Synchronous transmit-receive (STR) was an early IBM character-oriented communications protocol which preceded

Bisync Binary Synchronous Communication (BSC or Bisync) is an IBM character-oriented, half-duplex link protocol, announced in 1967 after the introduction of System/360. It replaced the synchronous transmit-receive (STR) protocol used with second gener ...

. STR was point-to-point only, and employed a ''four-of-eight'' transmission code, communicating at up to 5100 characters per second over half-duplex or full-duplex communication lines. The

IBM 1009 The IBM 1009 Data Transmission Unit was an IBM communications controller introduced in 1960. The 1009 used the Synchronous transmit-receive (STR) protocol to transfer data at 150 characters per second (cps) over a single point-to-point dial or le ...

Data Transmission Unit, the

IBM 1130 The IBM 1130 Computing System, introduced in 1965, was IBM's least expensive computer at that time. A binary 16-bit machine, it was marketed to price-sensitive, computing-intensive technical markets, like education and engineering, succeeding th ...

Synchronous Communications Adapter, the

IBM System/360 The IBM System/360 (S/360) is a family of mainframe computer systems that was announced by IBM on April 7, 1964, and delivered between 1965 and 1978. It was the first family of computers designed to cover both commercial and scientific applica ...

model 20 Communications Adapter, and the

IBM 2701 270x is a generic name for a family of IBM non-programmable communications controllers used with System/360 and System/370 computers. The family consisted of the following devices: * IBM 2701 Data Adapter Unit * IBM 2702 Transmission Control * I ...

Data Transmission Unit provided host computer support for STR. The

IBM 7701 The IBM 7701 Magnetic Tape Transmission Terminal was a communications device announced by IBM in 1960. It was designed to transfer the contents of a reel of magnetic tape over a leased or dial telephone circuit. The IBM 7702 was a similar device ...

, the

IBM 7702 The IBM 7701 Magnetic Tape Transmission Terminal was a communications device announced by IBM in 1960. It was designed to transfer the contents of a reel of magnetic tape over a leased or dial telephone circuit. The IBM 7702 was a similar device ...

Magnetic Tape Transmission Terminal, the

IBM 1013 The IBM 1013 Card Transmission Terminal was a device manufactured by IBM from 1961 which transmitted the data held on 80-column cards to a remote computer or another 1013. The speed was generally considered 100 cards per minute but could be f ...

Card Transmission Terminal, and the IBM 7710 and IBM 7711 Data Communication Units were among the remote devices supported by STR. STR was still supported as of 1972, although it had generally faded from use.

Four-of-eight code

The four-of-eight code was a 64 character set plus six control characters, encoded in eight bits. Like other kinds of

6b/8b encoding In telecommunications, 6b/8b is a line code that expands 6- bit codes to 8-bit symbols for the purposes of maintaining DC-balance in a communications system. The 6b/8b encoding is a balanced code -- each 8-bit output symbol contains 4 zero bits a ...

, exactly four bits of each character were one bits — there are 70 such possible bit combinations.

Data codes

This is the four-of-eight code. The first column of graphics is for the 7701, where no graphic is shown the code is unassigned. Plus zero and minus zero had distinct values for many machines, and are shown as +0 and -0. The second column of graphics is for the 1130 SCA, graphics are shown only if different from the 7701, mostly characters from the

EBCDIC Extended Binary Coded Decimal Interchange Code (EBCDIC; ) is an eight-bit character encoding used mainly on IBM mainframe and IBM midrange computer operating systems. It descended from the code used with punched cards and the corresponding six- ...

character set which were not present in BCD.

Control codes

These are the synchronous transmit-receive control codes. Control characters were reserved; there was no provision for transparent operation.

Longitudinal Redundancy Check

The ''Longitudinal Redundancy Check'' byte was the only character that could have other than four one bits. This byte contains a zero in each bit position where the data record contained an even number of one bits, and one where an odd number. "Idle characters and control sequences are not included in the LRS.

Line control

When no data is being transmitted the line is maintained in ''synchronize mode'' — a terminal transmits idle characters for approximately 1.3 seconds followed by ''end-of-idle'' sequence (CL Idle). It then listens for the same sequence for approximately three seconds. For four wire operation, when data is being transmitted the other wire pair is maintained in synchronize mode. At least one idle character is sent before each transmission. Ownership of the line is established by the transmitting station sending an ''inquiry sequence'' (TL INQ). The receiving terminal then responds with ''acknowledge'' (CL ACK2). Following this initiation sequence the transmitting terminal sends SOR1 TL followed by the first block of data. The data is terminated by an ''end-of-transmittal-record'' (TL LRS). The receiving terminal replies with CL ACK1, or CL ERR if the longitudinal redundancy check indicates a transmission error. This process continues with even/odd SOR and ACK, until the sending terminal has transmitted all the data. It sends the ''end-of-transmission'' sequence (CL EOT). The receiving terminal responds CL EOT and re-enters synchronize mode.

References

{{Reflist Link protocols IBM computer hardware