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Technical Articles (Current Issue 112 - May 2019)

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Hirose: More than 50,000 connectors

Modbus TCP

Modbus-IDA is a group of independent users and suppliers of automation devices that seeks to drive the adoption of the Modbus communication protocol suite and the evolution to address architectures for distributed automation systems across multiple market segments. Modbus-IDA will also provide the infrastructure to obtain and share information about the protocols, their application and certification to simplify implementation by users resulting in reduced costs.

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Modbus - Technical overview
MODBUS is an application-layer messaging protocol, positioned at level 7 of the OSI model. It provides client/server communication between devices connected on different types of buses or networks. The de-facto industrial serial standard since 1979, MODBUS continues to enable millions of automation devices to communicate. Today, support for the simple and elegant structure of MODBUS continues to grow. The Internet community can access MODBUS at a reserved system port 502 on the TCP/IP stack.

MODBUS is a request/reply protocol and offers services specified by function codes. MODBUS function codes are elements of MODBUS request/reply PDUs. This protocol specification document describes the function codes used within the framework of MODBUS transactions

Modbus allows for the administration of a net of devices, for example a system that measures temperature and humidity and communicates the results to a computer. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Versions of the Modbus protocol exist for serial port and Ethernet.

Modbus RTU is a compact, binary representation of the data. Modbus ASCII is human readable, and more verbose. Both of these protocols are serial based. The RTU format follows the commands/data with a cyclic redundancy check checksum, while the ASCII format uses a longitudinal redundancy check checksum. Modbus/TCP is very similar to Modbus RTU, but is transmitted within TCP/IP data packets.

An extended version, Modbus Plus (Modbus+ or MB+), also exists, but remains proprietary to Modicon. It requires a dedicated co-processor to handle fast HDLC-like token rotation. It uses twisted pair at 1 Mbit/s and has installations specs very similar to EIA/RS-485. However, it is NOT EIA/RS-485. MB+ includes transformer isolation at each node, which makes it transition/edge triggered instead of voltage/level triggered. A few EIA/RS-485 repeaters work with it by side-effect, but don't get your hopes up that you can support Modbus Plus with your computer's standard serial port.

Each device that intends to communicate using Modbus has a unique address. 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 checking information, ensuring that a command arrives undamaged. The basic Modbus commands can instruct a RTU to change a value in one of its registers, as well as commanding the device to send back one or more values contained in its registers.

There are many modems that support Modbus. Some of them were specifically designed for this protocol. Different implementations use wires, wireless communication and even SMS or GPRS. Typical problems the designers have to overcome include high latency and timing problems.

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