ProSoft MVI69E-MBS User Manual page 71

MVI69E-MBS ♦ CompactLogix Platform
Modbus Serial Enhanced Communication Module
In Slave Pass-Through mode, write commands from the Master are handled
differently than they are in Normal mode. In Slave Pass-Through mode, all write
requests are passed directly to the processor and data is not written directly into the
module's database.
This mode is especially useful when both a Modbus Master and the module's
processor logic need to be able to read and write values to the same internal
database addresses.
The following diagram shows the data flow for a slave port with pass-through enabled:
Step Description
1 Same as normal mode.
2 Same as normal mode.
3 a. In Pass-Through mode, if the Slave driver receives a read request, it looks for the data
in module's internal database, just as it would in Normal mode.
b. The data needed to respond to the read command is retrieved directly from the
internal database and returned to the Slave driver so it can build a response message.
c. In Pass-Through mode, if the Slave Driver receives a write request, it does not send
the data directly to the module's internal database. It puts the data to be written into a
special Input Image with a special Block ID code to identify it as a Pass-Through Write
Block and substitutes this special block in place of the next regular Read Data Block. The
special block is processed by the ladder logic and the data to be written is placed into the
WriteData controller tag array at an address that corresponds to the Modbus Address
received in the write command.
d. During normal backplane communications, the data from the WriteData array,
including the data updated by the Pass-Through Write Block, is sent to the module's
internal database. This gives the ladder logic the opportunity to also change the values
stored in these addresses, if need be, before they are written to the database.
Note: The ReadData array is not used in Pass-Through mode.
4 Same as normal mode.
5 Same as normal mode.
ProSoft Technology, Inc.
June 28, 2017
MVI69E-MBS Backplane Data Exchange
Page 71 of 154
User Manual