ZCOM Message Handling
Multiplexing
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
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