We have slave devices which talk modbus RTU on RS-485/422. We have gateway devices to convert RTU to Modbus TCP on ethernet. Now we want to put modbus over a dial-up link. I presume that RTU will be unreliable over dial-up because of the timing issues, so we're thinking about putting Modbus TCP over dial-up. Is this feasible? Are there any gateways/intelligent modems that will do this?

 

There is nothing wrong at all with using Modbus RTU on dial-up circuits as long as you do not use any of the fancy data compression and error checking stuff. Just turn it off and let the Modbus CRC do the error detection. With compression turned off, you should not have any unwanted "gaps" in the
messages.

Jerry Miille

 

Modbus RTU communications between remote hosts & PLC sysems has been a standard over Radio & Dial Up networks since 1978. There is no problem with the RTU proctocol. There are clear handshaking requirements which the serial port hardware MUST suppport to make the link work. Modem communications usually require support for pins 6 DSR,20 DTR 4 RTS,5 CTS (Pin 6 ensures modem & slave are powered up & ready, while RTS requests a modem connection, CTS is the handshake back that indicates modem is ready to receive). If your slave is some intelligent device (i.e. not a PLC) which has never been used in this manner, it often times NOT programmed to electronically handshake. You probably want to test this is a lab before deployment.