What is modbus?

Modbus is a serial communication protocol, which is developed by Modicon Company (now Schneider Electric? 1979, published by Schneider Electric, is used to communicate with programmable logic controller (PLC).

Modbus has become the industrial standard (Facto) of communication protocol in industrial field, and now it is a common connection mode between industrial electronic devices. Modbus is more widely used than other communication protocols because:

1, published in public, no copyright requirements.

2, easy to deploy and maintain

3. For suppliers, there are not too many restrictions on modifying the mobile local status or bytes.

Modbus allows multiple (about 240) devices to communicate on the same network. For example, one device measures temperature and humidity and sends the results to a computer. In data acquisition and monitoring system (SCADA), Modbus is usually used to connect monitoring computer and remote terminal control system (RTU).

Extended data:

Protocol version

Modbus protocol is currently used for serial ports, Ethernet and other networks supporting Internet protocols.

Most Modbus devices communicate through serial port EIA-485 physical layer.

For serial connection, there are two variants, which are slightly different in digital data representation and protocol details.

Modbus RTU is a compact and binary representation of data, and Modbus ASCII is a human-readable and lengthy representation. Both models use serial communication.

Subsequent commands/data in RTU format have the checksum of cyclic redundancy check, while ASCII format uses the checksum of longitudinal redundancy check. Nodes configured as RTU variants will not communicate with nodes set as ASCII variants, and vice versa.

For the connection through TCP/IP (such as Ethernet), there are several Modbus/TCP variants that do not need checksum calculation.

For these three communication protocols, the data model and function call are the same, but the encapsulation methods are different.

Modbus has an extended version of Modbus Plus(Modbus+ or MB+), but this protocol is proprietary to Modicon, which is different from Modbus. It needs a special coprocessor to handle high-speed token circulation similar to HDLC.

It uses 1Mbit/s twisted pair, and each node has a switch isolation device, which uses switch/edge trigger instead of voltage/level trigger. Connecting Modbus Plus to a computer requires a special interface, usually a board supporting ISA(SA85), PCI or PCMCIA bus.

limit

1 and Modbus are developed for PLC communication at the end of 1970. These limited data types can be understood by PLC at that time, but they do not support large binary object data.

2. For nodes, there is no standard method to find the description information of data objects. For example, it is determined whether the register data represents a temperature between 30- 175 degrees.

3. Because Modbus is a master/slave protocol, there is no way to require devices to "report anomalies" (except TCP/IP protocol based on Ethernet, which is called open-mbus)-the master node must query each node device circularly to find out the data changes. In applications where bandwidth may be important, this method will consume bandwidth and network time in applications, such as low-speed wireless links.

4.Modbus can only handle 247 addresses on a data link, which limits the number of devices that can be connected to the main control site (excluding Ethernet TCP/IP again).

5.Modbus transmission is carried out by buffering data between remote communication devices, which limits the continuity of communication and avoids the problem of buffering loopholes in transmission.

6.Modbus protocol itself does not provide security against unauthorized commands or intercepted data.

References:

Baidu Encyclopedia -Modbus Communication Protocol