Moxa MB3170 essentially is a tool to convert ModbusTCP packets (Ethernet) to ModbusRTU packets (serial). This YouTube vilog demonstrates what to look out for when a good communication between the Modbus Master (ModbusTCP) and Modbus Slave (ModbusRTU).
As you will see in the vilog, Modbus master will trigger a ModbusTCP request to Modbus slave via Moxa MB3170. Upon receiving the ModbusTCP packets, Moxa MB3170 converts it to ModbusRTU and sends it out to Modbus slave (via serial port, P1). If the Modbus slaves successfully received the ModbusRTU packets from Moxa MB3170, the Modbus slaves response back with a ModbuRTU packets which is shown as “RTU Resp.” in the MGate Manager Traffic Monitor tool (see YouTube vilog below). The Moxa MB3170 then converts the ModbusRTU packet into ModbusTCP packet and sends it to the Modbus master. After that, the cycle continues if Modbus master wants more data from Modbus slave.