ACC Programmer's Reference Guide

Chapter 2 27

ZCOM Message Handling

Message Queuing

Each program ZL U (either primary or auxiliary) has a s ingle input

queue on which messages are received and read on a first-in-first-out

basis. Becausethe ZLU is the input queuingmechanism 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 a re reset to zero via the ztimr routine (a zero “ time” value).

The application program uses the zread routinetoreadmessagesfrom

the input queue. All response and status messages are written to the

primary program ZLU. Data messages can be written to eit her the

primary or aux iliary pro gram 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

messagewassent.Themessageisaddedtotheendofthequeueforthe

specified ZLU.

Figure 2-1 Program Message Queueing

Application A

zread

Primary

Program

Points to the

Input Queue

MSGMSG

Input

Queue