What is the Bisync Protocol?

What is the Bisync Protocol?


Bisync is an abbreviation shortened from "binary synchronous". Sometimes you may also see the acronym BSC. Bisync is a block-oriented, error-correcting, synchronous data communications protocol introduced by IBM back in 1964 with the introduction of a product called the 270X Transmission Control Unit.

From the cover of the original IBM bisync publication (circa 1970) affectionately known as the "bisync bible". Click for a larger view.Furthermore, over the years, other terms have come into use and are often used interchangeably with bisync and BSC. For example, "3780 protocol", "3780 bisync", "2780 protocol", "2780 bisync", and "2780/3780 protocol". While technically these terms may be inaccurate, they do have a practical basis and convey meaning.

To be precise, 2780 and 3780 were model numbers of IBM remote job entry (RJE) data terminals -- namely the IBM 2780 Data Communications Terminal and the IBM 3780 Data Communications Terminal . These terminals used punch cards and consisted of a card reader, a card punch, and a line printer. They used the bisync protocol to transmit and receive data with an IBM mainframe computer. Usually dial-up or leased telephone lines and 2400 bps Bell 201C modems and then later 4800 bps Bell 208B modems were used to connect the terminal to the mainframe.

RJE was how programs, often referred to as jobs, were submitted to be run on mainframe computers back in the 1960 and 1970s. That was the era of keypunch machines and punched cards. The statements for a computer program, usually COBOL or FORTRAN, and the input data for the programs were punched onto cards using a keypunch machine. The resulting card deck was carried, often wheeled over on carts, to an RJE terminal, placed into a card reader hopper, a button was pushed, and the card images were transmitted to the mainframe.

An immediate response may have come back to printer, exchange, or card punch devices of the terminal. This output might have been the result of program just submitted, or from some previous run. Very frequently, the program and data were held at the mainframe for execution at a later time. This is known as batch processing.

The 3780 terminal was a later model than the 2780 terminal and used a more robust version of the bisync protocol -- hence the terms "3780 bisync" vs. "2780 bisync". Virtually all bisync in use today conforms with the 3780 version.

While it is true that "real" IBM 3780 and 2780 terminals are not in use today, the underlying bisync protocol became the defacto standard file transfer protocol for a wide array of devices in the days before the PC revolutionized computing. Much like Zmodem and FTP today, if you needed to get a file from one machine to another during that time, very often bisync was protocol used.

And bisync wasn't solely used in "true" computers. The bisync protocol ended up in ATM machines, check sorting machines, radar systems, cash registers, radio dispatching systems, telephone switches, and countless other devices.

This massive array of hardware is not about to disappear overnight. There is still a huge installed base of bisync-equipped machinery in North America and to a lesser extent in the rest of the world.

If somehow all of the bisync interconnected machines in the world were to disappear all at once, the results would be catastrophic. Many banks would cease to function. Some air traffic control systems would collapse. Many of the point-of-sale systems in retail stores would fail. Many credit and debit cards would become useless. EDI (electronic data interchange) networks that manage much of the business-to-business commerce would crash. There is no doubt that bisync is still a vital link in the chain of the world's computer infrastructure.

Bisync is dying away. Slowly. IBM forsook bisync years ago in favor of its SNA (Systems Network Architecture) connectivity model. (This led directly to the IBM 3770 data terminal family which was a successor to the 3780 -- a diskette drive was added and bisync was replaced with a new protocol compatible with SNA.) This abandonment, however, did not erase the bisync protocol nor its huge installed base. But it did open the door for specialty companies, like Serengeti Systems, to expand into this niche market.

To some, it is remarkable that any computer-related technology first used nearly forty years ago could still be in use today. New equipment relying solely on bisync connectivity probably has not been manufactured in more than a decade. Like the buggy whip, one day bisync will pass into history. But, no one is holding their breath just yet...


The following IBM publications may be helpful if you need to explore BSC/RJE communications in more depth:

General Information -- Binary Synchronous Communications (IBM publication: GA27-3004-2)
Component Description: IBM 2780 Data Transmission Terminal (IBM publication: GA27-3005-3)
Component Information for the IBM 3780 Data Transmission Terminal (IBM publication: GA27-3063-3)

Another good source was an article entitled "IBM Binary Synchronous Communications (BSC)" published by DataPro in July 1984, identified C07-491-301 Standards. (We have not been able to determine if this article is still in print.)
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Introduction to IBM 3780 BSC/RJE Communications

Who Uses 3780 BSC/RJE Communications Today?

What Did 3780/2780 BSC/RJE Terminals Look Like?

What is the Bisync Protocol?

3780 BSC/RJE Communications Frequently Asked Questions

3780 BSC/RJE Communications Glossary

Things to Consider When Purchasing 3780 BSC/RJE Emulation
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