Synchronizing access to PLC data block (Siemens S7 1200)

[drbitboy]

https://github.com/drbitboy/PLC_Modbus_client_server_50words

That is my test code for the Data Consistency check:

• S7-1200 is running
  • A Modbus server in the Main Program Cycle (OB1)
    • The holding registers are mapped to a 100-character string (51 registers, including one additional word at the beginning that appears to be the string length)
    • There is a boolean whether to execute a DIS_AIRT/EN_AIRT instruction pair wrapped around the MB_SERVER
    A 1kHz Cyclic Interrupt block (OB37) at a hight priority (24), alternating writes to the 100-character string (and Modbus server holding registers) of "AAA...AAA" and "BBB...BBB"
• CCW/Micro820 Modbus client is reading (Modbus command 15) the 100-character string as 50 holding registers from the S7-1200 at ~10Hz
  • For each read, CCW counts by 1 if there is a mismatch between the first word and the fiftieth,
    • The mismatch occurs if OB37 interrupts the MB_SERVER in OB1 while data are being transferred.
• Bottom line: without the DIS_AIRT/EN_AIRT pair enabled, about one transfer in a thousand shows a mismatch
  • I miss any mismatch from a read which immediately follows a read that yielded a mismatch
    • At one in a thousand, I am comfortable with the error, but feel free to correct the code.

Caveat: I used Modbus because my TIA V14 testbed does not have OPC UA, so YMMV.