ProSoft Technology MVI69-MCM - Manual (Page 111)

MVI69-MCM ♦ CompactLogix or MicroLogix Platform Reference
Modbus Communication Module User Manual
ProSoft Technology, Inc. Page 111 of 167
March 22, 2011
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 will be passed directly to the processor and data will not be written
directly into the database.
This mode is especially useful whenever 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 special Block ID code to identify it as a Pass-Through Write Block and
substitutes this speical 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
controller tag WriteData array at an address that corresponds to the Modbus Address
received in the write command.
d.During normal backplane communications, the data from the Write Data 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.
4 Same as normal mode.
5 Same as normal mode.