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Telecommunications Access Method (TCAM) is anaccess method, in IBM'sOS/360 and successors computer operating systems onIBM System/360 and later, that provides access to terminals units within a teleprocessing network.^[1]

TCAM provides similar functionality toQTAM, which it replaced. It was the access method for the initial version ofTime Sharing Option (TSO). With the advent of IBM'sSNA, TCAM was eventually superseded byVTAM.

TCAM was said to have the following enhancements over QTAM:^[2]

• Improved buffering, with more buffering options.
• Improved organization ofmessage queuing on disk including multiple volume support.
• Back-up copies of messages maintained on disk.
• Improved testing and debugging including off-line testing, improved debugging, online terminal testing, and logging.
• Improved line handling for inquiry applications.
• "Significantly increases speed and efficiency over QTAM."
• Improved operator monitoring and control.
• Binary Synchronous Support.
• Checkpointing.

TCAM consists of aMessage Control Program (MCP) and zero or moreapplication programs. The MCP handles communications with the terminals, identifies input messages and starts application programs to process them as required. This is similar in concept to the much laterinternet service daemon (inetd) in unix and other systems. It is also similar to QTAM, where the application programs are calledMessage Processing Programs (MPP).

TheMCP is assembled by the user installation from a set ofmacros supplied by IBM. These macros define the lines and terminals comprising the system, thedatasets required, and the procedures used to process received and transmitted messages.

The application programs, incorporating logic to process the various messages, are supplied by the installation, and use standard OS/360 data management macrosOPEN andCLOSE, and either theBasic macrosREAD,WRITE, andCHECK, or theQueued macrosGET andPUT. The use ofSAM macros allows application programs to be tested in abatch processing environment.

TCAM initially supported

• Binary Synchronous Communications (BSC) terminals
• Start/stop terminals
• 2260 terminals
• message queuing in storage and on disk

Support for3270 terminals was soon added.

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