Comtrol Modbus TCP User Guide - Manual (Page 18)

18 - Chapter 2. Programming Interface DeviceMaster UP Modbus/TCP User Guide: 2000447 Rev. I
What is Modbus/RTU?
2.1.2. What is Modbus/RTU?
Modbus/RTU is native Modbus in hexadecimal format. These are the base Modbus messages that contain
simple read and write requests. 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/RTU is used primarily for:
• Serial port connectivity. RS-485 is the most common serial mode, but RS-232 and RS-422 are also widely
used. Commonly used by both Master and Slave devices.
• Ethernet TCP/IP socket connections. This is not the same as Modbus/TCP (please see section on Modbus/
TCP), but does provide a very simple method of interfacing to remote devices. It is used by many
applications and some OPC servers.
Note: This communication method typically is not supported by PLCs.
2.1.3. What is 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 (please see section on 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.