ACC Programmer's Reference Guide

Chapter 1 19
ZCOM Subsystem
ZCOM Concepts
Program ZLUs
Each program may open one or more program ZL Us which provides an
input message queue to that program. Think of a program’s input
message queue as a mailbox where messages can be delivered. All
messages delivered are queued at the end of the i nput queue for a
program in the order in which they were received. Note that the message
is not actually delivered to the program, but simply placed into the input
queue. The program must issue a ZCOM library call to retrieve the
messages one-by-one from the input queue. The method used to cause a
message to be placed in to a program’s input queue will be different
depending on whether the message was created by another program or a
physical/logical device.
If a message is sent (generated) by another program, then the sending
program must specify the ZLU of t he program’s input queue to place the
message into. The receiving program need not do anything for t he
message to be successfully delivered to their mail box (program input
queue).
However, when a m essage is generated by a terminal ZLU, any program
that wishes to receive messages from that device (terminal ZLU) must
inform the ZCOM Subsystem first. The program should issue a ZCOM
call (zset_rcvr) to tell the ZCOM Subsystem that all data coming from
the specified terminal ZLU should be delivered to its input queue.
Whenever data arrives from the device, it w ill be placed into the input
queues of all programs that have requested this data.
Terminal ZLUs
Terminal ZLUs represent a physical or logical device and do not have
input queues of their own. Any data coming from a terminal ZLU will be
delivered to the input queues o f all applications t hat have requested it. If
there is no application interested in receiving the data, then the data is
thrown away and a message is logged to that effect.
Terminal ZLUs do however have three different output (transmit)
queues. When a program sends data to a terminal ZLU, it can select one
of three queues to place that data on. Each queue represents a different
level of priority. The Express Queue is used for extremely high-priority
protocol dependent requests. The High-Priority Queue is used for high
priority data and t he Low-Priority Queue is used for low priority data.
When a message is generated by an application, the buffers involved are
queued to the selected transmit queue and the ZCOM driver i s notified.