Reference Guide

ZCOM Message Handling
Message Queuing
Chapter 2 27
Message Queuing
Each program ZLU (either primary or auxiliary) has a single input
queue on which messages are received and read on a first-in-first-out
basis. Because the ZLU is the input queuing mechanism for timeouts and
statuses, as well as for data messages, all programs should request
(through the zopen routine) at least one program ZLU before using the
other ZCOM calls.
The ztimr routine may be used to enable or disable the timer on a
program ZLU. When enabled, a timeout message is added to the input
queue of the ZLU every n seconds (n being specified as a parameter to
the ztimr call). If there are already 10 or more messages on the queue, no
timer message is queued. Timer processing stops when ZTIMER and
ZCLOCK are reset to zero via the ztimr routine (a zero “time” value).
The application program uses the zread routine to read messages from
the input queue. All response and status messages are written to the
primary program ZLU. Data messages can be written to either the
primary or auxiliary program ZLUs.
When a zsend routine call is executed, the message to be sent is moved to
the ZCOM kernel buffer which is queued to the ZLU to which the
message was sent. The message is added to the end of the queue for the
specified ZLU.
Figure 2-1 Program Message Queueing
Application A
zread
Primary
Program
Points to the
Input Queue
MSGMSG
Input
Queue