RS485 Lockup - S7-1200 Modbus

The Mixer came from China with a S7-200CN, 224XP CPU.
3 Chinese VFDs. V&T E5-H. Wired together with RS485, 2 wire.
9600, no parity. 3 different sizes. I think 20-50 HP. 480 service at 400 Amps.

The PLC was changed to S7-1200, 1215C CPU and RS485 serial comms module. CM1241, the big one that attaches to the right of the CPU.

Runs most of the time, but overnight it will lockup.
I suspect noise on the RS485 communications. No LED's flashing for communications. 24DC Power cycle restores comms.
I can detect fault in software. Modbus reports code 8200 for all VFDs.
Is there a way to do a soft reset of the RS485 module with code?

I put two counters on it to count faults with read and write.
Overnight, writes were 285, reads were over 3000. This is with the VFD's idle.

I'm going to rewire with shielded, and check for a terminating resistor. Even so, I don't want another lockup. The S7-200 didn't - what did I do wrong?

I would make sure the shield of the communication wire is only attached at one end. Usually the PLC side.

http://www1.siemens.cz/ad/current/content/data_files/automatizacni_systemy/mikrosystemy/simatic_s71200/manual_s7-1200_2012-04_en.pdf

I have not done this but can you not cycle the Modbus Master Initial code again to reset the network?

Just a thought,
Garry
http://www.accautomation.ca

Update. The problem was timing. Was using the "DONE" bit from the FB to trigger the next FB. Needed to add delay to allow replies from VFD's to finish. That code was working in the Chinese 200, but failed with the 1200.

Was concerned that multiple errors could "lock up" RS485 module.
Software resets didn't work. Had to power cycle the PLC.
Reported issue to Siemens. Copy of what I sent:
Contents of request
----------------------------------------------------------------------
Product: Communication module CM 1241, RS485
Reference: Timing Issue - Lockups
Description:
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Hardware:
S7-1200 PLC 1215 CPU DC/DC/DC, 6ES7215-1AG40-0XB0
RS422/485 Module, 6ES7241-1CH32-0XB0
Connector, 6ES7972-0BA42-0XA0

There is three VFDs. Six MB_MASTER commands perform 3 reads and 3 writes. They are run by a sequencer using set/reset of M bits. The "DONE" signal triggers the sequencer. It's bit enables the next "SEQ" input. This code was copied from working code in the S7-200CN, 224XP.
Many errors. I added error counters. Thousands of errors per day, lockup overnight. PLC appears normal. No error on serial module, but comm lights aren't flashing. It will be hung on one of the MB_MASTERs, waiting for a DONE signal. Most of the time, power cycle is the only cure.

Noticed the flashing comm lights didn't have a constant pulse. Sometimes, they went dark for almost 1 second. Suspected a timing issue. Added timers to each SEQ input. Started with 500 ms. That fixed it. I have it down to 10 ms. No errors after 1 hour.
No failure last night.

Problems that need to be reported to Siemens:
1) Multiple errors (thousands) can cause the module to lockup without reporting a fault, and requires power cycle to reset.
2) It looks like the DONE signal from MB_MASTER is a bit premature. Using DONE to signal the next MB_MASTER causes them to "step all over each other", generating read and write faults.

At one point, STATUS word was 8200. Sorry I didn't monitor/log individual STATUS codes.