AM3270/TR3271 Reference Manual

1 Bisync Protocols—
Concepts and Terminology
086705 Tandem Computers Incorporated 1–1
This section explains:
Basic bisync concepts, terms, and procedures.
Types of message formats.
Bisync control characters.
Standard procedures, such as polling and selecting.
For introductions to Tandem’s AM3270 and TR3271 access methods see Section 2,
“Overview of AM3270 and TR3271 Access Methods.”
What are
Bisync Protocols?
Binary-synchronous protocols, referred to variously as “bisynchronous,” “bisync,”
“byte-sync,” and “BSC” protocols, are link-level, block-mode, character-oriented
protocols. This manual refers to them as “bisync” protocols.
Bisync protocols control the transmission of digital data on a half-duplex line (or one
way on a full-duplex line) that connects two or more block-mode devices. In a block-
mode terminal network, the terminals send complete messages, rather than the
character-by-character transmission supported by an asynchronous protocol.
Character-oriented block-mode protocols use a particular character code set such as
ASCII or EBCDIC for transmission. Some of the characters are reserved for control
functions such as synchronization and error control. To send binary or text messages
that may appear to contain any of these reserved characters, a transparent
transmission mode is used.
Note AM3270 and TR3271 do not support line-level transparent transmission. They do support subdevice-level
transparency, when the line is configured for EBCDIC.
Comparing Bisync
and Async
Bisync transmission can send large blocks of data without pausing, and does not
require the start and stop bits of asynchronous communication. All bit intervals are
equal, including bit intervals between blocks of data. Bit and character
synchronization occur by preceding each block with synchronization (SYN) characters.
The bit pattern of the SYN character depends on which character code set is used.
Bisync protocols are limited to half-duplex transmissions (or one-way on full-duplex
lines), and they require positive acknowledgement of every data block sent.