Modbus

Modbus is a client/server data communications protocol (previously referred to as a master-slave protocol) that is based on a request-response model. Modbus has become a de facto standard communication protocol for communication between industrial electronic devices in a wide range of buses and network.

Modbus is used for transmitting information between devices that are connected to buses or networks over serial lines or Ethernet and, increasingly, using wireless.

Modbus is the most widely used network protocol in the industrial manufacturing sector. The most common use case is the communication between a human-machine interface (HMI) or supervisory control and data acquisition (SCADA) system and a sensor, programmable logic controller (PLC), or programmable automation controller (PAC). It provides a hardware-agnostic, cost-effective way to enable the interoperability of disparate automation equipment.

Modern systems require that Modbus can support the connection of multiple server devices, facilitate faster data transfers, and provide data transfers over greater distances. To do this, RS-485 and RS-422 are used. RS-485 and RS-422 (less commonly used) facilitate the use of multidrop wiring methods (connecting multiple server devices to client devices), extend the range of transmissions to up to 4000 feet before a repeater is needed, and have a data rate of 10 megabits per second (Mbps) up to 100 kilobits per second.

Examples of devices that output Modbus would be flow meters, pressure sensors, air quality sensors, power meters, etc.

A good example would be a generator that may have a Modbus RS485 output. This allows an environmental monitoring appliance to interface with the generator or power meter and pull data from it, i.e. power, voltage, frequency, runtime hours of the generator, etc.

An environmental monitoring appliance can monitor these parameters and provide alerts, graphing, and web based access to them. The environmental monitoring appliance effectively become a Modbus to SNMP TCP/IP gateway for these devices so they can be remotely monitored.

As well as monitoring, there are also Modbus set registers, so for example, based on a sensor status an environmental monitoring appliance can become a SCADA type system, where the environmental monitoring appliance can send a Modbus command to turn something on or off.