Bus and Tag

{{Short description|IBM peripheral interface}}

File:IBM 360 Bus Tag Cables.png

Bus and Tag is an "IBM standard for a computer peripheral interface",{{cite web |last1=IBM Corporation |title=IBM Terminology "O" |url=https://www-01.ibm.com/software/globalization/terminology/o.html |website=IBM Globalization |accessdate=Sep 8, 2018}} and was commonly used to connect their mainframe computers to peripheral devices such as line printers, disk storage, magnetic tape drives and IBM 3270 display controllers. The technology{{cite book

| title = IBM System/360 and System/370 I/O Interface Channel to Control Unit Original Equipment Manufacturers' Information

| id = GA22-6974-4

| publisher = IBM

| edition = Fifth

| date = January 1978

| url = http://bitsavers.org/pdf/ibm/370/channel/GA22-6974-4_360_370_IO_Interface_Channel_to_Control_Unit_OEM_Information_Jan78.pdf

| mode = cs2

}}

uses two{{efn|Three for a channel with the Two-byte Interface feature.}} sets of thick, multi-connector copper cables, one{{efn|Two for a channel with the Interface feature.}} set, carrying data, called the bus, and the other set, carrying control information, called the tag.{{cite web |last1=PC Magazine |title=Definition of: bus and tag channel |url=https://www.pcmag.com/encyclopedia/term/39055/bus-and-tag-channel |website=Encyclopedia |accessdate=Sep 8, 2011}}

File:ChannelTerminator.JPG

Bus and Tag cables are "daisy chained"; and one interface can attach up to eight peripheral control units. The last control unit in the chain must have a terminator plug. Each control unit can attach a maximum number of devices, "sixteen is a typical number."{{cite web |last1=IBM Corporation |title=Mainframe hardware: Evolving design |url=https://www.ibm.com/support/knowledgecenter/zosbasics/com.ibm.zos.zmainframe/zconc_mfhwdesign.htm |website=IBM® IBM Knowledge Center |accessdate=Sep 8, 2018}} There is an architectural limit of 256 devices per channel, and initially a limitation of {{convert|200|ft|m}}, later extended to {{convert|400|ft|m}}, between the mainframe and the control unit.{{cite web |last1=Dickens |first1=Steven |title=Evolution of z Systems Channel – A Parallel Universe |url=https://mainframedebate.com/2017/03/17/evolution-of-z-systems-channel-a-parallel-unniverse/ |website=MAINFRAME DEBATE |accessdate=Sep 8, 2018}} Bus and Tag channels handle data rates up to 4.5 MB per second.{{cite web |last1=PC Magazine |title=Definition of: parallel channel |url=https://www.pcmag.com/encyclopedia/term/48818/parallel-channel |website=Encyclopedia |accessdate=Sep 8, 2018}} Only one device can transfer data at a time.

Bus and Tag architecture was also used by other computer manufacturers to attach IBM peripherals to their systems. It was later published by the US National Technical Information Service (NTIS) as FIPS PUB 60-2, I/O Channel Interface.{{cite report

| title = I/O Channel Interface

| id = FIPS PUB 60-2

| date = July 29, 1983

| url = https://nvlpubs.nist.gov/nistpubs/Legacy/FIPS/fipspub60-2.pdf

| publisher = National Technical Information Service

| access-date = May 18, 2023

}}

Bus and Tag was introduced with System/360 in 1964, and was also used with System/370. With the introduction of serial, fiber optic ESCON in the 1990s Bus and Tag channels were re-christened "parallel channels", and were gradually superseded. "Parallel channels are not available on the newest mainframes and are slowly being displaced on older systems." Equipment is available to allow connection of older devices using Bus and Tag to mainframe FICON{{cite book |last1=IBM International Technical Support Organization |title=IBM Z Connectivity Handbook |date=April 2018 |isbn=978-0738443089 |page=193 |url=https://www.redbooks.ibm.com/redbooks/pdfs/sg245444.pdf |accessdate=Sep 8, 2018}} or ESCON channels.