant nor predictable.It is better to rely on techniques that are insensitive to the channel characteristics.

author: moffy
author_rank: Super Contributor
timestamp: February 07, 2025, 12:29:52 am
content: Quote from: tggzzz on February 06, 2025, 10:35:12 pmThe scope traces look pretty similar to simple RC, maybe an initial step on the edge but that can be taken into account, and if it is a fixed cable length installation you can legitimately optimise the filter/drive for that circumstance. But baseband signalling is BW wastefull and a modulation scheme has advantages but one still might need to take into account the cable characteristics."I think that’s called ‘pre-emphasis’ right?"Sounds good, you might want to drop the overall signal level though, to give headroom for the +/- 15V supplies.

author: tatteredsocks
author_rank: Newbie
timestamp: February 07, 2025, 12:51:27 am
content: Quote from: jbb on February 06, 2025, 10:30:45 pmI haven't seen any implementation that is not FPGA coded. I've seen some IC's will adjustable pre-emphasis built in but they are low temp. I need higher temp than standard 85c

author: tggzzz
author_rank: Super Contributor
timestamp: February 07, 2025, 01:41:22 am
content: Quote from: moffy on February 07, 2025, 12:29:52 amWithout knowing the cable type and where it is installed, it is difficult give firm advice.Cable insertion loss is normally regarded as being proportional to sqrt(f), but maybe not in this case.There is no statement about what, if any, EMI/EMC regulations might be relevant. Those might push it to being necessary to have coax cable, or wires for power plus optical fibre for data.QuoteErrr.. usually

author: coromonadalix
author_rank: Super Contributor
timestamp: February 07, 2025, 02:21:42 am
content: mono cable at >10km i never saw anything outside differential pair like canbus rs422 rs485 .... and with length you slow down the speed 200 kb at this length, i have big doubtsyou have other means, cellular network, even a microwave dish, we had one for simultaneous 4x video streams .... but microwave loll it may roast your nutsyou can have long wireless network systems ??fiber optics ??alas many are priceycan you use some mesh sytems like xbee ? repeaters at x intervals ?other ideas : we did use xbee stuff to control a lot of appliances in a mesh fashion, redundancy or other uses ...https://www.digi.com/products/embedded-systems/digi-xbee/rf-modules/sub-1-ghz-rf-modules/xbee-pro-900hpup to 28 miles range w high gain antennas etc .. under 60$ it's a stealerand theses can do 200Kbpswhat do you want to communicate with ? what is the equipment use at this cable end ?

author: David Hess
author_rank: Super Contributor
timestamp: February 07, 2025, 02:49:42 am
content: I have gotten excellent results driving long RS-232 lines with a current output driver which saturates at RS-232 voltage levels. This could be adapted to drive a long line terminated into 100 ohms through a coupling capacitor.Quote from: tatteredsocks on February 07, 2025, 12:51:27 amPre-emphasis long predates FPGAs and digital signaling, for instance with analog video over longer cable lengths. It could be implemented with an analog network, and variable pre-emphasis is possible in the analog domain using circuits like video faders.

author: moffy
author_rank: Super Contributor
timestamp: February 07, 2025, 03:51:28 am
content: Quote from: David Hess on February 07, 2025, 02:49:42 amRS-232 can have a DC offset unlike Manchester coding, so going through a capacitor could be problematic. The idea of a low value terminating resistor though, is well worth exploring.

author: David Hess
author_rank: Super Contributor
timestamp: February 07, 2025, 11:50:55 am
content: Quote from: moffy on February 07, 2025, 03:51:28 amI assume the OP is taking that into account. The DC offset of RS-232 can be removed by transmitting a character, and then a DC balance character, if transmission is continuous. Alternatively, if only the edges of the RS-232 signal are sent, then a set-reset latch or flip-flop at the end can restore the RS-232 levels; this has the disadvantage of having lower noise immunity.

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 07, 2025, 01:10:21 pm
content: Quote from: tatteredsocks on February 06, 2025, 06:22:31 pmMakes sense. Choice modulation scheme, NRZ vs RZ, etc is separate from what I'm asking about though (the filtering on the pulse output).QuotePreemphasis is an option for dealing with it, but it shouldn't be necessary in the first place IMO. There must be something else missing from these schematics to explain the lowpass filtering, otherwise it should have rise/fall times well under 1us.Maybe it's something to do with this mysterious 180V bias voltage? The first schematic says "180V ~100ohm ~1.5uF", could you show an actual schematic of what this refers to?

author: Marco
author_rank: Super Contributor
timestamp: February 07, 2025, 03:30:39 pm
content: Quote from: mtwieg on February 07, 2025, 01:10:21 pmIt's high pass, but the output (left of the capacitor) isn't being measured. The output responds instantly and then droops with RC time of 20 us.

