Difference between MODBUS Modes (RTU , ASCII) and MODBUS PLUS


Thread Starter

Maya Ali

any body can explain me that what is Difference between MODBUS Modes (RTU , ASCII) and MODBUS PLUS.

Does Modbus plus also these Modes(RTU /ASCII)?

I have seen that one modsoft(version 2.32) progamming termianl(PC)with SA-85 modbus plus card communicate with 984 series PLCs through MODBUS PLUS. But in Modsoft the configration for communication has RTU mode. It makes me confused.

Help me to understand that.

Maya Ali

Fritschal Terheijden

In a nutshell:

ModbusASCII and ModbusRTU usualy use standard serial links on which to communicate, running on 19k2 Bd maximum. Modbus+ uses a 2-wire Belden Cable (network) running at 1 MBit/s. The difference between ModbusRTU and ModbusASCII are: RTU uses hexadecimal coding and critcal timing schemes, ASCII uses ASCII coding and non-critical timing schemes. When communicating through Modems, ASCII should be used. RTU will definitely give you problems when used with Modems and telephone lines. RTU is inherently faster than ASCII using the same Baud Rates. Modbus+ uses a subset of the standard Modbus functions and is primarily used to communicate large chunks of data between Modicon PLC's and SCADA-systems. PC's need an additional I/O-Card to be able to link to a Modbus+ network. Hopefully this gives you some additional info...