BISYNC (BSC) Abbrev. for binary synchronous communications (protocol). A line protocol created by IBM for synchronized communication between mainframe computers and remote job-entry terminals. BISYNC is a character-oriented protocol: it uses special control characters to mark the beginning and end of a message, to acknowledge previous messages, to request retransmission of missing or damaged messages, etc. The BISYNC protocol may be used with the 6-bit Transcode, 7-bit ASCII, or 8-bit EBCDIC character codes, and multidrop or point-to-point communication lines.

The protocol is inherently half duplex: a message is sent, a reply is sent, the next message is sent, etc. Thus BISYNC communication usually uses half duplex communication lines and modems. Full duplex communication lines and modems may be used but most of the additional capacity is wasted.

BISYNC has been largely replaced in computer communications by newer data link control protocols, such as SDLC and HDLC. BISYNC's retransmission and acknowledgment scheme does not work efficiently over connections with long delay times. This is particularly important in the US and other areas where the telephone system is converting to satellite transmission systems for voice and data traffic.

BISYNC (ˈbaɪˌsɪŋk) Computing binary synchronous communications