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Do termination resistors help with EMI immunity?

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strantor

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I am trying to communicate with a device via RS-485. point-to-point, only 2 nodes.
The bus media is 200m of shielded twisted pair 22AWG inside a multiconductor cable.
in the other conductors of the cable flows 8-20kHz high voltage PWM from a VFD.
There is an exceptional amount of noise in the cable.

I am not using any termination resistors currently.
I can communicate just fine with my remote device when the VFD is not running.
As soon as I start the VFD, communication is destroyed.

All that I have read about RS-485 termination resistors talks about transmission line impedance and the resistors are to correct propagation delay.

My question is, in addition to affecting propagation delay, do the resistors do anything to help with the EMI? Would I see any benefit of implementing them, considering my setup works just fine without them, in the absence of EMI?
 
Hi,

Your noise is likely caused by capacitive-coupling between adjacent wires in the cable. Doing anything to reduce the impedance from your RS485 wires to each other (or to ground) will help.

Figure 5 (right) of **broken link removed** have some suggestions that will improve your communications...
 
All that I have read about RS-485 termination resistors talks about transmission line impedance and the resistors are to correct propagation delay.
No, the termination resistors prevent reflections from the end of the cable.
If you look with a scope you will see some odd steps in the waveform as it is reflected from the un-terminated far end of the cable.

in the other conductors of the cable flows 8-20kHz high voltage PWM from a VFD.
There is an exceptional amount of noise in the cable.
An unhappy situation.
If you could run a separate screened cable for the RS485, that would help considerably.
Is this an umbilical to go subsea for your modified motor? In which case I guess you are stuck with what you have got.

What is your signalling speed?
If it is not too high, you could consider putting low pass filters at each end of the cable to attenuate the noise from the PWM and leave the RS485 intact.

JimB
 
An unhappy situation.
If you could run a separate screened cable for the RS485, that would help considerably.
Is this an umbilical to go subsea for your modified motor? In which case I guess you are stuck with what you have got.
Yes it's the subsea umbilical. No option for a separate cable.
What is your signalling speed?
If it is not too high, you could consider putting low pass filters at each end of the cable to attenuate the noise from the PWM and leave the RS485 intact.

JimB
I will try that as well as the 3-resistor biasing circuit in the app note posted by MikeML, if other options don't pan out. I don't need any more than 9kbps.

So far I've tried RS485, CAN bus and DSL. I expected RS485 to work the best, followed by CAN bus, and I figured I would get a free take-home for the DSL modem pair. But much to my surprise, RS485 and CAN bus both fell on their faces while the DSL modem pair was almost a great success.

The DSL modem pair performs flawlessly, like it doesn't even know it's transmitting through the worst EMI environment conceivable, 95% of the time. Zero data corruption, zero difference in latency between VFD running or not running. But There are occasional hang-ups where 4-5 seconds elapse with no throughput, then back to normal. Or not; a couple of times I've had to reset the modems.

I really expected more out of RS485; In light of the proof of concept given by the DSL modems, I suspect I'm doing it wrong, hence this thread. I want to make sure all the nails are in the coffin before I bury it.

In the morning I'm going to test out a homebrew opto-isolated digital current loop serial interface. I've read about old teletype machines using these types of communication. 20mA or 60mA ON, 0mA OFF. I tested 4, 20, 60, and 100mA current levels today, and all of them showed equally nasty spikes (>30V) on the scope when the VFD was running. I was surprised that even with 100mA flowing through the wire, the EMI was enough to cause these spikes. But even with the spikes, there was no change in the current level as read by my DMM. I think that current-based (high current,60mA maybe) digital comms has a fighting chance.
 
The DSL modem pair performs flawlessly, like it doesn't even know it's transmitting through the worst EMI environment conceivable,
Could it be that the DSL modem is operating at a reasonably high frequency which is above the noise frequencies from the VFD and has high pass filters which attenuate much of the noise from the VFD?

Current loop comms, I have not seen that used in a long time, but worth considering if it gets you over the noise problems.

JimB
 
Hy strantor,

Can you answer some questions:

(1) Can the 200m cable be changed or is it a given?
(2) If the cable is a given can you post the exact type or ideally a link to the specification?
(3) What is the voltage levels of the 8KHz to 20KHz PWM signal
(4) Is the PWM signal a single line rather than differential
(5) What is the driving impedance of the PWM signal
(6) What is the receiving impedance of the PWM signal
(7) Ideally, could you post an exact schematic showing the connections, especially screening and earths for the complete link

spec
 
Last edited:
Hy strantor,

Can the 200m cable be changed or is it a given? If it is a given can you post the exact type or a link to the specification?

spec
I'm currently testing with the umbilical from another tool. The actual umbilical that this tool will use in the future does not yet exist, but is expected to be similar. Unfortunately I cannot post the cable spec document as it has warnings plastered all over it stating that it is property of the manufacturer and not to be disclosed to 3rd parties. But I could answer any specific questions about its properties.
 
Could it be that the DSL modem is operating at a reasonably high frequency which is above the noise frequencies from the VFD and has high pass filters which attenuate much of the noise from the VFD?
I guess so, but it sounds a bit funny considering the usual answer for comms problems is to decrease the baud rate.
 
