Publication 1747-RM001G-EN-P - November 2008
SLC Communication Instructions 12-19
Modbus RTU Master protocol is implemented with support for 32-bit
registers.
When Modbus command 3, 4, 6 or 16 is selected, a selector for register
size appears. There are two different word orders for 32-bit registers.
For example, a value of aabbccdd h may be represented as:
– aabb ccdd h, or
– ccdd aabb h
To support different processors and file types, the word-swap option
for 32-bit registers is made available.
Message Type File Type Element Size Maximum Number of
Elements per Message
Modbus
Commands
B, N (command 5) 1-bit 1
B, N, F (command 6) 1-word or
32-bit register
(float file type)
1
B, N (commands 1, 2,
and 15)
1-bit 1856 Modbus bit elements
(116 words)
Commands 1 and 2 are
read-only, command 15 is
write-only.)
B, N, F (commands 3
and 16)
B, N (command 4)
multi-register 116 16-bit Modbus
register elements (or 58
32-bit Modbus register
elements)
TIP
When reading and writing with 32-bit registers, verify with
RSLogix 500 if the value appears as intended after the
transfer. Toggle word-swap if necessary.
Word-swap is required when reading and writing to Float
file type with another MicroLogix processor.
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