Has anyone successfully used a V.92 modem as a Modbus RTU master to talk with a slave PLC. I have no problems with the earlier modem versions (e.g. 56k, etc.) as master and I have no problem with the V.92 as the slave modem. I just can't get the V.92 modem to talk when it is the master. The modems connect, but they don't talk. Even when disabling all the following (V.92 mode, compression, buffering, and error correction) via AT commands it still doens't work.

 

I used a Multi-tech (great straight-forward modem) Multimodem ZDX V.92 modem, but I defaulted to ModBus ASCII. In Modicon.com you'll find ModBus/Modem documents that detail how to do this and why "RTU" does not mix well with newer modem compression technologies. If you have access to a local High-TECH Schneider Automation distributor, they migth get you a copy of Kenneth Jennings' ModBus Modem Connectivity guide. Good luck.

 

Ted,

Schneider has recently (Feb 6, 2003) updated resolution 0763 named "Modems for Modbus". Ask for it at customercentral <at> modicon.com or e-mal me to meirs <at> fil.co.il and I'll send it to you. It might help you.
Meir Saggie

 

I had a similar issue several years ago when setting up a P190 programmer to talk over ethernet to a VAX computer serial port for doing plc program documentation. I had to get the latest hardware that supported a 9 bit data format. MODBUS RTU uses 8 data bits plus a parity bit. MODBUS ASCII uses 7 data bits and a parity bit.

 

I already have a copy of the Modbus Modem Connectivity guide. It is a great reference for gettting modems to talk. However, it doesn't address the newer V.90/V.92 modems. I have tried all the tricks mentioned in the guide, as well as all I can think of.