Issues with Modbus communication between Yokogawa CENTUM VP and PLCs
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The DCS system’s communication module is the ALR121, which uses Modbus-RTU for communication with the unit’s PLC. DCS only needs to read data from the PLC, without the need to write data, and uses a \"two-wire\" system. ALR121 has four terminals: TX+, TX-, RX+, and RX-. How should they be wired? I only connected the two wires of the cable to RX+ and RX-, but indicators such as SND and RCV do not light up. What is the definition of Size in CommDataWW within the configuration? What amount is appropriate? This is the first time I’ve encountered Modbus communication in actual work, so I don’t have much experience. I hope those who are more experienced can offer some guidance.
This post was last edited by jlshnlhj on 2023-12-22 08:55. Is the \"two-wire system\" used just for reading data from the PLC? For the communication “handshake”, it is necessary to send data for determining which data to read, as well as to send a confirmation message upon receipt. So all 4 wires need to be connected. For CommDataWW, consult the Yokogawa manual or online help.