IBM I/O channel - Revision history

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2024-04-09T15:20:17Z

‎Channel details: Go via redir; will be article some day

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‎Channel details: Link some terms

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A pretty good start

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'''IBM I/O channels''' (also called the '''Bus and Tag''' interface), starting with the [[IBM System/360]] (although its precursors stretch back to the [[IBM 709]]), were intended to connect all manner of [[peripheral]]s to the systems, from low speed devices like [[line printer]]s and [[punched card]] readers and punches, to high speed [[mass storage]] devices like [[disk]]s and [[magnetic tape drive]]s. The channel interface allowed a [[device controller]] to connect to a [[channel]] on an IBM [[mainframe]]; the channel could transfer data to the [[main memory]] of the machine with [[Direct Memory Access|DMA]], and [[interrupt]] the [[Central Processing Unit|CPU]].

==Channel details==

Channels were programmable, but were limited in functionality; they were finite state machines, with conditional branching capability. The individual operations are called [[channel command word]]s; they were stored in [[main memory]], and were organized into [[channel program]]s. Some channels provided multiple 'subchannels', which gives the illusion of a separate channel controller for each subchannel (each subchannel had its own channel program). (In reality, a single channel controller is shared between the subchannels.)

Channels came in '''selector''', '''byte multiplexer''' and '''block multiplexer''' forms; they supported different levels of concurrency among the devices attached to the channel. (The variants were apparently the same at the interface level; they differed in how they were handled in the channel controller.)

A selector channel handles high-speed devices such as disks and magnetic tape drives, and supports only a single subchannel, which operates a single device at a time; transfers pass an entire block in one operation. A byte multiplexer channel handles a group of slow-speed devices (on up to 255 subchannels), and interleaves operations to each of them, with each transfer handling single small data item (e.g. a byte). The later block multiplexer channel had aspects of both; it was shared among several subchannels, but each device monopolized the channel during a block transfers. It was particularly suitable for devices which interspersed bursts of high-speed transfers with periods of quiet; e.g. during rotational delays of disks.

On low-end systems, the channel was implemented by [[microcode]] in the CPU; on high-end systems, the channels were discrete physical processors.

==Interface details==

At the hardware level, the channel interface was asynchronous, and transfers between the channel and the device controller initially used a four-way handshake. That limited the speed, especially on physically large systems; it was later changed to use a two-way handshake. Initially, it was 8 bits wide; a later variant doubled that to 16 bits, to increase transfer speeds.

It is physically instantiated as a pair of large multi-[[conductor]] cables, one of which (called the 'bus') carried data (two of these in the 8-bit wide variant), and the other of which (called the 'tag' - hence the name) carried control [[signal]]s. The cables run from one controller to another; the last holds a [[terminator]]. The controllers were only logically separated from the device(s); they might be integrated into a single unit.

The interface specification allowed [[Original Equipment Manufacturer|OEMs]] to provide peripherals that could be connected to a System/360 machine. ([[IBM]] made the channel interface specification publicly available when the System/360 was announced, for this purpose.) The channels later appeared on [[IBM System/370]]s and compatible machines, as well as others; some manufacturers provided them on their machines as a way to connect IBM-compatible peripherals from OEMs to them.

The channel interface was standardized by [[ANSI]], and also became a [[FIPS]] standard.

==See also==

* [[DX10 Data Channel]]
* [[DX20 Programmed Device Adapter]]
* [[DX11-B System 360/370 Channel to PDP-11 Unibus Interface]]

==External links==

* [http://www.bitsavers.org/pdf/ibm/360/A22-6843-3_360channelOEM.pdf IBM System/360 I/O Interface Channel to Control Unit Original Equipment Manufacturers' Information] (A22-6843-3)
* [https://bitsavers.org/pdf/ibm/370/channel/GA22-6974-4_360_370_IO_Interface_Channel_to_Control_Unit_OEM_Information_Jan78.pdf IBM System/360 and System/370 I/O Interface Channel to Control Unit Original Equipment Manufacturers' Information] (GA22-6974-4)
* [https://nvlpubs.nist.gov/nistpubs/Legacy/FIPS/fipspub60-1.pdf I/O Channel Interface] (FIPS PUB 60-1; ANSI X3T9/600 Rev. 2)
* [https://nvlpubs.nist.gov/nistpubs/Legacy/FIPS/fipspub60-2.pdf I/O Channel Interface] (FIPS PUB 60-2)

[[Category: Interface Standards]]
[[Category: International Business Machines]]

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 ==Channel details==
-Channels were programmable, but were limited in functionality; they were finite [[state machine]]s, with [[conditional branch]]ing capability. The individual operations are called [[channel command word]]s; they were stored in [[main memory]], and were organized into [[channel program]]s.
+Channels were programmable, but were limited in functionality; they were [[finite state machine]]s, with [[conditional branch]]ing capability. The individual operations are called [[channel command word]]s; they were stored in [[main memory]], and were organized into [[channel program]]s.
 Some channels provided multiple 'subchannels', which gives the illusion of a separate channel controller for each subchannel (each subchannel had its own channel program). In reality, a single channel controller is shared between the subchannels.

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