Communication Modules burning up

[Manthan]

Hi All,

System Intro:
We have a labelling system running on the production line . For controls, B&R PLC (X20CP1584) is used and HERMA 400 for labelling application. They are communicating via Modbus RTU. Modbus RTU master (X20CS1030) is used to Tx and Rx data to/from Herma 400.

Communication Set up:
Modbus Master sends signal to Herma 400 (x13 port), then port x14 goes to herma 400_2 (x13 port) and so on. We have total 4 Labelers on this bus topology.

Herma 400 Power connection:
Herma is supplied 230VAC from the panel. Then it has filtering circuit and controllers for Panasonic servo motor. At the end, we have AC to DC convertor circuit which supplies 24VDC to communication card.


So, the issue is that - communication module (where x13 and x14 connects) inside herma 400 getting burnt up, frequently.

Incident 1: 2 weeks ago, all 4 of them were affected at the same time. All labelers lost communication to PLC. Once we replaced communication cards, all started running fine.

Incident 2: After that incident, we bypass no 1 labeler and now communication signal is going trough no2, then 3 and then 4. But, yesterday, operation of no 2 was affected the way. Communication failure.


It is hard to understand what is causing this. Is it from the PLC side (Master Modbus) or the 230VAC supply or the micro board circuit(AC to DC).

Surge protector and breakers are fine. And in working condition. Hope somebody here can point in right direction.

 

[mendonsy]

The first thing to do is check the grounding. If the power source comes from different transformers you could possibly have a big difference in the grounds.

 

[Manthan]

Shouldn't it blow the fuses if drawing more current? In the circuit we have two 3.4Amp Fuses, servo designed to run at 2.5 FLA. And why it is not affecting servo module or I/O module and only communication one.

 

[plvlce]

+1 for likely being at least a grounding issue.

If different points of the communication circuit have different ground potential you will get current flow along the path between them. It doesn't need to be anywhere near enough to blow a main fuse in order to damage a communication card. Comm cards are not designed with the expectation they will experience current beyond the trivial amounts they use themselves.

 

[danw]

At the end, we have AC to DC convertor circuit which supplies 24VDC to communication card.

1. If this is not stand-alone 24Vdc power supply from a major vendor, I would replace it with quality 24Vdc power supply from a major vendor.

2. You can spend a lifetime tracking down ground faults that cause common mode ground loops, or you can buy RS-485 repeater/isolators. An isolator is RS-485 in/RS-485 out, but electrically isolated from input to output.

I live in a thunderstorm area where close lightning strikes take out RS-485 networks all the time. I insist on at least 2 isolators, one at the main master because that's usually an integrated RS-485 port on an expensive CPU or COMM module and another isolator half way down the network bus. I suggest protecting every port with an isolator but nobody ever does.

So the master is protected and at least half of the bus is protected. After the first strike when half bus dies, they buy the remaining isolators and isolate every port.

The cost of doing business.

 

[TheWaterboy]

+1 for the isolators. You need to find that floating ground, but I always isolate when different supplies are involved and often even when they aren't.

 

[Manthan]

Hi,

Spent a much more time to find ground fault. But that doesn't seem to be an issue. All points are grounded related to that system. All circuit boards are as well connected to ground point. Labeler outer body and inner body are well coted and don't let charge flow through it. We have total 16 labelers, 4 on each 3 of the production lines. Each line has encountered this issue, in the past. But, not too frequently.


In terms of 24V power supply, we have already complained to manufacturers to look into the design. But, they are most certain that circuit is well designed and delivering required voltage to the com module.

But, as @danw mentioned, since electrical fault on COM line could damage 485 driver, we will stick two RS-485 repeater/isolators for trial. And see if situation improves or not.

Thank you for all the responses.

 

[sparky66]

Please clarify what you mean by "burning up". Is there a burnt smell, or do they just stop working?

 

[Manthan]

Out of the blue, stops working. Sometimes 3 in a row (connected via Profibus) at the same time. And sometimes first one (Bus Add. 1) after Master module. HMI says, communications not established to the labellers. As soon as we replace that circuit board, it comes alive and starts working.