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Basic Telecommunications Access Method (BTAM)[1] was a low-level programming interface specified by IBM for use on the IBM System/360 for sending and receiving data through telecommunication lines. Later, IBM specified higher-level interfaces (QTAM, VTAM) and entire architectures (TSO, SNA).
BTAM requires the application program (or transaction processing system) to handle almost every detail of the protocol. This is harder than using a higher-layer protocol, but it permits interfacing to non-standard devices in non-standard ways. At the time BTAM was introduced, there was little standardization anyway. Like most of the System/360 programming interfaces, BTAM continued to be supported in later iterations of the system architecture. IBM finally withdrew support for BTAM in 2000.
BTAM was an access method for interactively communicating with local or remote terminals - usually connected on Channel 0 (System/360) or Channel(s) 0 and/or 4 (System/370 and later). BTAM dynamically built CCW's for reading, writing and "polling" terminals and dealt with specific responses from those terminals, according to the success or failure of the communication channel.
BTAM was a key component in IBM's early transaction processing systems such as MTCS, CICS and IMS and underpinned the rise of online transaction processing for many large commercial Banks and Insurance companies. It was not unusual for BTAM and later developed access methods (such as VTAM) to co-exist, supporting different devices simultaneously under the same transaction processing system.
Amongst many other devices supported, BTAM handled: