ProSoft Technology MVI69-MCM - Manual (Page 129)

MVI69-MCM ♦ CompactLogix or MicroLogix Platform Reference
Modbus Communication Module User Manual
ProSoft Technology, Inc. Page 129 of 167
March 22, 2011
5.5.5 Pass-Through Control Blocks
If one or more of the slave ports on the module are configured for formatted
pass-through mode, the module will send input image blocks with identification
codes of 9956, 9957, 9958 or 9959 to the processor for each write command
received. Any MODBUS Function 5, 6, 15 or 16 command will be passed from
the port to the processor using a block identification number that identifies the
Function Code received in the incoming command. Ladder logic must exist in the
process to handle the receipt of all MODBUS write functions and to respond as
expected to commands issued by the remote MODBUS Master device.
Important: MVI69-MCM modules with firmware version 1.21 and newer cannot use ladder logic
written for earlier firmware versions. Please use the ladder logic or Add-On Instruction specifically
labeled for your MVI69-MCM module’s firmware version. Firmware version 1.21 includes the
following changes to the pass-through control blocks:

Mutual exclusion on Pass-Through Block IDs 9956, 9957, 9958, and 9959 from both ports - If
both ports are configured as slave ports, when both of the slave ports receive write commands
with the same Function Code, which would need to use the same block identifier from the
above list, the module will process the command from the port which first received the
command and will return an Exception Code error code 6 (node is busy - retry command later
error) from the other port that received the command last. The Master will retry the command
on the busy port after a short delay. This prevents Pass-Through blocks on both ports from
overwriting each other.

The Pass-Through Block ID is now written by the module into the first word, the (0) offset, of
the processor's backplane input image. Previously this location contained a 0 (zero) value.
Ladder logic for earlier firmware versions will not work with MVI69-MCM firmware version 1.21
or later.