AirLink RV50 Communications Message

We are in the process of replacing a Digi WR31 cell modem with a Sierra Wireless AirLink RV50.

For some reason, we are not able to get the correct message out of our Sierra Wireless RV50 and into our RTU.

The healthy/correct message from our Digi looks like this:

49 01 00 00 00 04 32 41

A portion of that message appears in the message that comes out of the Sierra Wireless, but it has six leading bytes and it cuts off the last two. The message from the AirLink looks like this:

00 00 00 00 00 06 49 01 00 00 00 04

Any idea what would be causing the addition of the first six bytes, as well as the truncation of the last two? I’m assuming it’s something in the configuration, but we have been unable to locate it.

Never mind. The message is coming from the master in Modbus TCP. Looks like the Digi was able to translate Modbus TCP into Modbus RTU, something we need the Sierra Wireless RV50 to do for us as well. (Not sure it’s functionally able to though.) The leading six bytes are header bytes part of the Modbus TCP message, while the aforementioned two trailing bytes are calculated CRC values used by Modbus RTU.