Tuesday, February 22, 2011

Modbus Omron Protocols Support

The T100M + PLC supports a subset of the Omron and Modbus (both ASCII and RTU modes are now supported) compatible communication protocols so that it can be easily linked to third party control hardware or software products such as Supervisory Control and Data Acquisition software, touch panel etc. the PLC recognizes automatically the type of command format and will respond with the correct response. These were accomplished without user intervention and without any need to configure the PLC at all.

Both Omron and Modbus protocols use the same device ID address (00 to FF) as the native protocol. Since the internal variables and addresses of I/O in the T100M + PLC are organized very differently from the Modicon or Omron PLCs, we need to map these addresses to the corresponding memory areas in the other PLCs do that they can be easily accessed by their corresponding protocols. All I/Os, counter, timers, internal relays, and data memory DM[1] to DM[4000] are mapped, however, 32-bit variables and string variables are not mapped since they fundamentally quite different in their implementation among different PLCs. Internal variables which are not mapped can be still be accessed by copying the contents of these variables to unused data memory, so that they can be accessed by these third party protocols.

The rev D of the T100MD+ or T100MX+ PLCs also supports the Modbus RTU protocol. The difference between the ASCII and RTU protocols is that the latter transmits binary data directly instead of converting one byte of binary data into two ASCII characters. A message frame is determined by the silent interval of 3.5 character times between characters received at the COMM port. In other hand, the memory mapping and function codes are identical to the Modbus ASCII protocol.

The new rev D T100M+ PLC can also act as a Modbus RTU master. The same WRITEMODBUS and READMODBUS commands can be used to send and receive Modbus RTU commands. What you need to do is to add decimal (10) to the COMM port number to signal to the processor that you wish to use Modbus RTU commands on COMM1.

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