Installation manual

Publication 1747-RM001G-EN-P - November 2008
SLC Communication Instructions 12-21
Similar to the Read Register command,
– If the source file is a Floating file (Example 3), word-swap is
recommended.
– If the source file is an Integer file (Example 4), word-swap is
unnecessary.
Here, it is assumed the peer device does not swap received word order.
You need to validate the word order prior to implemention on the
network.
• Channel
FRN 11 of SLC 5/03, SLC 5/04, and SLC 5/05 supports Modbus RTU
Master on Channel 0 only. Channel 0 is the RS-232 port.
• Message Timeout
This value defines how long, in seconds, the message instruction has to
complete its operation once it has started. Timing begins when the
false-to-true rung transition occurs, enabling the message. If the timeout
period expires, the message instruction will generate a specific error (see
MSG Instruction Error Codes on page 12-41). The amount of time that
is acceptable should be based on application requirements and network
capacity and loading. The default value is 2 seconds for Modbus
commands. The maximum timeout value is 255 seconds.
Example 4: A 32-bit Write Request for a Float (F) command
RSLogix 500 Data View (dec) SLC Processor
(hex)
MLX Processor
(hex)
RSLogix 500 Data View (dec)
123456
47F1 2000
Write no swap
W
r
i
t
e
w
o
r
d
-
s
w
a
p
47F1 2000
2000 47F1
error
1.086583e-19
123456
IMPORTANT
Since a 32-bit register is not defined in the Modbus
protocol specification, you are advised to first test if the
word order is decoded correctly. Some devices may require
word-swap and others may not.
RSLogix and SLC firmware do not determine word order
based on element file type. You need to specify if a 32-bit
word-swap is desired.