Communications Shielding/Wiring

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fifty60

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USA
I know that it is preferable to always shield as close to the electronics component as possible. I have a an HMI screen that I am wiring RS232 and RS485 communications to. I have 2 questions about this:

1) Is it ok to use the same comms cable for both RS232 and RS485. Would a single 22/8 cable work? I know the RS232 can be +/- 14V at extremely low amps, but is this enough to not have it ran in a single cable? Both communications terminate at the same DB-9 at the HMI screen.

2) There is not a good place to ground at the HMI and still keep the ground wire at 2" or less. Is it ok to ground at the on the initial DB-9 and RJ-11 side of the communications, not the HMI side? What is the trade-off here? I do not see how this would affect common mode or differential noise..
 

dfmischler

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Facilities Manager
1) Is it ok to use the same comms cable for both RS232 and RS485. Would a single 22/8 cable work? I know the RS232 can be +/- 14V at extremely low amps, but is this enough to not have it ran in a single cable? Both communications terminate at the same DB-9 at the HMI screen.
The EIA-232 standard calls for controlled slew rates on signal transitions, so there should be no fast edges to cause extra problems for the other signals. I can't think of any reason they can't run in the same cable unless the manufacturer objects (unlikely if they terminate on the same DE9 connector). Make sure you assign proper pairing - i.e. twisted pairs if possible - to any differential signals (EIA-485). This will help protect the EIA-485 signal from EIA-232 transitions and vice-versa.

2) There is not a good place to ground at the HMI and still keep the ground wire at 2" or less. Is it ok to ground at the on the initial DB-9 and RJ-11 side of the communications, not the HMI side? What is the trade-off here? I do not see how this would affect common mode or differential noise..
I am not familiar with your HMI equipment, but there are generally two issues with grounding cable shields: you want a quiet ground (which argues for a low-impedance path to earth - as short as possible with big wire or even braid to big metal in extreme cases, and connecting both ends of the shield), and you don't want to introduce ground loops (which argues for only connecting the shield at one end of the cable). These two goals are clearly in conflict. The EIA-232 signals are single-ended and referenced to the signal ground so weird things could happen if the ground is noisy enough or if there are ground potential differences, but for short runs there is usually a lot of voltage margin due to the wide voltage swing (I believe the minimum is 6V p-p). The EIA-485 signals are differential and so are much more noise immune in every way, and less concerned about ground reference, although the receiver could possibly break down if the common mode voltage is too high.

Years ago I worked for a company who used mostly optically isolated differential signalling on runs of up to several thousand feet (e.g. crossing a bridge over a major river) to avoid issues of noise and ground potential differences.
 
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gar

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Location
Ann Arbor, Michigan
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141210-0947 EST

fifty60:

What is an HMI?

You may need to ask some other questions. For example ---
1. What is the distance between source and destination?
2. What is the RS232 baud rate?
3. Where and how are the EGCs of source and destination located and related?

An early RS232 receiver, MC 1489, has a positive nominal threhold of 1.3 V and min-max of 1 and 1.5 V. On the negative side these values are -1.0, -0.75, -1.25 V. All at 25 C. This was based on early RS232 specifications.

Well designed RS232 receiver ports do not exist on some equipment. Some just use a transistor base emitter junction as the threshold device, not much hysteresis.

For short distances Belden 8723 is a good choice. Cable length is inversely proportional to cable length. Cable capacitance is the major problem.

Ground potential differences are a great problem, and somewhat related to separation distance between source and destination on the common or ground path.

We make optically isolated RS232 modules that provide capability of 115.2 kbaud up to about 4000 ft. As a test I have applied a 1000 V RMS sine wave to one of the interconnect wires between the two interface modules while transmitting data with no data errors, or damage to either RS232 driver/receiver.

.
 

fifty60

Senior Member
Location
USA
The distance is very small, so I probably do not even need to ground the cable. The maximum run would be less than 20ft, and usually only around 10ft. Would I be better off not grounding at all RS232 runs of 20ft or less?


Baud rate is 9600, so capacitance should not be an issue. From what I understand length is never the actual issue, but capacitance is the limiting factor.

Would I be better off not grounding at all RS232 runs of 20ft or less?

Is it correct that for RS485 I do not even have to ground my comms cable?

It seems like there is not a one cable fits all solution to combine RS485 and RS232...
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
141211-1003 EST

fifty60:

At 20 ft and 9600 baud Belden 8723 will work fine. This has a drain wire that I would connect at one or both ends. Keep in mind that the RS232 common is already connecting the chassis at each of the communication path unless there is some form of isolation somewhere in the path.

Where your problem exists is how the EGCs are derived. This is true for both RS232 and RS422/485 where there is no isolation in the communication paths between the two ends. The AC power including EGC for both ends of the communication path should derive from a single source such that the power wiring first goes to one end of the comm path, and then proceeds to the other end. This provides the shortest most direct path for the EGC between the two comm ends.

If each communication end has its own AC power and EGC run back to the main or sub panel, then a large voltage difference (up to about 1/2 the hot voltage) can occur on one EGC comm end to the other EGC comm end from a hot to EGC short at one comm end. Lightning can also cause a similar difference.

This voltage difference can burn out all RS232 and RS422/485 chips, and possibly others.

.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
141211-1621 EST

Since HMI is simply an interface, and assuming that it requires 120 V power, then simply run the AC power cord along with the 8723 communication cable, and derive power and EGC from the machine.

If there is no 120 available in the machine, then install an appropriate control transformer to provide 120. This will solve most "ground" problems with the exception of a welder putting his ground electrode on the machine and welding on the HMI support.

.
 
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