Binary Synchronous Communication (BSC or Bisync) is an IBM character-oriented, half-duplex link protocol, announced in 1967 after the introduction of System/360.
The intent was that common link management rules could be used with three different character encodings for messages.
Transcode disappeared very quickly but the EBCDIC and USASCII dialects of Bisync continued in use.
Long messages have SYN bytes inserted approximately every second to maintain synchronization.
This ability to include intermediate check sums in a long data frame allows a considerable improvement of the error detection probability.
In this environment, packets or frames of transmission are strictly unidirectional, necessitating 'turn-around' for even the simplest purposes, such as acknowledgments.
In typical full-duplex, data packets are transmitted along one wire pair while the acknowledgements are returned along the other.
The other device can reply ACK0 to accept the bid and prepare to receive, or NAK or WABT to refuse.
In some cases connection of a terminal to multiple hosts is possible via the dial telephone network.
A master station, normally a computer, can sequentially poll terminals which are attached via analog bridges to the same communication line.
This is accomplished by sending a message consisting only of an ENQ character addressed to each device in turn.
Some non-IBM hardware vendors such as Mohawk Data Sciences used Bisync for other purposes such as tape-to-tape transmission.
IBM offered assembler language macros to provide programming support.
Teleprocessing monitors such as IBM's CICS and third-party software such as Remote DUCS (display unit control system) and Westi platforms used Bisync line control to communicate with remote devices.
used BSC protocol for communication between Regional Center and Institution (bank) server over leased line.
Some important systems use Bisync data framing with a different link control protocol.
Houston Automatic Spooling Priority (HASP) uses Bisync half-duplex hardware in conjunction with its own link control protocol to provide full-duplex multi-datastream communication between a small computer and a mainframe running HASP.