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Concepts

Modbus concepts

FairCom Edge has a Modbus service that allows FairCom Edge to connect to and interact with Modbus devices. This service is embedded within the FairCom jsonAction API. It can be enabled and disabled within the FairCom Edge server. You must enable the Modbus service before creating a Modbus input.

The FairCom Edge server can capture Modbus data for forwarding to other systems.

Modbus comes in two forms:

  • Modbus RTU

    Usually serial-based over RS422

  • Modbus TCP

    TCP/IP based over Ethernet or Wi-Fi

Modbus uses a client/server architecture. Normally a hardware device acts as a Server and is connected to sensors or other equipment and provides a way to read or set registers or coils.

FairCom Edge can act as a client to a Modbus server. When you provide access details to the Modbus server and a description of the registers and coils you are interested in, FairCom Edge will connect to the Modbus server and interact with the registers and coils specified.

Coils vs. registers

  • Coils are:

    • Either on or off.

    • Represented by 1 or 0.

  • Registers:

    • Can be 1, 2, or 4 bytes.

    • For 2 and 4-byte registers, byte ordering can be specified.

Coils and registers are identified by number or by address. The address is usually one less than the number — for example, the first register would be register number 1 and would have an address of 0, and register number 2 would have an address of 1, and so forth. If your Modbus device states it has a register at address 4000, that would be register number 4001.

When specifying Modbus data in FairCom Edge. use the 0-based address.