Industrial Abstract: PLC as a high-performance control unit, in a distributed system, the more widely used. In this way, the upper control system is an important part of. PLC can be a variety of ways, such as using the existing configuration of the monitoring software and the host supervisor Diego machine communication, but for small-scale control system, to find a cost-effective method of communication, has a positive practical significance. 

 Hardware description of a communication device 

 Realization of the channel between the PLC and PC, allows two complementary functional deficiencies, PLC for convenient and reliable control, PC, graphics, data processing, printing of statements and Chinese display has a strong function. Therefore, the PLC manufacturers have developed various models of PLC and PC communication interface module for the company. Developed by the Mitsubishi FX-232AW interface module for the FX2 series PLC and computer communication. Connected to the Ethernet interface module AJ71E71 connection with the MAP network interface module AJ71M51-S1, and FAISMAP network connection interface module AJ71M51M1 and so on. Different means of communication, has a different cost price and a different scope. In this method describes a PC's RS-232 port to communicate with the PLC. The recovery of the PLC. 

 FX2 series PLC programming interface RS-422 standards, the computer's serial port using RS-232 standard. As PLC computer communications interface circuits, the RS-422 standards must be converted to RS-232 standard. 

 RS-232 and RS-422 standards in the signal transmission, the average is not the same logic. 

 RS-232 single-ended receiver and single-ended transmitter, only one signal line to transmit information, and depending on the level of the signal line to determine the size of common signal ground level logic "1" ( -3 to-15V) and "0" (3 to 15V); 

 The standard RS-422 transmission standard for a balanced manner, the double-end send and end receive, according to the potential difference between the two transmission lines to determine the logic state. RS-422 circuit from the transmitter, the balance connection cable, the cable terminal load and a receiver. Balanced transmitter and the differential receiver logic level and the potential difference between the conversion (2V "0",-2V "1"). Recycling of Siemens the PLC. 

 The choice of MAXIM's MAX202 RS-232 and TTL level conversion. MAX202 internal voltage multiplier circuit and the converter circuit, only a 5V supply for operation, easy to use; selection MAX490 RS-485 and TTL conversion. Each tablet MAX490 has a transmitter / receiver full duplex communication, it takes two MAX490 addition, only four external capacitors can. 

 PLC's RS-422 interface is connected with the DB-25 connector, a PC for general use DB-9 connector. 

 RS-232, RS, CS short, so that the computer sends data, the PLC is always in the ready state. In other words, the computer can at any time the data to the PLC. DR, ER cross-connect the computer to receive data, you must wait until the PLC is in the ready state. PLC recycling. 

 Two communication devices, software description 

 2.1FX2 between the PLC and the computer communication protocol 

 FX2 series PLC and computer communication between the RS-232 standard, its transmission rate is fixed at 9600bps, the parity bit with even parity. Data frame for the unit to send and receive. More than one character frame as shown by the force of five parts, which checksum is added to the command code ETX character ASCII code (hexadecimal number) to obtain income and a minimum of two digits . STX and ETX standard of the start of the character frame starting and end signs. Siemens PLC recycling. 

 FX2 series initial command issued by the host and the communication between the computer and the PLC to respond to them to communicate. A total of 0,1,7,8 four commands, the host computer to the PLC to read and write and forcibly set. ENQ, ACK and NAK, the coordination of the host computer and the PLC communication response. 

 2.2 The communication process 

 BlandC the preparation of the host PLC communication program. Must first initialize the COM1 port, baud rate is 9600bps, the parity bit with even parity, seven valid data. When the computer receives characters ACK response from the PLC, you can enter the data communication. 

 Computer soft devices within the PLC to read, write and enforce the ON / OFF operations. Open the PLC timer and counter settings using constant, and the data in the file register FX2 series PLC digital input, output, and software device is transparent to the computer. Operation character frame format shown in Figure 3. Different operations in different "character". For example, the computer soft PLC equipment Y20 ~ Y37 read operation, check the device address table 00A2 to read 2 bytes of data. Acquisition of the PLC. 

 Errors such as sending a command, the PLC returns a NAK signal, the operation failed to re- 

 In order to ensure that the host and the PLC communication is accurate, the PC must also be communication protocols and school. Information received is incorrect, then re-read. Such as repeated three times is still OK, and then displays an error message. 

 Using the C language is easy to achieve more than programming. 

 The host can also write data to the PLC, a single point of strong near the set and reset. 

 Reliable operation of the program running rapidly under control, and laboratory environments, cost-effective, small size, but for longer distance communication, a bad environment to ask questions, need to add optical isolation and other measures. 
 Regulator (http://www.pandeng.org.cn)