Modbus RTU (RS485) Master is BUSY after port configuration

S7-1200 + CM1241(RS485)

TIA portal V17 update 6

Hello!

I'm trying to communicate my PLC with Simex weight module SWI-94 ( PDF manual SWI-94 page 43). I faced some problems so i started debugging everything with USB RS485 adapter.

First i checked if it is possible to get data from weight module - that worked without a problem.

Then i tried to connect PLC to PC and i can't make it work. After I run MB_COMM_LOAD block, MB_MASTER stays in BUSY . There is no timeout. RX/TX led isn't blinking.

There is my configuration:

Wiring:

CM1241 is configured as RS485 2-wire, protocol free port, Non bias

Baud 9600

Parity none

8 bit / char

Stop bit 1

Wiring and should be OK, because once MB_MASTER was timeout, and was sending RX every 1000ms. Then i was able to read RX communication in "Modbus Slave" simulator program.

When PLC start to send data, it send complete garbage packets as seen on pic below:

Looks like i have an problem in configuration of MB blocks.

Thanks for the answer!This is quite valuable knowledge.

Also, I faced another problem: (And got it fixed :D)

After I established connection, PLC sends packet with additional 2 Bytes:

Rx:000641- 01 03 00 01 00 0C 14 0F 00 00

 When it should packet like that: (Simulated in Master Pool software)

01 03 00 01 00 0C 14 0F

 FIX:

I just used better quality cable with twisted pair, and add terminators on both ends of transmission line.

PS: I found the Modbus Parser website, quite helpful when looking for transmission errors:

Modbus Parser/Analyzer

Thanks for help again!