RS485 multidrop network drops after few days

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Thread Starter

P Paul

I have a multidrop connection as follow:
AB PLC->Prosoft Card(RS-232)->FO Converter(B&B)->FO Converter(B&B)->Serial Converter(to RS485)->4 Slave devices (EATON - Diesel Pumps).

Initially the communication works fine, but after few days it drops for all slave devices and when I go to one slave device and change any modbus setting for example, changing parity from EVEN to NONE and then back to EVEN the communication will resume to all salves right away. But then again it will drop after few days.

Please help!
 
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Lynn August Linse

Based on 25 years experience with RS-485, sounds to me like you have a floating ground between the final converter and the pumps, so you're building up enough of a ground potential that over voltage diodes kick in & shunt signal (so for example a pump putting out '7 volts' might look like '15 volts' to the converter). So in other sites I've seen RS-485 run fine for a week or so, then be offline for hours (days), then come back online for another week and so on. Exact behavior depends on hardware design of the products.

I'd assume the act of changing setting is somehow causing the potential difference to discharge? This might happen for sure if you connect something to the pump like a laptop.

I can't really explain HOW to properly ground the pumps in relation to RS-485(Eaton might have an app note), but for sure you rarely can just use 2-wires or 4-wires when MIXING vendor equipment; you'll need 2-wires plug 3rd ground wire or 4-wires plus 5th ground wire. If the B&B has an isolated/floating RS-485 ground, at a minimum you'll want to run the ground between the converter to the first pump, assuming Eaton somehow handles a common ground between pumps.

Worse-case you might need to install some more RS-485 isolated repeaters ... but I'd start with Eaton pump documentation about use of RS-485 on multiple pumps. If they explain running the separate ground wire, then you should be able to solve your problem.
 
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