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{{Refimprove|date=December 2010}}
'''Telecommunications Access Method''' ('''TCAM''') is an [[access method]], in IBM's [[OS/360 and successors]] computer operating systems on [[IBM System/360]] and later, that provides access to terminals units within a teleprocessing network.<ref>{{cite book|title=Messaging and Queuing Using the Mqi: Concepts & Analysis, Design & Development|author1=Burnie Blakeley |author2=Harry Harris |author3=Rhys Lewis |name-list-style=amp |pages=9|year=1995|publisher=McGraw-Hill|isbn=0-07-005730-3}}</ref>

==Features==
TCAM provides similar functionality to [[Queued teleprocessing access method|QTAM]], which it replaced. It was the access method for the initial version of [[Time Sharing Option]] ('''TSO'''). With the advent of IBM's [[Systems Network Architecture|SNA]], TCAM was eventually superseded by [[VTAM]].

TCAM was said to have the following enhancements over QTAM:<ref>{{cite book|publisher=IBM|title=OS TCAM Concepts and Facilities|id=GC30-2022-2|date=January 1973|url=http://www.bitsavers.org/pdf/ibm/360/os/tcam/GC30-2022-2_OS_TCAM_Concepts_and_Facilities_Rel_21.6_Jan73.pdf}}</ref>
* Improved buffering, with more buffering options.
* Improved organization of [[Message queue|message 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 Communications|Binary Synchronous]] Support.
* Checkpointing.

== Structure ==
TCAM consists of a '''Message Control Program (MCP)''' and zero or more '''application 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 later [[inetd|internet service daemon (inetd)]] in unix and other systems. It is also similar to QTAM, where the application programs are called ''Message Processing Programs'' (MPP).

The '''MCP''' is assembled by the user installation from a set of [[Macro (computer science)|macros]] supplied by IBM. These macros define the lines and terminals comprising the system, the [[Data set (IBM mainframe)|datasets]] 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 macros <code>OPEN</code> and <code>CLOSE</code>, and either the ''Basic'' macros <code>READ</code>, <code>WRITE</code>, and <code>CHECK</code>, or the ''Queued'' macros <code>GET</code> and <code>PUT</code>. The use of [[Sequential access|SAM]] macros allows application programs to be tested in a [[batch processing]] environment.

==Device support==
TCAM initially supported
*[[Binary Synchronous Communications]] ('''BSC''') terminals
*[[Asynchronous serial communication|Start/stop]] terminals
*[[IBM 2260|2260 terminals]]
* message queuing in storage and on disk
Support for [[IBM 3270|3270 terminals]] was soon added.

== References ==
<references />

{{Mainframe I/O access methods}}

[[Category:IBM mainframe operating systems]]

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Telecommunications Access Method - Revision history

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