Modbus 通信規約基本上是遵循Master and Slave 的通信步驟，有一方扮演Master 角色採取主動詢問方式，送出Query Message 給Slave 方，然後由Slave 方依據接到的Query Message 內容準備Response Message 回傳給Master。即使目前硬體通信已經可以達到雙方互相主動通信的能力，但是於Modbus 通信規約的規定，必須一方為Master，另一方為Slave 不能互換角色。一般使用上，監控系統(HMI)都為Master，PLC、電表、儀表等都為Slave，HMI 系統一直Polling Slave 的各種elay and register 最新數值，然後做顯示及各種邏輯計算及控制調整等處理。-Modbus communication protocol is basically follow the Master and Slave communication procedure, one party to take proactive role to play Master asks way, sending Query Message to the Slave side, and then received by the Slave side based content preparation Query Message Response Message back to the Master. Even if the current hardware communication initiative can already achieve mutual communication abilities, but the provisions of the Modbus communication protocol must be one of the Master, the other as Slave can not swap roles. General use, the monitoring system (HMI) are Master, PLC, meter, meters so as Slave, HMI systems have been Polling Slave various elay and register the latest values, and then do the show, and various logic calculation and control adjustments, etc. Managers.