Bus and Tag
| Company type | Computer peripheral interface standard |
|---|---|
| Industry | Computer hardware |
| Founded | 1964 |
| Founder | IBM |
| Products | Peripheral interface cables and channel technology |
Bus and Tag represents an IBM-established standard for a computer peripheral interface, primarily utilized to link mainframe systems with peripherals such as line printers, disk storage, magnetic tape drives, and IBM 3270 display controllers. This technology employs two distinct sets of robust, multi-connector copper cables: one set, known as the bus, transmits data, while the other, termed the tag, conveys control signals.
The Bus and Tag cables are arranged in a daisy chain configuration, permitting a single interface to connect up to eight peripheral control units. A terminator plug is necessary on the final control unit in the chain. Each control unit generally manages up to sixteen devices. Architectural constraints include a maximum of 256 devices per channel and an initial cable length limit of 200 feet between the mainframe and control unit, later extended to 400 feet. Data transfer speeds can reach up to 4.5 MB per second; however, only one device can transmit data at any given moment.
This design was also embraced by other manufacturers for interfacing IBM peripherals and was formally documented by the US National Technical Information Service as FIPS PUB 60-2, titled I/O Channel Interface.
Introduced alongside the System/360 in 1964 and utilized with System/370, Bus and Tag channels were later rebranded as "parallel channels" following the introduction of serial fiber optic ESCON technology during the 1990s. Since then, these parallel channels have been gradually discontinued. By 2018, new mainframes no longer supported parallel channels, and older systems have been progressively transitioning away from them. Equipment is available that enables legacy Bus and Tag peripherals to connect to contemporary mainframe channels such as FICON or ESCON.
Evolution
The initial System/360 architecture incorporated two channel types: byte multiplexor channels and selector channels. IBM has since expanded on this channel architecture.
In 1970, IBM launched the 2880 block multiplexor channel for the 360/85 and 360/195 systems, designed to support devices like the IBM 2305 fixed-head disk. This channel introduced disconnected command chaining, allowing high-speed peripherals to temporarily release the channel during operations without terminating the channel program. Additionally, it featured an optional two-byte interface—referred to as a bus extension—to achieve data rates of up to 3.0 MB/s. That same year, IBM announced the System/370, which incorporated block multiplexor channels.
As DASD device speeds increased beyond what original channel protocols could handle efficiently—and as implementing two-byte interfaces became financially impractical—IBM developed data streaming technology. This advancement initially supported transfer rates of 3.0 MB/s and was later enhanced to reach speeds up to 4.5 MB/s.
Example
The diagram below depicts a sophisticated system configuration featuring two CPUs interconnected with various peripheral devices through bus and tag cabling.
External links
- OEMI