modbus rtu modem


Thread Starter


Looking for an external modem that will allow modbus rtu 8,e,1 data communications.

I need to communicate at the following:
Modbus rtu, 8 data, even parity, 1 stop, 9600
Required to communicate from laptop to modicon 984-A145 PLC network. Any suggestions??
Ever looked in to Control Microsystems products? We use their bell202 modems on leased lines with high reliability. I believe they may have a higher speed modem, but i'v e been wrong before. They are out of Canada and offer rtus and related plc products; seem to be excellent quality.
Phoenix Contact offers a low cost industrial modem. It is rs232 to dialup or leased line modem. The product designation is PSI Modem. It allows even, odd or no parity, as well as all other standard protocol settings. I have personally tested this with modbus and the setting you require. Note: most commercial modems do not support anything other than no parity. This seems to be true for all current commercial modems.

Automation Linse

There are hundreds of companies making such things.

The "keyword" you need to watch for is "supports 11-bit asynchrounous characters". Some commercial modems only support 10-bit, meaning they support 8-bit data with really no parity bit or 7-bit data with parity bit.

For 8,E,1 you need 1 start + 8 data + 1 parity + 1 stop bit, which equals 11 bits total.

Problem is Modbus/RTU even as 8,N,1 (and also Allen-Bradley DF1 as 8,N,1) is really sending a fixed "1" parity bit, not "no" parity. So they still move as an 11-bit async char.

best regards
- LynnL,