Logical Terminal Tables

The Logical Terminal Table (LTT) pages contain the logical configuration information for each terminal ZLU in the ZCOM system. Each terminal table consists of the basic table (which is fixed for all terminals) and a table extension (which is of configurable size specified by LOGICAL-SIZE parameter in the TTGEN configuration file).

The basic table contains common information for all terminal ZLUs. It is logically divided into two sections; an area reserved for use by the ZCOM subsystem and a user area that may be manipulated by application programs (via the zltup routine).

The table extension may be used to store any application defined data structure and can be manipulated by the application program that allocates it (via the zltstore routine).

The layout of a Logical Terminal Table (zltt_type) basic table is shown in Table 3-9 “Logical Terminal Table (Reserved Area)” and Table 3-10 “Logical Terminal Table (user maintainable Area)”. Note that the section reserved for use by the ZCOM system is immediately followed by the user area.

This is followed by LTUSER - the user maintainable area - which consists of the fields shown in Table 3-10 “Logical Terminal Table (user maintainable Area)”.

LTZLU - Terminal ZLU number

This is the ZLU number of the owning terminal.

LTZCS - Owning ZLU checksum

This is the ZLU checksum of the owning terminal.

LTMUX - Mux interface number for this terminal

This is the Mux interface number that this terminal is configured on.

LTPORT- Port number within the Mux card

This is a one byte field that indicates which port this terminal is attached to within the Mux card.

LTTERM - Terminal number within port

This is a one byte field that indicates which terminal within a port this terminal is configured on. This field has been replaced by the ltmxtm field. This field continues to be valid for 2/8-channel MUX cards for backwards compatibility reasons.

LTPTERM - Pointer to the physical terminal table (zptt_type *)

It contains the memory address of the related physical terminal table. This field is used as a linkage to find physical terminal information.

LTGZLK - ZLU of next terminal in group

Contains the circular linkage of ZLUs linked to the same physical terminal. The linkage is to allow de-multiplexing of inbound messages. For a normal 1:1 terminal, the entry will contain its own ZLU number. Note that there is no checksum associated with this ZLU.

LTDEVC - Terminal device type

Unique identification of the actual terminal connected to the system. Two different terminals can have the same screen format and physical protocol, but the device type may not necessarily be the same; it relates to the symbolic device type specified in the TTGEN configuration file.

LTTYPE - Logical terminal type

The logical terminal type is used to describe the message format expected from the terminal. For screen-type terminals this will identify the terminal as belonging to a class of terminals with the same screen formatting attributes and message delimiters (e.g., IBM.3278 or NCR.501).

LTFLAG - System status flags

Currently the system status flags are used to indicate whether the outbound multiplexing and the inbound multiplexing are enabled.

Figure 3-2 System Status flags

Both are set by TTGEN according to the device type specified. These flags may be modified dynamically by use of the zltmx routine call.

LTZMXP - ZCOM address of multiplexing program

This field contains the node, ZLU number, and checksum of the outbound multiplexing program. If the ZCOM_LTFLAG_OMX bit in ltflag is set, the responsible multiplexing program should set up this field with its primary ZLU (via the zset_rcvr routine, mode 2). All out-going messages will then be queued to its primary ZLU.

LTZPRCVR - ZCOM address of primary indirect receiver

This field contains the node, ZLU number, and checksum of the program to receive data messages from an inbound multiplexed terminal. If the ZCOM_LTFLAG_IMX bit in ltflag is set, the system sets up this field on a zset_rcvr (mode 0) call. All incoming messages will be received by the inbound multiplexing program, which will then route the data to the receiver specified in this field.

LTZSHRCVRS - Pointer to a linked list of indirect shared receivers

This field contains a pointer to a linked list of shared receivers that wish to receive data messages from an inbound multiplexed terminal. Each entry in the linked list represents one program. If the ZCOM_LTFLAG_IMX bit in ltflag is set, the system sets up this field on a zset_rcvr (mode 0) call. All incoming messages will be received by the inbound multiplexing program, which will then route the data to each of the receivers specified in this linked list.

LTHLDQ - Sub-packet holding queue

This queue is a holding queue for partial messages to an inbound multiplexed terminal.

LTDHQN - Data holding queues

For use by an application program to store temporary data for each terminal. Only the queue header is stored in these fields, as the data itself is stored in the ZCOM subsystem buffers.

LTUSER - User-maintainable area

The fields in the user-maintainable area may be modified by the zltup call. The area consists of the following fields (up to the end of the terminal table):

LTADDR - Logical terminal address

Holds the logical terminal addressing information needed to recognize the correct logical terminal in a group of multiplexed terminals. This is a 10-byte field, however the multiplexing program may use these bytes according to the specific requirements of the protocol.

LTSTAT - Logical status

Contains the status of the logical terminal. This entry is maintained by a multiplexing program and will remain unaltered for a normal 1:1 terminal.

Figure 3-3 Logical Status Structure LTSTAT

LTCWCT, LTTXCT, LTRXCT, LTERCT - Message and error counters

Allow message counters for control writes, transmit data, receive data, and error events to be maintained by the multiplexing application program, for multiplexed logical terminals.

LTAPNO - System application number

This field is initialized by TTGEN based on the value specified in the Term or Lterm statement in the TTGEN configuration file. This field is not used by the ZCOM subsystem. It allows applications to recognize the terminals which are assigned to it. Each ZCOM application which is a direct receiver of terminal messages should access only those terminals with its allocated application number.

LTINST, LTBRCH, LTWORK, LTAREA - Terminal logical ID

These logical terminal ID fields describe the owner and location of the terminal at an application program level. At this level the physical parameters are not relevant. Because these fields are at an application level, their contents are largely dependent on the particular application.

LTNAME - Terminal name

This is a 30-byte ASCII description of the terminal. It is used by the ZCOM utility programs as well as application programs. The field is initialized from the desc parameter of the Term or Lterm statements in the TTGEN configuration file.

The logical terminal table extension follows the basic table, and is used exclusively by the application programs. The size of this area is specified in the HLTDSZE field in the ZCOM header table (zheader_type).