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Binary synchronous communications, or BISYNC, is a character (byte)-oriented form of communication developed by IBM in the 1960s. It was originally designed for batch transmissions between the IBM S/360 mainframe family and IBM 2780 and 3780 terminals. It supports online and RJE (remote job entry) terminals in the CICS/VSE (Customer Information Control System/Virtual Storage Extended) environment.

BISYNC establishes rules for transmitting binary-coded data between a terminal and a host computer's BISYNC port. While BISYNC is a half-duplex protocol, it will synchronize in both directions on a full-duplex channel. BISYNC supports both point-to-point (over leased or dial-up lines) and multipoint transmissions. Each message must be acknowledged, adding to its overhead.

BISYNC is character oriented, meaning that groups of bits (bytes) are the main elements of transmission, rather than a stream of bits. The BISYNC frame is pictured next. It starts with two sync characters that the receiver and transmitter use for synchronizing. This is followed by a start of header (SOH) command, and then the header. Following this are the start of text (STX) command and the text. Finally, an end of text (EOT) command and a cyclic redundancy check (CRC) end the frame. The CRC provides error detection and correction.

Most of the bisynchronous protocols, of which there are many, provide only half-duplex transmission and require an acknowledgment for every block of transmitted data. Some do provide full-duplex transmission and bit-oriented operation.

BISYNC has largely been replaced by the more powerful SDLC (Synchronous Data Link Control).

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