Configuration as a Modbus Slave MVI56-MCM ♦ ControlLogix Platform
Modbus Communication Module
ProSoft Technology, Inc. Page 55 of 159
July 24, 2008
Pass Thru should only be used when required. If a master issues a Write
command to the module, the module must build a special block of information.
Then, it waits for confirmation from the ladder logic that the block has been
processed.
Note: If the module is waiting for the block to be processed by the ladder, and the master device
issues another Write command, the module will return an Error Code of 6 (module busy). This
results in the data written by the master not to be processed.
5.3.1 Pass Thru Coil Adjustment
The Sample Ladder Logic will only allow for the first 416 coils (26 registers) to be
processed. This is due to the size of the Coil Array within the
_UTIL.Passthru.MBCoil setup, and the Sample Ladder Logic. To increase this
array the following steps must be performed.
1 Increase the Coil array under the User Defined data type of _CoilArray
(should be a value divisible by 32).
2 Change the ladder within _PassThru rung #2 to reflect this new change. This
must be changed in 2 places. First, the LES statement should be equal to the
new array size.
3 The COP statement that copies the new status of the bits must have the
length changed to Equal to the number of bits defined in the array 416 bits in
array / 16 = 26.
These changes are necessary for the proper operation of the logic when the
ladder logic is processing the new write data from the master.
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