Modbus, developed by Schneider/Modicon, has been the defacto standard for interfacing remote I/O and PLCs into the process DCS system for the past 15 years. Because of this strong history, most legacy control systems interface conveniently with the Modbus standard. Numerous actuator and instrument manufacturers support the protocol, therefore, it is used extensively as a “fieldbus” network as well. The following discussion focuses primarily on the protocol use for field networking purposes.

Modbus features

  • Interfaces conveniently into most existing plant control architectures.
    Networks up to 32 devices on a 4-wire network (2-wires for signal and 2-wires for power).
  • Trunk network may extend up to 4,000 feet (1220 meters) per segment.
  • Dramatically cut wiring costs and commissioning over conventional applications.
  • Protocol has been proven in thousands of “mission critical” process applications over the last 20 years.
  • Capable of supporting both simple discrete devices as well as sophisticated analog applications.
  • May be supported without additional training since most plants are already using the protocol extensively.
  • Popular among instrument manufacturers for a wide variety of applications.

Modbus economic analysis

Since many PLCs and DCSs integrate a Modbus master with Modbus drivers there is minimal cost for plugging in the Modbus line and mapping I/O to the application software. A conventional 24VDC power supply may be used for powering the field devices. (Power for the master is typically incorporated into the PLC or DCS rack.)

With the exceptional distance capabilities of the Modbus RS485 protocol there are dramatic wiring savings as noted above. Analog input capabilities further improve the economic benefits. Modbus modules from StoneL have a 4 to 20 mA input which digitizes the signal with a resolution to 0.1%. Power for the circuit is available from the bus power pair wired to the module. Process flow, temperature, pressure, and any other 4-20mA input signal, may be input directly into the bus, eliminating wiring and input modules at the controller!