ACC Programmer's Reference Guide

20 Chapter1
ZCOM Subsystem
ZCOM Concepts
The driver is then responsible for handling the message. Messages on the
express q ueue are sent before those on the high priority queue which are
sent before those on the low priority queue.
Because the ZCOM Subsystem itself takes responsibility for the delivery
of messages, the sending program is able to specify a number of options
when it passes the message to the system. These options include:
• Continue execution whether the message is sent successfully or not.
• Continue execution but report any error status (status messages are
queued to the ZLU of the calling program).
• Continueexecution; a d efinite response will be queued to theprogram
ZLU when the send has completed.
• Suspend execution until the send has completed (i.e.,
acknowledgment that the message was transmitted).
On completion, the message buffer may also be passed back to the
sending application program if required.
Each message on the ZLU has a ‘type’ attribute so that the program
reading the ZLU is able to differentiate between status messages,
timeout messages and data messages from terminals or other programs.