Synchronous transmit-receive
| Industry | Computer hardware, Communications |
|---|---|
| Founded | June 16, 1911 |
| Headquarters | Armonk, New York , United States |
| Products | Synchronous transmit-receive (STR), Bisync protocol, IBM 1009 Data Transmission Unit, IBM 1130 Synchronous Communications Adapter, IBM System/360 model 20 Communications Adapter, IBM 2701 Data Transmission Unit, IBM 7701 Magnetic Tape Transmission Terminal, IBM 7702 Magnetic Tape Transmission Terminal, IBM 1013 Card Transmission Terminal, IBM 7710 Data Communication Unit, IBM 7711 Data Communication Unit |
| Website | www |
Synchronous transmit-receive (STR) was an early character-oriented communications protocol developed by IBM that preceded the Bisync protocol. STR functioned on a point-to-point basis, employing a four-of-eight encoding scheme, and supported transmission rates up to 5100 characters per second over either half-duplex or full-duplex communication lines.
Support for STR on host systems was available through several IBM devices, including the IBM 1009 Data Transmission Unit, the IBM 1130 Synchronous Communications Adapter, the Communications Adapter for the IBM System/360 model 20, and the IBM 2701 Data Transmission Unit.
On the remote end, STR compatibility was provided by devices such as the IBM 7701 and IBM 7702 Magnetic Tape Transmission Terminals, the IBM 1013 Card Transmission Terminal, as well as the IBM 7710 and IBM 7711 Data Communication Units.
While STR was largely phased out over time, it continued to be supported as late as 1972.
Four-of-eight code
The four-of-eight code is a character encoding scheme comprising 64 distinct characters plus six control codes. Each character is represented by an eight-bit pattern containing exactly four bits set to one. This approach is a variant of 6b/8b encoding, enabling a total of 70 unique bit combinations.
Data codes
The following table illustrates the four-of-eight code utilized by hardware such as the IBM 7701 and IBM 1130 Synchronous Communications Adapter. Variations in displayed characters correspond to differences in supported character sets like EBCDIC and BCD.
| Graphic | 1130 Graphic | four-of-eight code |
Hex | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| N X O R 8 4 2 1 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| space | 1 1 1 1 0 0 0 0 | F0 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
| +0 | ¢ | 0 1 1 0 1 0 1 0 | 6A | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| . | 1 0 0 0 1 0 1 1 | 8B | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ⌑ | < | 0 1 1 0 1 1 0 0 | 6C | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ( | 0 1 0 1 0 1 1 0 | 56 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
| + | 0 0 1 1 0 1 1 0 | 36 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
| group mark | | | 1 0 0 0 1 1 0 1 | 8D | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| & & | }
(The table continues detailing character mappings specific to STR's four-of-eight encoding.) Control codesWithin STR, certain control characters were specifically assigned to synchronous transmit-receive control functions. The protocol did not support transparent operation for these control codes.
Longitudinal Redundancy CheckThe Longitudinal Redundancy Check (LRC) byte was unique in permitting more than four bits set to one. It holds zeros in bit positions where the corresponding data record contained an even count of ones, and ones where the count was odd. Idle characters and control sequences are not included in the LRC calculation. Line controlDuring idle periods, the line remains in synchronize mode: a terminal sends idle characters for approximately 1.3 seconds, followed by an end-of-idle pattern ( Control of the line is gained when the sender issues an inquiry sequence ( References |