Network Print Facility interfaces with VTAM

To do printing, a VTAM® application program typically starts a session with a printer in its SNA network and sends the print data during that session. The Network Print Facility replaces the SNA-network printer with a logical printer within an NPF VTAM capture point application program.

For each logical printer LU:

VTAM must activate an APPL definition statement
The NPF VTAM capture point application must open a matching ACB

The NPF VTAM capture point application as a whole is not defined to VTAM in any way.

It is possible to run multiple instances of the NPF VTAM capture point application at the same time. Each must run in a different MVS™ address space and must do processing for a different set of logical printer LUs. There are several reasons why this might be useful:

To improve throughput by spreading processing across multiple address spaces.
To reduce region size problems by spreading the logical printer LUs across multiple regions.
To enable different sets of logical printer LUs to be started and stopped at different times.

The set of logical printer LUs to be supported by any given instance of the NPF VTAM capture point application is selected as follows:

The routing file entry for each logical printer LU contains a class identifier, the LUCLASS parameter, which specifies one or more numbers in the range from 1 to 64. LUCLASS is specified via the ROUTING function of the EZAPPFL macro. See EZAPPFL TYPE=ROUTING for a complete definition of the LUCLASS parameter.
The JCL used to start each instance of the NPF VTAM capture point application contains an LUCLASS sub-parameter, also specifying one or more numbers in the range from 1 to 64.
Each logical printer LU whose routing file record's LUCLASS value matches the JCL's LUCLASS value is selected. The LUCLASS values are considered to match if they have one or more class identifiers in common; it is not necessary to have all specified class identifiers the same.

When running the VTAM capture point application, the routing file is required for logical printer name selection, even if the coding of a general routing exit makes it unnecessary for actual routing functions.