ProSoft Technology MVI69L-MBS - Manual (Page 73)

MVI69L-MBS ♦ CompactLogix Platform MVI69L-MBS Backplane Data Exchange
Communication Module User Manual
ProSoft Technology, Inc. Page 73 of 154
January 6, 2014
In Slave Pass-Through mode, write commands from the Master are handled
differently than they are in Normal mode. In 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.