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JZ20-J-T40 with JZ-RS4

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Hi, everybody,

 We have strange situation - we prepared Jazz PLC (JZ20-J-T40) for local boiler room control and for data collection to BMS system (for that we configured Jazz as a slave with JZ-RS4 comm module (RS-485)). BMS was made by other company(they used Schneider SmartX controller AS-P). They get all the information, but in our cabinet all LED pilot lights and LED indication lights of relays that connected to PLC's outputs are flickering. If we disconnect modbus cable, everything is fine. When connect back - LEDs flickers. BMS company (their modbus cable) says the cable is fine, properly grounded and etc. What could be the cause of that strange behaviour and what could we do to prevent that? Thank you!

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It definitely sounds like an incorrect power configuration issue.  Amongst other possibilities, the flicker could be your power supply constantly approaching it's shutdown limit, but not quite getting there.  I'd check a few things.

1).   Are the 2 systems being powered from different phases of a 3 phase supply?   I ALWAYS ensure that all controls are on the same phase.  And associated with this, ensure that the neutral to your gear's location is ok as well, and the actual volts at your end are correct.

2).  Have you got the modbus cable earthed at your end?  If so, if you connect, but with the earth disconnected, does the problem go away?

3).  If the problem persists after checking 1 & 2, measure the current draw on your power supply with a good quick acting DC clamp directly at the output, with your system running with and without the modbus cable connected.  If you can't get a good enough DC clamp, put an analogue ammeter in the line.  If you find that there is a significant difference, then there is definitely a power interaction issue.   Tracking the difference down the various branches will help pinpoint things.

cheers, Aus

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