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Modbus > Where It Is Used - Pg. 59

Modbus 59 checksum of the underlying TCP/IP protocol, whereas most current Modbus TCP implementations include the original Modbus checksum within the TCP/IP pay- load, as shown in Figure 4.4. Modbus Plus or Modbus Modbus Plus is proprietary to Modicon, which sends embedded Modbus messages over an RS-485 communication link. Modbus Plus supports some interesting fea- tures, including Modbus bridging to allow multiple buses to interconnect, extending the number of supported nodes indefinitely. Where It Is Used Modbus is typically deployed between PLCs and HMIs, or between a Master PLC and slave devices such as PLCs, HMIs, Drivers, Sensors, I/O devices, etc., as shown in Figure 4.5. Typically up to 247 devices are supported in a single, non- bridged bus. A common deployment uses Modbus on TCP/IP within a SCADA DMZ or Supervisory LAN, where master HMIs provide a central management capability to a number of Master PLCs, each of which may connect serially over a bus topology