During the use of weighers, we often need to communicate with computers, PLC and DCS systems to achieve automatic control and computer management. The communication of the weighing instrument is also a often fault. In this article, the communication failure, reasons and exclusion methods commonly used in RS232 and RS485 serial interfaces commonly used in weighing instrumental communication have been introduced. I hope that it will help the installation and mediation personnel of the weighing instrument.
2 Communication bus and communication protocol
According to the electrical characteristics of the communication interface, it can be divided into RS-232 and RS-485. The RS-232 interface of the weight-to-purpose meter is usually a simplified version. Only the two data cables and the ground line GND are used only to send TXD, receiving RXD data cables, and ground line GND. The RS-232 uses an unbalanced transmission method. The signal line-3V to -15V indicates logic 1, +3V to +18V indicates logic 0. The unbalanced transmission method of the interference inhibitory ability is poor, and it is easily affected by co -ground noise and external interference. It is generally used for low -speed and short -distance communication. The communication distance ≤15m.
The RS-485 adopts a two-line balance transmission method, which is generally called the same phase A, and the anti-phase is B. When the data of the sending terminal is 1, the A level is the BRIDE+2V ～+6V, and when the A level is 0, the A level is relatively B. -6V than the B level. When the flat is higher than 200mv, the output is 1, and when the A level is lower than 200mv than the B level, the output is 0; the covarurative voltage range of the signal line is -7V ~+12V. The RS485 bus has a strong anti -co -model interference capacity, the maximum transmission rate is 10Mbps, the transmission rate is inversely proportional to the transmission distance. Below the transmission rate of 100Kbps, it can reach 1200M communication distance. Instrument. The RS-485 bus generally supports the maximum 32 nodes. If the special 485 chip is used, it can reach 128 or more nodes. RS485 generally adopts the terminal matching of the terminal matching. Do not use a ring or star network. When the transmission rate is high and the communication distance is long, the terminal resistance is required to prevent errors from reflecting. Because the characteristic impedance of most twisted cables is between 100Ω and 120Ω, the terminal resistance is generally 120Ω.
Weighing instrument communication interfaces generally use asynchronous serial methods, and 4 important parameters must be consistent with both sides: Potter rate, number of data bits, number of stop digits, and puppet verification. The data position is generally 7 or 8 bits, and the stop bit is 1, 1.5 or 2 bits. The puppet school checks without verification, puppet verification, strange verification or verification location 1. Set 0 and other methods. The transmission rate of the communication interface is generally 300PBS, 600PBS, and 1200pbs until MPBS.
The communication agreement is like the language communicated by the communication between the communication, and the two parties must be consistent to communicate correctly. Many of the re -instruments have their own communication protocols, so they must be communicated according to the agreement format prescribed by the instrument production plant. In order to ensure the correct data transmission, many protocols use verification sum, and the calculation methods and representations methods of verification and verification and methods are also different. The upper machine communication software programming needs to be carefully considered.