Hai,

 I have s7-1200 installed with rs485 module which is working as modbus master. it will communicate with analyzer device(modbus slave).There are 3 modbus slave in loop, so PLC will send request to 1st slave and wait for done/error feedback, then trigger request to next. Sometimes error/done bit is not generating, so i made 7 second delay and trigger request to next slave. ie send request to 1st slave, wait for error/done feedback;if not coming after 7 second it will remove existing request and trigger request to next slave. This setup is working and am getting value.

But after some days, values in PLC is not updating. 1st modbus slave communication block is showing error code as 8200(port busy). But in modbus slave display i can see the values are updating properly. Problem is random and frequency of problem also random. If we do PLC restart it will work again.

Software used- tia v10.5 (i understood its old)

MB_COMM_LOAD, MB_MASTER are used for communication. receiving request is for 40 byte.

please suggest.

I think the problem is that your software is to old, I would update my software as the first step.

Hi!

You could use a modbus sniffer to "see" what's going on when the error happens.

I use Modbus Tester from modbus.pl but this probably won't help you. At ModbusTools.com they have some sharewares which might help.

check out also this     http://www.modbus.org/tech.php