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 07, 2025, 03:50:58 pm
content: Quote from: Marco on February 07, 2025, 03:30:39 pmThe waveforms posted above contradict this...

author: moffy
author_rank: Super Contributor
timestamp: February 07, 2025, 11:48:18 pm
content: This LTSpice simulation borrows from David Hess's idea of a terminating resistor, using it to increase the effective bandwidth.If you think of R1/C1 as the cable, then adding R3 through C3 increases the BW by about x10 while also reducing the signal amplitude by same. This is illustrative only as the actual effective series resistance and capacitance of the cable are unknown, but the time constant should be roughly similar to that shown in the oscilloscope captures.

author: Marco
author_rank: Super Contributor
timestamp: February 08, 2025, 09:26:03 am
content: Quote from: mtwieg on February 07, 2025, 03:50:58 pmIf you assume the 180V power supply is a low impedance load, lets say 1 Ohm, then it will see the signal immediately and then droop with a RC time of 21 us.All you are seeing on the traces is the exact results you would expect from the RC circuit.

author: Marco
author_rank: Super Contributor
timestamp: February 08, 2025, 09:30:44 am
content: Quote from: tatteredsocks on February 06, 2025, 04:52:03 pmBTW, this doesn't exist of course ... what is the signal return exactly? A cable shield for a multi-conductor cable? The ground beneath a suspended power cable? What?

author: voltsandjolts
author_rank: Unknown
timestamp: February 08, 2025, 09:40:28 am
content: I would assume there are inductors at both ends of the cable to isolate the comms from the 180Vdc.For this discussion, we could also assume the cable specifications are akin to this:https://www.camesawireline.com/products/emc/products-info/view/product/id/1k22-monoconductor

author: voltsandjolts
author_rank: Unknown
timestamp: February 08, 2025, 09:44:10 am
content: Quote from: Marco on February 08, 2025, 09:30:44 amI suspect they mean simplex/unidirectional comms, from transmitter (of sensor data) to receiver (datalogger/display) over a cable similar to that I linked in the post above. 180Vdc powers the remote sensor package and transmitter.This very short yt video shows a typical application of such a system.https://youtu.be/_3mzQTMwkV4?feature=shared

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 08, 2025, 08:39:26 pm
content: Quote from: Marco on February 08, 2025, 09:26:03 amSo you're suggesting that the other side of C24 is connected to a 180V voltage source with just 1ohm of impedance? In this case yes one would expect a ~20us time constant on the right terminal of C24. That would also mean the signal is attenuated by a factor of ~21, and is highpass filtered with the same time constant. I guess maybe the attenuation might be allowable if the receiver is designed properly...So what specifically keeps this from working at higher baud rates? Are you also working on the same project as tatteredsocks?Quote from: voltsandjolts on February 08, 2025, 09:40:28 amYes, that should certainly help. But again, no schematic showing how this source is connected at either end...

author: benseno
author_rank: Regular Contributor
timestamp: February 08, 2025, 10:48:43 pm
content: Did you consider ready made power line communication chips?https://www.ti.com/document-viewer/lit/html/snoaab6

author: voltsandjolts
author_rank: Unknown
timestamp: February 09, 2025, 11:52:33 am
content: Quote from: mtwieg on February 08, 2025, 08:39:26 pmIsolating the dc power source/sink from the communications driver/receiver is necessary, and should be simple enough that any suggestion you have could be implemented, if it improves communications. Feel free to experiment. In this type of application, current draw on the dc supply is typically less than a couple of amperes. 180Vdc would be the target voltage at the sensors/transmitter and cable resistance means the dc source end of the cable would be say 100V higher, or whatever.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 10, 2025, 01:16:52 am
content: Quote from: tatteredsocks on February 06, 2025, 04:52:03 pmManchester wastes 50% of bandwidth but gives an average of 0 for each token.If you want a DC average of 0, so that you can AC couple this,you can use RS232 with a subset pattern, and some RS485 type help.There are 70 symbols in the possible 256, that have a DC average of 0, and start and stop bits average 0, if you disable your drivers after the stop bit, aka RS485 schemes.Some MCUs have hardware support for this RS485 disable signal. ie long inter-byte pauses do not drive the cable.That averages to zero within each character, so you can send slightly over 2^6 bits in an 8 bit byte in a RS232 10 bit time slot.You can encode 6 bits per RS232 byte and have 6 other tokens for framing information.