DFHTCT logical device codes: VTAM non-3270

Certain types of logical unit may be used to gain access to more than one resource within a subsystem. For example, a card punch device may be attached to a 3770 logical unit: the CICS® application program can direct punch output, through BMS, via the 3770 to the card punch device. The facility provided by CICS to permit communication to devices within logical units of this type is the logical device code (LDC).

Although these are VTAM® units, they require macro definition, unlike other VTAM devices.

The logical units that support LDCs are:

To reference such a device in a CICS application program, or in the CMSG transaction for message switching, you specify an LDC mnemonic which CICS translates into a numeric LDC value. When CICS sends an output data stream to the logical unit, it includes the LDC value in the function management header (FMH). When the logical unit receives the data stream, it uses the LDC value to determine which component is to receive the output, or to perform some standard action.

Each LDC mnemonic to be referenced must be defined in the TCT, optionally with its associated LDC value and certain device characteristics for use by BMS functions. Such LDC information is contained in either the system LDC table, or in an extended local LDC list. You code the following DFHTCT macros to specify the system LDC table or an extended local LDC list:
When you are defining a logical unit in the TCT, you can specify its LDCs in either of two ways:
  1. Code a DFHTCT TYPE=LDCLIST macro to define a local list of LDC mnemonics (and optionally their LDC values); for example,
    LDC2 DFHTCT TYPE=LDCLIST,
                LDC=(DS,JP,PB=5,LP,MS)
    In the DFHTCT TYPE=TERMINAL macro defining the logical unit, you specify in the LDC operand the name of the local list as defined by the DFHTCT TYPE=LDCLIST macro. For example:
         DFHTCT TYPE=TERMINAL,
                TRMTYPE=3600,
                LDC=LDC2, ...

    has associated the LDCs DS, JP, PB, LP, and MS with the 3601 logical unit that you are defining. The LDC values either may be specified in the local list, or are obtained from the system LDC table. If BMS uses these LDC mnemonics, their page size and page status must also be available from the system LDC table.

    Note: A local list defined by a DFHTCT TYPE=LDCLIST macro may be shared by a number of 3601, LUTYPE 4 and batch logical units.
  2. In the DFHTCT TYPE=TERMINAL macro defining the logical unit, you specify in the LDC operand the name of an extended local LDC list. For example:
    LDC1 DFHTCT TYPE=LDC,LOCAL=INITIAL
         DFHTCT TYPE=LDC,LDC=BCHLU
         DFHTCT TYPE=LDC,DVC=BLUPRT,LDC=AA,
                PGESIZE=(6,30)
         DFHTCT TYPE=LDC,DVC=BLUPCH,LDC=BB,
                PGESIZE=(1,80)
         DFHTCT TYPE=LDC,DVC=BLUCON,LDC=CC,
                PGESIZE=(1,132)
         DFHTCT TYPE=LDC,LOCAL=FINAL
         DFHTCT TYPE=TERMINAL,TRMTYPE=BCHLU,
                LDC=LDC1, ...

    has associated the LDCs CO, R1, H1, P1, AA, BB, and CC with the batch logical unit that you are defining. Their LDC values and device characteristics for BMS functions are described in the extended local LDC list, which is named LDC1.

When CICS requests an output or message switching operation using a particular LDC mnemonic for a logical unit, it tries to resolve the mnemonic from the list (whichever form) specified by the LDC operand of the DFHTCT TYPE=TERMINAL macro. If the LDC is not located in the local list or in the extended local list, the LDC specified is not valid for that terminal entry. In this case, X'00' is inserted in the logical device code portion of the FMH, and no destination name is inserted.

When a BMS function is requested for an LDC, and the LDC mnemonic is successfully resolved, the device characteristics (for example, device name and destination name) are accessed for the BMS function. If the LDC is in an extended local LDC list, these characteristics lie in the located extended local list entry. Otherwise, the system LDC table is searched for the LDC and the associated device characteristics.