MODBUS RTU over RS485 problem with Schneider SEPAM Protection Relay

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Thread Starter

asaria

Hi All,

Any of you experience in setting up MODBUS RTU communication with Schneider SEPAM Protection Relay?

I configured 2 wires multidrop communication (my PLC as Master and 3 SEPAM Relays as slave) with 9600 baud rates, 8 bits data, None parity, and 1 stop bit.

I found that after a day the transaction occurred then I got communication error with it.

I suspected that the problem could be there's an overload occurred in transaction. Because when I tried to troubleshoot with initialize whole data and start new transaction, it worked fine.

Any better solution? Since i'm newbie in this stuff.

Thanks.
 
Asaria,

Which PLC are you using as a Modbus Master? If you are using any of the smaller PLC models, such as Allen Bradley MicroLogix 1100, GE VersaMax, or Siemens S7-200, make sure that you are sending only one Modbus message instruction on the Modbus port at one time. I had this issue particularly when using GE VersaMax. My communication over RS485 was intermittent because I was not waiting till previous instruction on the port was complete.

Although you are talking at relatively slow baud rate, try installing terminating resistors (at both ends of the network cable), with the resistor value corresponding to the impedance of the network cable (typically, 120ohm 1 Watt).

Monitor response time for all the nodes. Check if the response time out configured in the message instructions is sufficient for the nodes for sending the response.

Hope this helps.
 
Amit,

Yes, I used 120 Ohm termination resistor at Master Data+ and Data-, and at last node of slave I used RCX switch (for terminator).


 
L

Lynn August Linse

> I found that after a day the transaction occurred then I got
> communication error with it.

Are you running the RS-485 third signal return wire? RS-485 is never a 2-wire signal: you either have to have a shared ground path via the power supply or chassis-frame (so 'bicycle-electronics), or a third ground wire.

An RS-485 2-wire circuit with usually works, but stops for say 10 minutes or an hour every few days is a classic sympton of an unreferenced RS-485 line. I've worked with RS-485 for 20+ years and have even been paid to go to sites to troubleshoot RS-485 systems as consultant.

General rule of thumb - pure 2-wire (no 3rd ground) only is reliable with a single vendor's products, so master and slave all belong to a 'system' and that vendor has handled the ground indirectly via the power system.

Pure 2-wire frequently doesn't work with multi-vendor mixed systems. Some folks get lucky and their complex 2-wire works, but then some people drive their car drunk or without wearing seat-belts and don't die ... doesn't make either a good idea.
 
B
This excerpt is from a RS485 primer published by Weidmuller a supports what Lynn has written.

If only the two signal wires are used to connect the nodes then the commons of the electronic devices may be floating in relation to each other. If the relative voltage exceeds the common mode specifications, then not only may communications be disrupted, but permanent damage to the integrated circuits may also occur. At its most frustrating, communications ay be intermittent.
 
J
Also is not correct to run the ground "wire" through the cable shield (as i saw in many installations).

Also not correct to connect the shields extremes in both ends of the cable. Not correct to route the DC power wires in the same cable as data wires. And be careful to run the data cable to close to the power cable (as you are working with a high voltage/high current environment).
 
I have similar problem where the 3rd part meters over modbus getting failed after 2-3 days. So what wiring precaution to be taken to wire RS-485. Please advice some a steps as advised by Mr. Lynn August.

Regards,
Prasoon Tewari
 
Hi All,

Have you found any solution for this communication? I´m having a similar problem to communicate Micrologix with Sepam Relay over Modbus RS-485. The MSG instruction always have the Error Hex: 37.
 
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