Somehow you're going to have to come up with some scheme that will allow your communication signal to "compete" with the crosstalk interference. A couple of possible approaches come to mind, that could be used alone or in combination:

  1. Drastically increase your signaling voltage, possibly with a custom-made line driver that will put out ±15V (or even more), together with de-sensitizing the receiving end to bring you back to logic levels.
  2. Use differential signaling.
  3. Use diode clamps at both ends of the cable, to clip any crosstalk transients to ±15V (or whatever the driving signal amplitude is).
  4. Low-pass filter the received signal as much as possible.
  5. Use your communication signal to FSK modulate a high-frequency carrier (perhaps a couple of MHz), and de-modulate at the receiving end.

Given the constraints you're working under, that's about all I can think of.
 
Hy strantor,

Can you answer some questions:

(1) Can the 200m cable be changed or is it a given?
(2) If the cable is a given can you post the exact type or ideally a link to the specification?
(3) What is the voltage levels of the 8KHz to 20KHz PWM signal
(4) Is the PWM signal a single line rather than differential
(5) What is the driving impedance of the PWM signal
(6) What is the receiving impedance of the PWM signal
(7) Ideally, could you post an exact schematic showing the connections, especially screening and earths for the complete link

spec
I barely caught your edit just now.
1,2 - see reply #7
3 - 0-3,000V (not a typo)
4 - It's 3-phase PWM "sine" AC for a motor
5,6 - I don't know
7 - I will draw one tonight
 
I barely caught your edit just now.
1,2 - see reply #7
3 - 0-3,000V (not a typo)
4 - It's 3-phase PWM "sine" AC for a motor
5,6 - I don't know
7 - I will draw one tonight

Thanks srantor.

Answer (3) is 3KV, WOW, no wonder you are having cross talk problems. Just to confirm being three phase that will be three independent wires?

I know you said that you can't post data about the cable but can you give the type, or has the cable been custom made for you?

It will be a help to see your circuit.

spec
 
Are you doing the PWM at the bottom or at the sea-level? It would be best if you could feed the setting of the PWM from the top, have the pwm controller at the bottom and have enough filtering down there so that you don´t have any crosstalk issues.
 
I would agree that termination resistors could help the problem. The resistors will absorb some of the interference power and should reduce it's amplitude more than it reduces the signal voltage.
 
I have no idea what you are doing Strantor but for some reason it still intrigues me. The idea of developing electronics for oceanographic applications just sounds so adventurous. Going out to see for a test run, working along side scientist, it all just sounds fun to me :cool:. Perhaps when you are finished you will be able to share your ocean going adventure with us with some video footage or what not.
 
Agree with the terminations. Without them you could have some real ugly stepped waveforms causing false triggers.
 
Ok I tried out my 40mA digital current loop circuit and it worked great, on the bench. Up to 56kbps.
Then I tried it out with the cable, no dice. Had to lower current down to 4mA and lower baud rate down to 600bps. Then it worked. Even with the VFD running it worked. But 600bps is too slow.

So I went back to RS485, trying the termination resistor schemes described in Mike's app note. RS485 still cannot compete with the noise.

I guess all that's left are the suggestions in post #9 and that's a rabbit hole I don't have time to explore.

I really want to just go with the DSL modems but I can't guarantee reliability. I think I am going to have to default to fiber optics at this point. I really wanted to avoid fiber.
 
Are you doing the PWM at the bottom or at the sea-level? It would be best if you could feed the setting of the PWM from the top, have the pwm controller at the bottom and have enough filtering down there so that you don´t have any crosstalk issues.
Yeah I know, and that would solve some other problems too, but unfortunately that's not an option; PWM comes from topside and it must stay that way. The whole point of this exercise is to shrink the subsea electronics capsule down from the size of a duffel bag to the size of a cigar box. and what you're suggesting would increase the capsule size to that of a shower stall.
 
I have no idea what you are doing Strantor but for some reason it still intrigues me. The idea of developing electronics for oceanographic applications just sounds so adventurous. Going out to see for a test run, working along side scientist, it all just sounds fun to me :cool:.
Yeah it is fun, when it isn't stressful, and it's always stressful.

The company I work for makes subsea saw tools. They are made in Europe, designed for ultra deep water, and they cost (us) millions. Unfortunately we cannot sell them for millions, here in the Gulf of Mexico.
There is no ultra deep water in the gulf and no need for an expensive ultra deep water saw. So I'm trying to cut down on the bells and whistles, make everything simpler, cheaper, and better suited for the GOM market.
The company only employs one Engineer here in the states, and that's me. I'm supposedly a Control Systems Engineer, but I spend more time doing anything but that. Mechanical Engineer, Procurement Specialist, Forklift driver, R&D etc, too many hats to keep track of. I'm the closest thing to a scientist around here, and sometimes it does feel like I'm working alongside myself.


Perhaps when you are finished you will be able to share your ocean going adventure with us with some video footage or what not.
Sure thing.
For now, here's the motor I designed, almost assembled enough to test next week.
20160419_091753_resized.jpg 20160419_093258_resized.jpg 20160419_093547_resized.jpg 20160420_074214_resized.jpg 20160420_074655_resized.jpg
 
Wow, that is awesome, did you do the machine work yourself?
 
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