ZLU Definition

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).

Mapped ZLUs

All messages addressed to a particular ZLU will be redirected to the alternate ZLU specified in the zmapr routine call if the original ZLU has been mapped to a different ZLU. One use of mapped ZLUs is for providing high availability systems. For example, if a hardware failure was detected in one of the ACC Mux cards, the application could map all of the ZLUs associated with the failed card to a spare unused card in the system.