Reference Guide

ZCOM Message Handling
ZLU Definition
Chapter 2 25
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 auxiliary 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 that particular terminal’s ZLU (using the zset_rcvr
routine call). Any number of programs can be set up as a receiver for a
given terminal at any one time. 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 physical entity associated
with a terminal ZLU is dependent on the particular protocol being used.
For example, with a TTY protocol a Terminal ZLU would be associated
with a physical terminal or terminal like device. However, with a
protocol such as X.25, a terminal ZLU might represent a virtual circuit
or the HDLC line (X.25 level 2).