- Hewlett-Packard Printer User Guide

Using the VPO Application Programming Interfaces
The VPO Interfaces
Chapter 4176
Modifying Message IDs
Generally speaking, if an MSI instance modifies a message, a new
message-ID is generated when the instance sends the message back to
the MSI. However, there are exceptions: no new message-ID is generated
if the message has either been diverted to the MSI (that is, no other copy
of the message exists either inside or outside the MSI) or an MSI
instance sends an unmodified message back to the MSI. If two parallel
MSI instances receive the same message and both subsequently send it
unchanged back to the MSI, multiple versions of the same message with
the same message ID exist in the MSI. As a consequence, any MSI
instance connected further down the serial chain receives multiple copies
of the message with the same message ID.
NOTE It is recommended you use the serial MSI feature if you or your
applications require read/write access to messages.
You can control the generation of message IDs by setting the variable,
OPC_MSI_CREATE_NEW_MSGID in the opc[sv]info file to the following
values:
1 enable VPO A.04.00 behavior
2 enable VPO A.05.00 and higher behavior (Default)
3 custom: no automatic creation of new message IDs
You can suppress the generation of new message IDs
generally (for example, parallel messages, copied
messages, and so on) using opcdata_generate_id()
or opcdata_set_str(). For more information on these
calls, see the VPO Developer’s Reference.