+P S -P S 1 2 3 4 TDY +P S -P S 1 2 3 4 TDY +P S -P S 1 2 3 4 TDY +P S -P S 1 2 3 4 TDY 离散信号 离散信号 420mA 信号 420mA 信号 Process Measurements Process Measurements NET Concentrator System Station #1 NET Concentrator System Station #2 MODBUS RTU (双绞线RS-485)