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Copyright © Roboteq Inc. 2018. All Rights Reserved. 1
1. Introduction
The Roboteq Modbus Implementation User manual contains information about how Roboteq
implemented Modbus protocol in controllers.
1.1 What is Modbus
Modbus is a serial communication protocol developed by Modicon published by Modicon in 1979 for use
with its programmable logic controllers (PLCs). In simple terms, it is a method used for transmitting
information over serial lines between electronic devices. The device requesting the information is called
the Modbus Master and the devices supplying information are Modbus Slaves. In a standard Modbus
network, there is one Master and up to 247 Slaves, each with a unique Slave Address from 1 to 247. The
Master can also write information to the Slaves.
The official Modbus specification can be found at www.modbus.org/specs.php.
1.2 Modbus object types
The following is a table of object types provided by a Modbus slave device to a Modbus master device:
Object type
Access
Size
Coil
Read/Write
1 bit
Discrete Input
Read Only
1 bit
Input Register
Read Only
16 bits
Holding Register
Read/Write
16 bits
1.3 Protocol versions
Versions of the Modbus protocol exist for serial port and for Ethernet and other protocols that support the
Internet protocol suite. There are many variants of Modbus protocols:
• Modbus RTU: This is used in serial communication and makes use of a compact, binary
representation of the data for protocol communication. The RTU format follows the
commands/data with a cyclic redundancy check checksum as an error check mechanism to ensure
the reliability of data. Modbus RTU is the most common implementation available for Modbus. A
Modbus RTU message must be transmitted continuously without inter-character hesitations.
Modbus messages are framed (separated) by idle (silent) periods.
• Modbus ASCII: This is used in serial communication and makes use of ASCII characters for protocol
communication. The ASCII format uses a longitudinal redundancy check checksum. Modbus ASCII
messages are framed by leading colon (":") and trailing newline (CR/LF).
• Modbus TCP/IP or Modbus TCP: This is a Modbus variant used for communications over TCP/IP
networks, connecting over port 502. It does not require a checksum calculation, as lower layers
already provide checksum protection.