Modbus/Ascii - Comtrol DeviceMaster EtherNet/IP-Modbus EIP-MOD User Manual

1.6.2. Modbus/ASCII

Modbus/ASCII is native Modbus in ASCII format. This protocol is used primarily by legacy devices and is no
longer supported as widely as Modbus/RTU.
Like Modbus/RTU, Modbus/ASCII contains the base Modbus messages that contain simple read and write
requests. The differences between Modbus/ASCII and Modbus/RTU are:
1. The message data is sent in ASCII format, so the message length is twice as long. It requires two ASCII
characters for each byte of data.
2. An 8-bit LRC is attached to verify the message instead of a 16-bit CRC. The LRC is also transmitted in
ASCII format.
3. There are defined starting and ending characters to determine a Modbus/ASCII messages.
The format is as follows:
Where:
• The terms Master or Client are used to identify the sender of the message.
• The terms Slave or Server are used to identify the devices responding to the message.
Modbus/ASCII is used primarily for:
• Serial port connectivity
RS-485 is the most common serial mode, but RS-232 and RS-422 are also used. Used primarily by legacy
Slave devices.
• Ethernet TCP/IP socket connections
This is not the same as Modbus/TCP, but does provide a very simple method of interfacing to remote
devices. It is used by some applications and some OPC servers.
Note: This communication method typically is not supported by PLCs.
DeviceMaster EIP-MOD User Guide: 2000664 Rev. A
Modbus/ASCII
Chapter 1. Introduction - 17