Reference Guide

ZCOM Message Handling
Multiplexing
Chapter 230
Multiplexing
The multiplexing feature of ZCOM allows more than one logical terminal
to be mapped to a single physical terminal. This mechanism is
implemented in ZCOM by allowing messages to and from a terminal
ZLU to be intercepted by a program (which is able to alter the data
content, e.g., adding or removing headers) and then forwarded on to the
destination physical terminal or its receiver program.
Some fields in the logical terminal table and in the physical terminal
table are mentioned in the following descriptions; more information on
these fields can be found in the logical terminal table and the physical
terminal table information in the section on Tables and Data Structures.
The outbound and the inbound multiplexing are separate, so although in
most situations the multiplexing program will perform both functions, it
is possible to perform only inbound multiplexing or only outbound
multiplexing in any program.
The ltflag field of the logical terminal table (zltt_type) contains two flag
bits ZCOM_LTFLAG_OMX and ZCOM_LTFLAG_IMX, which indicate
whether messages to or from the logical terminal respectively, are to be
handled by a multiplexing program. These bits are set by TTGEN
according to the device type.
When more than one logical terminal is mapped to a single physical
terminal, the ltgzlk field (<zltt_type>.ltgzlk) linkage forms a circular list
of terminals in the same multiplexed group (i.e., mapped to the same
physical terminal). The ltqzlk field contains the ZLU number (16-bit) of
the next terminal in the group. For a non-multiplexing terminal, this
field contains the ZLU number of the current terminal (i.e., pointing
back to itself, a special case for circular list).