Since the first PLC was successfully used on GM's automobile production line, PLCs have been widely adopted due to their convenience, reliability, and low price. However, a PLC is ultimately a "black box," unable to provide real-time, intuitive monitoring of the control process, and lags significantly behind a DCS (Distributed Control System). The development and popularization of computer technology have provided new technical means for PLCs, enabling real-time monitoring of the PLC's control process and results, greatly enhancing its 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.
American manufacturers
Rockwell AB
Rockwell's PLCs primarily include types such as PLC2, PLC3, PLC5, SLC500, and ControlLogix. PLC2 and PLC3 are earlier types. Currently, the most commonly used small PLC is SLC500, medium-sized PLCs are generally ControlLogix, and large PLCs 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, as well as network media such as DH, DH+, DH485, and ControlNet. Details of the DF1 protocol can be downloaded from AB's database.
AB's PLC also provides OPC and DDE. Its 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 functions, allowing users to implement their own communication protocols through programming.
GE
The GE 90-70 and 90-30 series PLCs are currently the most widely used in China. Both of these PLCs support the SNP protocol, and the specific details of the SNP protocol can be found in their PLC manuals.
GE's PLCs can now also be connected via Ethernet. GE does not disclose the details of its Ethernet protocol, but it provides an SDK development package that allows communication.
European Series
Siemens
Siemens PLCs primarily include the earlier S5 and the current S7-200, S7-300, S7-400, and other types of PLCs. The earlier S5 PLCs supported the 3964R protocol, but due to its limited use in China, it is rarely used for data communication except for a few retrofit projects.
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 provides comprehensive communication functions. 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 disclosed. 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 offers a wide variety of PLC types, which are widely used in China. Their primary communication methods support the MODBUS and MODBUSPLUS protocols.
The MODBUS protocol has been widely used in the industrial control industry. It is no longer just a communication protocol for PLCs; it is also widely used in many intelligent devices such as smart instruments and frequency converters. With further development, MODBUS now has the MODBUSTCP method, 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.
Japanese PLC
Omron
Omron PLCs are widely used in China. Regarding communication methods, OMRON currently primarily uses two methods:
The HostLink protocol is a communication method that uses serial ports for data transmission. When the PLC enters MONITOR mode, the host computer can communicate with the Omron PLC. When communicating with Omron, it's important to allow sufficient time between communication sessions; excessively fast communication can easily lead to PLC communication anomalies.
ControlLink is a fast communication method for Omron PLCs. ControlLink facilitates data communication via circuit boards, which have data exchange areas to facilitate data exchange and ultimately complete the data collection function. Using this method requires an Omron driver.
Mitsubishi
Mitsubishi PLCs are widely used in China. Mitsubishi offers a variety of PLC types, primarily including the FX, A, and Q series. Mitsubishi PLCs utilize multiple communication protocols, with each series having its own. For example, the FX series supports communication via the programming port, RS-232, and RS-485. The A and Q series can communicate via Ethernet. Additionally, Mitsubishi PLCs can communicate via 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.
