Since the first PLC was successfully applied on GM's automobile production line, PLCs have been widely used due to their convenience, reliability, and low price. However, a PLC is ultimately a "black box," unable to provide real-time, intuitive observation of the control process, and thus lags significantly behind a DCS (Distributed Control System). The development and widespread adoption of computer technology have provided new technical means for PLCs, allowing for the monitoring of the control process and results via computers, greatly enhancing their capabilities. However, the communication media and protocols of different PLCs vary. The following is a brief introduction to the communication media and protocols of major PLCs.
1
American manufacturers
Rockwell AB
Rockwell's PLCs mainly include models such as PLC2, PLC3, PLC5, SLC500, and ControlLogix. PLC2 and PLC3 are earlier models. The most commonly used small PLCs now are SLC500, medium-sized ones are generally ControlLogix, and large ones use the PLC5 series.
The DF1 protocol is a communication protocol supported by all Rockwell PLCs. DF1 can transmit data via serial media such as RS-232 or RS-422, or via network media such as DH, DH+, DH485, and ControlNet. Details of the DF1 protocol can be downloaded from AB's resource library.
AB's PLCs also provide OPC and DDE. The integrated software RSLogix includes DDE and OPCSERVER, which can be used for data communication.
AB's mid-to-high-end PLCs also offer advanced language programming capabilities, allowing users to implement their own communication protocols through programming.
GE
GE currently uses the 90-70 and 90-30 series PLCs most commonly in China. Both of these PLCs support the SNP protocol, the details of which are available in their PLC manuals.
GE's PLCs can now also be connected via Ethernet. GE does not publicly disclose the details of its Ethernet protocol, but it provides an SDK development package that allows communication based on this package.
2
European Series
Siemens
Siemens PLCs mainly include the early S5 and the current S7-200, S7-300, S7-400 and other models. The early S5 PLCs supported the 3964R protocol, but because it is rarely used in China now, except for a very few retrofit projects, there are very few data communication with it.
The S7-200 is a small PLC from Siemens. Due to its low price, it has been widely used in China and supports MPI, PPI, and free communication port protocols.
The Siemens 300 PLC supports MPI and can also communicate with computers via PROFIBUS and industrial Ethernet bus systems. For point-to-point communication, the CP340/341 can be used.
As a large-scale PLC from Siemens, the S7-400 offers comprehensive communication capabilities. It can communicate via the S7 standard MPI, as well as via C-bus, PROFIBUS, and Industrial Ethernet. For point-to-point communication, the S7-400 requires the CP441 communication module.
Siemens' communication protocols are not publicly available. Many configuration software programs, including Zijinqiao configuration software, support communication methods such as MPI and PPI. PROFIBUS and industrial Ethernet generally communicate data through Siemens software.
Schneider Electric (Modikon)
Schneider Electric offers a wide variety of PLC models, which are widely used in China. Their communication primarily supports two protocols: MODBUS and MODBUSPLUS.
The MODBUS protocol has been widely used in the industrial control industry. It's no longer just a communication protocol for PLCs; it's also widely used in many intelligent devices such as smart meters and frequency converters. With further development, MODBUS now offers MODBUSTCP, which transmits data via Ethernet, resulting in even faster communication speeds.
MODBUSPLUS offers faster transmission speeds and longer distances compared to MODBUS. However, this communication method requires the installation of the SA85 card provided by MODCON on the computer, along with the card's driver, to enable communication.
In addition to the two methods mentioned above, Modicon's PLCs also support various communication methods such as TCP/IP Ethernet, Unitelway, FIPWAY, FIPIO, AS-I, and Interbus-S.
3
Japanese PLC
Omron
Omron PLCs are widely used in China. Regarding communication methods, OMRON currently primarily uses two methods:
The HostLink protocol is a serial communication method for data transmission. When the PLC enters MONITOR mode, the host computer can communicate with the Omron PLC. When communicating with the Omron PLC, it is important to allow sufficient time between communication sessions; excessively fast communication can easily cause PLC communication errors.
ControlLink is a high-speed communication method for Omron PLCs. ControlLink facilitates data communication through interconnected boards, which exchange data to complete the data acquisition function. Using this communication method requires configuring the Omron driver.
Mitsubishi
Mitsubishi PLCs are widely used in China, particularly in small models. Mitsubishi offers a wide range of PLC models, primarily including the FX, A, and Q series. Each Mitsubishi PLC series utilizes multiple communication protocols. For example, the FX series supports communication via a programming port, RS-232, or RS-485. The A and Q series can communicate via Ethernet. Additionally, Mitsubishi PLCs can also communicate using the CC-LINK protocol.
Matsushita
Panasonic PLCs and computers can communicate via serial port and Ethernet. The communication protocol used is the MEWTOCOL protocol. Like most Japanese PLCs, the MEWTOCOL protocol is relatively simple. Many software programs, including Zijingqiao configuration software, can directly read data from the PLC.
The above describes the mainstream communication methods for PLC devices abroad. Besides the manufacturers mentioned above, there are many other PLC manufacturers:
