ACC Programmer's Reference Guide

Chapter 2 25

ZCOM Message Handling

ZLU Definition

There are three types of ZLUs. These are:

•Program

• Terminal (or remote device via a communication link)

•Mapped

Program ZLUs

A Program ZLU may be either a primary ZLU or an auxiliary ZLU. All

programs using ZCOM must have a primary ZLU defined. Other ZLUs

may then be defined as auxiliary if required. If a message is sent to

another program, then the destination ZLU is the other program’s

primary or aux iliary ZLU.

Terminal ZLUs

Terminal ZLUs are used to direct messages to terminals. Before a

program can receive messages from a terminal, a program must set itself

up as receiver for t hat particular terminal’s ZLU (using the zset_rcvr

routine call). Any numb er of programs can be set up as a receiver for a

given terminal at any one t ime. It is possible, of course, to set up a

program to route messages to other programs from the terminal ZLU.

NOTE Terminal ZLU is a generic term. The actual physi cal entity associated

with a terminal ZLU is dependent on the particular protocol being used.

For example, with a TT Y protocol a Terminal ZLU would be associated

withaphysicalterminalorterminallikedevice.However,witha

protocol such as X.25, a t erminal ZLU might represent a virtual circuit

or the HDLC line (X.25 level 2).