How bridge facility virtual terminals are autoinstalled

Bridge facility virtual terminals are defined to CICS® Transaction Server for z/OS® as LU2 devices. They are created by the 3270 bridge mechanism to support the execution of a CICS 3270 application in a bridged environment, where all terminal interaction is intercepted and passed to the 3270 bridge mechanism.

The 3270 bridge mechanism uses a model 3270 terminal definition (facilitylike) to build the bridge facility, creating both an eight-byte token to identify it and a four-character terminal identifier, which is used as both TERMID and NETNAME.

For bridge facilities created with the START BREXIT command, the token and name are unique within the region creating the bridge facility, and the TERMID takes the form }AAA, where AAA is an alphabetic sequence that ascends serially.

For bridge facilities created by the Link3270 bridge, the token and name are unique across the CICSplex, and the TERMID is of the form AAA}. Uniqueness is achieved by using a shared file to control allocation of names.

Bridge facility terminal names and netnames are normally allocated dynamically by the bridge mechanism, but if the AIBRIDGE system initialization parameter is set to YES, the terminal autoinstall control program is called and can be used to assign installation specific names.

Using the terminal autoinstall control program for bridge facilities

If you specify AIBRIDGE (YES), then the autoinstall control program is called when a bridge facility is allocated or deleted. The autoinstall control program is passed a parameter list ( the communications area) described in The communications area at INSTALL for bridge facility virtual terminals and The communications area at DELETE for bridge facility virtual terminals. This indicates whether the program was called for a Link3270 or a START bridge facility.

Autoinstall of a START bridge facility

Apart from the information contained in the communications area, you can obtain the following information:

The autoinstall control program can use the USERID and TRANSID values to derive new TERMID and NETNAME values and return them in the communications area.

Autoinstall of a Link3270 bridge facility

Installation specific terminal names can be allocated in one of two ways:

  1. Names can be defined by the client program and passed on the initial Link3270 request. The autoinstall control program can then allow, change or reject these names.
  2. Names can be defined by the autoinstall control program

The names suggested by the client program are passed in the communications area. The autoinstall control program can also use EXEC CICS ASSIGN USERID to obtain the USERID and use this to validate the suggested TERMID and NETNAME.

The client defined TERMID and NETNAME fields can also be used to pass some installation specific data to the autoinstall control program, to be used to generate the required names.

Bridge facility name uniqueness

Some applications use the termid to allocate a unique resource. This relies on the name being unique within the CICSPlex®. Bridge facility names have the same namespace as termids. However, CICS is unable to ensure that the bridge facility name returned by the autoinstall control program is not the same as a termid somewhere in the CICSPlex. Neither the termid nor netname returned by the autoinstall control program are validated.

Related concepts
How Client virtual terminals are autoinstalled
Related tasks
Rewriting user-replaceable programs
Assembling and link-editing user-replaceable programs
Writing a program to control autoinstall of terminals
Writing a program to control autoinstall of consoles
Writing a program to control autoinstall of APPC connectionsWriting a program to control autoinstall of shipped terminals
Related reference
The autoinstall control program at INSTALL
The autoinstall control program at DELETE
Default actions of the sample programs
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