Rockwell Automation AADvance Controller Configuration Guide - Manual (Page 191)

 
Document: 553633
ICSTT-RM405f-EN-P
Issue 07
9-5
Note: The original Modbus standard defines the address field as a four-digit field, but
with a prefix which related to the data type. The areas shown hatched in the
illustration show how original-style, five-digit Modbus addresses (for example, a holding
register at 40,001) relate to the AADvance memory map.
The addresses used for Modbus data transfer listings start at one; the first AADvance
Workbench variable network address is 1 and the first coil is 00001. An address of 0
for either field is illegal.
Modbus Message Scheduling
In operation, all of the Modbus master objects defined for a project (be they Modbus
RTU or Modbus TCP) function independently of each other. Each master polls its
associated slave devices, to send the messages scheduled for the slaves.
For Modbus RTU, a master object polls its slave devices one at a time, executing one
message for each slave in turn. Different slaves attached to the same master will usually
need different numbers of messages. To accommodate this, the master proceeds to
work through the messages listed for the slaves, one message for each pass, returning
to the first message for a slave after it has sent the last one. This means that some
messages (for slaves with fewer scheduled messages) will be sent more than once
during a poll cycle. The poll cycle completes when the Modbus master has made an
equal number of polls to each slave and has sent every message; the cycle then starts
again.
Example