Installation manual

Publication 1747-RM001G-EN-P - November 2008
13-90 SLC Communication Channels
Modbus RTU Protocol
This section shows the configuration parameters for Modbus RTU (Remote
Terminal Unit transmission mode) protocol. For more information about the
Modbus RTU protocol, see the Modbus Protocol Specification (available from
http://www.modbus.org).
SLC 5/03, SLC 5/04, and SLC 5/05 support Modbus RTU Master from
Series C FRN11 onwards.
Modbus RTU Master
Message instructions are used to transfer information between the data files in
the Modbus RTU Master and the Modbus RTU Slaves. See the section,
Configuring a Channel for Modbus RTU Master, for detailed information
about configuring a MSG instruction for Modbus Communications.
Modbus addressing is limited to 16 bits per memory group, each with a range
of 1 to 65,536. There are four memory groups, one for each function:
• coils (generally addressed as 0xxxx)
• contacts (1xxxx)
• input registers (3xxxx)
• holding registers (4xxxx)
Coils and contacts are addressed at the bit level. Coils are like outputs and can
be read and written to. Contacts are like inputs and are read-only. Input
registers and holding registers are addressed at the word level. Input registers
are generally used for internally storing input values. They are read-only.
Holding registers are general purpose and can be both read and written to.
The most significant digit of the address is considered a prefix, and does not
get entered into the Modbus Data Address field when configuring the message
instruction.
When the message is sent, the address is decremented by 1 and converted into
a 4-character hex number to be transmitted via the network (with a range of
0-FFFFh); the slave increments the address by 1, and selects the appropriate
memory group based on the Modbus function.