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Modbus RTU to Modbus ASCII translation - MOXA

A recent scenario that we were asked about at ECS is how to translate between the protocols Modbus RTU to ASCII or ASCII to RTU.

In order for this translation to occur, you will need a ‘Protocol Converter or Gateway’ (not to be confused with an internet gateway).

Our examples use the MOXA MGate MB3170 (1 port) & MB3270 (2 port) devices.

Example 1 – this simple solution using 2 x MB3170 units, translates Modbus RTU (Master) to Modbus ASCII (Slave) via Modbus TCP connected devices.

Example 2 – utilizes 1 x MB3270 with Modbus Serial RTU (Master) connected to 1 port, and Modbus Serial ASCII (Slave ID-1) connected to the other port.

Two possible architectures, are local and remote translation:

Local being, working within the boundaries of the physical limitations of the RS232/422/485 protocols.

  • *RS232 has a physical cable length limitation of 15 meters.
  • *RS422 has a max cable length of 1500 meters.
  • *RS485 has a max able length of 1200 meters.

Remote being, to ‘extend’ the protocol by means of Modbus TCP over common TCP protocols.

  • *Modbus TCP can work over ethernet, TCP/IP, LAN’s, VLAN’s, WAN’s, VPN’s or anywhere you can route TCP/IP.

The MB31XX/32XX series has over 20 different model offerings, with combinations of:

  • 1 Port or 2 Port (RS232/422/485 connection points)
  • Normal or Wide Temperature – 0 to 55 deg or -40 to +75 deg respectively.
  • Isolated (2kV), or non-isolated
  • Multi-mode or Single-mode fibre, with SC or ST connectors.
  • IECEx hazardous location certified options.


This is just one solution for a myriad of Modbus use cases, for your own customised solution please contact the Automation & Network Engineering support team at ECS on 0800 849 2211.

To purchase or view our range of MOXA Modbus Protocol Gateways and other Netorking products go to:


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