Hello,

I have a problem with periodical occurrence of errors in the Modbus RTU communication using the CP processor for ET200SP. Sometimes (eg after switching off a slave device or, if it was incorrectly connected - RX / TX transmission lines were crossed), the MB_MASTER function stops in the "BUSY" state and can be reset only by switching off the communication module (power module). It seems to me a known problem that sometimes occurred in the past in the WR_REC function for S7300 (?). It does not look like an error in the program, simply the MB_MASTER function returns the "BUSY" bit without the "REQ" set. Even if I had an error in the program, this situation should not happen.

Is it possible to program the actual request clearance or function reset (applying TRUE on REQ input of MB_COMM_LOAD does not have any effect)? If this error occurs, the entire communication bus is down. Has anyone encountered this problem and knows how to solve it? In the attachment, I send printscreen of data blocks online when an error occurs.

I found similar problem here: https://support.industry.siemens.com/tf/ww/en/posts/cm1241-modbus-rs-485-crash/141922/?page=0&pageSize=10

not solved

Hello,

I have maybe some ideas, try test them:

1. you can try prepare own port_cfg block and execute it. I expect in this time MB blocks in iddle state. After that you need disable this block to not mix up with MB blocks.

2. Look which classic SEND/RECV blocks has Reset input and try set it with block execution. Your MB blocks need to be also iddle with this test. Again with disabling block to not mix up with MB blocks.

3. something going to my head that some TAG MB_STATE exists in instance DB, setting this TAG value to 0 can reset state of Master block to init value. simply check if problematic situation occur and try this reset value.

Hope some of that ideas can be solution.

Hi,

Thank you very much for your answer. I checked and actually resetting the MB_State  in modbus instance DB solves the problem - "BUSY" is OFF and the communication is starting again with new request. This is so easy but should be well known to other programmers.

Best regards

Hello,

glad gave solution and it works. As Kaulquappe mentioned, it's not clear solution and therefore is no way to give this as recommend solution for programmers. This solution is simply experience during tests, looking deeply in matter.

Hi,

Once more thanks for help and quick response. I uderstand, that making any changes in instance db is not good idea but it helps at the moment - I will wait for Siemens response.

I send all data (including link to the post here) to our Polish Simatic support.

Yesterday we tested the communication in our office. We tested by connecting and disconnectiong wires of RS485 and always after a few times we got the malfunction of MB_MASTER with continuous "BUSY" state "ON". For me Siemens MB_MASTER function dosn't work correctly. If i get any answer I will post it here.

Best regards