DFHTCT - Terminal Control Table

Enhanced Definition

The Terminal Control Table (DFHTCT) is a fundamental CICS control block that defines all terminals, logical units, and associated communication characteristics known to a CICS region. It provides CICS with the necessary information to manage communication sessions with connected devices, enabling users to interact with CICS applications. The `DFHTCT` (Terminal Control Table) is a crucial CICS control block that defines all terminals, printers, and logical units (LUs) with which a CICS region can communicate. It acts as a directory, providing CICS with the necessary information to manage terminal input/output operations and user interactions.

Key Characteristics

- CICS Control Block: DFHTCT is an essential internal CICS table, traditionally assembled from source code, though largely superseded by dynamic resource definition.
- Terminal Definition: Each entry in the TCT describes a specific terminal, printer, or logical unit (LU), including its type, model, screen size, and communication method.
- Communication Management: CICS uses the TCT entries to establish, maintain, and terminate sessions with terminals, handling input/output operations.
- Autoinstall Support: Modern CICS leverages terminal autoinstall, where a minimal TCT entry (or RDO definition) acts as a model, allowing CICS to dynamically create full terminal definitions upon connection, reducing manual configuration.
- Resource Attributes: Entries specify attributes like the VTAM APPLID, terminal capabilities (e.g., extended attributes), and associated security profiles.
- Internal Use: Primarily for CICS's internal management of terminal resources, though some information can be queried by CICS application programs using EXEC CICS ASSIGN commands.

Use Cases

- User Login: When a 3270 terminal connects to a CICS region, CICS uses the TCT (or autoinstall) to identify the terminal, validate its characteristics, and establish a user session.
- Printer Output: Defining and managing printers for CICS applications to direct output, such as reports, invoices, or labels, to specific physical devices.
- LU-to-LU Communication: Configuring CICS to communicate with other CICS regions or external systems using protocols like APPC, though modern inter-region communication often uses DFHCONN and DFHSESSION RDO definitions.
- Terminal Autoinstall Configuration: Setting up a generic TCT entry (or RDO TYPETERM) to enable CICS to dynamically define new 3270 terminals as they connect, simplifying administration for large numbers of similar terminals.
- Terminal Monitoring and Troubleshooting: CICS system programmers use CICS commands (e.g., CEMT INQUIRE TERMINAL) to view TCT-derived information for monitoring terminal status, active sessions, and diagnosing connectivity issues.

Related Concepts

The DFHTCT works in conjunction with VTAM (Virtual Telecommunications Access Method), which is the underlying network access method managing the physical and logical connections to terminals. While VTAM handles the network layer, the TCT provides CICS with its application-specific view of these resources. Modern CICS largely replaces static TCT assembly with Resource Definition Online (RDO), where terminal definitions are stored in the CICS System Definition Data Set (CSD) and dynamically loaded. The DFHSIT (System Initialization Table) contains parameters that control CICS initialization, including whether terminal autoinstall is enabled, directly impacting how the TCT is utilized.

Best Practices:

Prioritize Terminal Autoinstall: For 3270 terminals, always enable and configure CICS terminal autoinstall to dynamically define terminals, significantly reducing manual TCT maintenance and improving system flexibility.
Use RDO for Persistent Definitions: For terminals or logical units requiring specific, persistent attributes not covered by autoinstall (e.g., dedicated printers, APPC LUs), define them using CICS Resource Definition Online (RDO) and store them in the CSD.
Minimize Static TCT Assembly: Avoid assembling large, static DFHTCTs unless absolutely necessary for very specific, legacy, or non-autoinstallable devices that cannot be managed via RDO.
Implement Security: Ensure that terminal definitions, especially autoinstall models, are configured with appropriate security