Editing IBM System/360 (section)

===={{anchor|ByteMuxChan|SelectChan}}Byte-multiplexor and selector channels====<!-- anchor added for intra-article linking -->
There were initially two types of channels; byte-multiplexer channels (known at the time simply as "multiplexor channels"), for connecting "slow speed" devices such as card readers and punches, [[line printer]]s, and communications controllers, and selector channels for connecting high speed devices, such as [[disk drive]]s, [[tape drive]]s, data cells and [[drum memory|drums]]. Every System/360 (except for the Model 20, which was not a standard 360) has a byte-multiplexer channel and 1 or more selector channels, though the model 25 has just one channel, which can be either a byte-multiplexor or selector channel. The smaller models (up to the model 50) have integrated channels, while for the larger models (model 65 and above) the channels are large separate units in separate cabinets: the IBM 2870 is the byte-multiplexor channel with up to four selector sub-channels, and the IBM 2860 is up to three selector channels.

The byte-multiplexer channel is able to handle I/O to/from several devices simultaneously at the device's highest rated speeds, hence the name, as it [[multiplexing|multiplexed]] I/O from those devices onto a single data path to main memory. Devices connected to a byte-multiplexer channel are configured to operate in 1-byte, 2-byte, 4-byte, or "burst" mode. The larger "blocks" of data are used to handle progressively faster devices. For example, a 2501 card reader operating at 600 cards per minute would be in 1-byte mode, while a 1403-N1 printer would be in burst mode. Also, the byte-multiplexer channels on larger models have an optional selector subchannel section that would accommodate tape drives. The byte-multiplexor's channel address was typically "0" and the selector subchannel addresses were from "C0" to "FF." Thus, tape drives on System/360 were commonly addressed at 0C0–0C7. Other common byte-multiplexer addresses are: 00A: 2501 Card Reader, 00C/00D: 2540 Reader/Punch, 00E/00F: 1403-N1 Printers, 010–013: 3211 Printers, 020–0BF: [[270x|2701/2703]] Telecommunications Units. These addresses are still commonly used in z/VM virtual machines.

System/360 models 40 and 50 have an integrated 1052-7 console that is usually addressed as 01F, however, this was not connected to the byte-multiplexer channel, but rather, had a direct internal connection to the mainframe. The model 30 attached a different model of 1052 through a 1051 control unit. The models 60 through 75 also use the 1052–7.

[[File:IBM 360 Bus Tag Cables.png|thumb|300px|Cable used as Bus or Tag cable for IBM System/360]]
[[Image:IBM 360 Bus Tag Terms.JPG|thumb|300px|Bus and tag terminators]]

Selector channels enabled I/O to high speed devices. These storage devices were attached to a control unit and then to the channel. The control unit let clusters of devices be attached to the channels. On higher speed models, multiple selector channels, which could operate simultaneously or in parallel, improved overall performance.

Control units are connected to the channels with "bus and tag" cable pairs. The bus cables carried the address and data information and the tag cables identified what data was on the bus. The general configuration of a channel is to connect the devices in a chain, like this: Mainframe—Control Unit X—Control Unit Y—Control Unit Z. Each control unit is assigned a "capture range" of addresses that it services. For example, control unit X might capture addresses 40–4F, control unit Y: C0–DF, and control unit Z: 80–9F. Capture ranges had to be a multiple of 8, 16, 32, 64, or 128 devices and be aligned on appropriate boundaries. Each control unit in turn has one or more devices attached to it. For example, you could have control unit Y with 6 disks, that would be addressed as C0-C5.

There are three general types of bus-and-tag cables produced by IBM. The first is the standard gray bus-and-tag cable, followed by the blue bus-and-tag cable, and finally the tan bus-and-tag cable. Generally, newer cable revisions are capable of higher speeds or longer distances, and some peripherals specified minimum cable revisions both upstream and downstream.

The cable ordering of the control units on the channel is also significant. Each control unit is "strapped" as High or Low priority. When a device selection was sent out on a mainframe's channel, the selection was sent from X->Y->Z->Y->X. If the control unit was "high" then the selection was checked in the outbound direction, if "low" then the inbound direction. Thus, control unit X was either 1st or 5th, Y was either 2nd or 4th, and Z was 3rd in line. It is also possible to have multiple channels attached to a control unit from the same or multiple mainframes, thus providing a rich high-performance, multiple-access, and backup capability.

Typically the total cable length of a channel is limited to 200 feet, less being preferred. Each control unit accounts for about 10 "feet" of the 200-foot limit.