Modbus TCP


Modbus is a serial communications protocol published by Modicon (now Schneider Electric) for use with programmable logic controllers. It is a standard communication protocol and a commonly available means of connecting industrial electronic devices. Modbus enables communication among many devices connected to the same network and communicates the results to a computer. Modbus is often used to connect a supervisory computer with an RTU in SCADA system. Many of the data types are named from its use in driving relays: a single-bit physical output is called a coil, and a single-bit physical input is called a discrete input or a contact.

Each device intended to communicate using Modbus is given a unique address. In serial and MB+ networks, only the node assigned as the Master may initiate a command. On Ethernet, any device can send out a Modbus command, although usually only one master device does so. A Modbus command contains the Modbus address of the device it is intended for. Only the intended device will act on the command, even though other devices might receive it. All Modbus commands contain checksum information, to allow the recipient to detect transmission errors. The basic Modbus commands can instruct an RTU to change the value in one of its registers, control or read an I/O port, and command the device to send back one or more values contained in its registers.

Portland Engineering works on many modems and gateways that support Modbus, as it is a very simple protocol and often copied. Some of them were specifically designed for this protocol. Different implementations use wireline, wireless communication, such as in the ISM band, and even short message service (SMS) or General Packet Radio Service (GPRS). One of the more common designs of wireless networks makes use of Mesh networking. Typical problems that designers have to overcome include high latency and timing issues.

No matter the system, if it speaks ModbusTCP, PEI understands it.

 

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