VTAM (Virtual Telecommunications Access Method) is an IBM application program interface (API) for communicating with telecommunication devices and their users. VTAM was the first IBM program to allow programmers to deal with devices as "logical units" without having to understand the details of line protocol and device operation. Prior to VTAM, programmers used IBM's Basic Telecommunications Access Method (BTAM) to communicate with devices that used the binary synchronous (BSC) and start-stop line protocols.
VTAM became part of IBM's strategic Systems Network Architecture (SNA) which in turn became part of the more comprehensive Systems Application Architecture (SAA). As the computer industry turned to open standard architectures, IBM began to deemphasize its proprietary architectures in favor of becoming a participant and leader in developing open standard architectures. However, most of its customer base retains a large investment in legacy applications and system skills based on VTAM, SNA, and SAA.
VTAM's interface consists of "macro instructions" that set up connection control blocks and then do SENDs to devices and either synchronous or asynchronous READs from them. Typically, legacy programs written in COBOL, PL/1, and assembler language use VTAM to communicate with interactive devices and their users. Programs that use VTAM macro instructions are generally exchanging text strings (for example, online forms and the user's form input) and the most common interactive device used with VTAM programs was the 3270 Information Display System.
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