Editing GPIB (section)

== Characteristics ==
IEEE 488 is an [[8-bit]], electrically [[parallel communication|parallel]] bus which employs sixteen signal lines — eight used for bi-directional data transfer, three for [[Handshake (computing)|handshake]], and five for bus management — plus eight ground return lines.

The bus supports 31 five-bit primary device addresses numbered from 0 to 30, allocating a unique address to each device on the bus.<ref name="NI-488.2">{{Cite book | title = NI-488.2 User Manual | date = February 2005 | publisher = National Instruments Corporation | id = NI P/N 370428C-01 | page = A-2 | chapter = GPIB Addressing | quote = The primary address is a number in the range 0 to 30. | chapter-url = http://www.ni.com/pdf/manuals/370428c.pdf | access-date = 2010-02-16 }}</ref><ref name="Agilent_82350B">{{Cite book|chapter-url=http://literature.cdn.keysight.com/litweb/pdf/82350-90004.pdf|title=Agilent 82350B PCI GPIB Interface: Installation and Configuration Guide|date=2009-07-20|publisher=Agilent Technologies|page=26|chapter=Table 1-1: 82350 GPIB interface card configuration parameters|id=Agilent P/N 82350-90004|quote=any address in the range 0 - 30, inclusive, may be used|access-date=2010-02-16}}</ref>

The standard allows up to 15 devices to share a single physical bus of up to {{convert|20|m}} total cable length. The physical topology can be linear or star (forked).<ref>{{Cite web | title = GPIB Instrument Control Tutorial | publisher = National Instruments | date = 2009-08-24 | quote = connected in either a daisy-chain or star topology | url = http://zone.ni.com/devzone/cda/tut/p/id/2761 | access-date = 2010-02-16 }}</ref> Active extenders allow longer buses, with up to 31 devices theoretically possible on a logical bus.

Control and data transfer functions are logically separated; a controller can address one device as a "talker" and one or more devices as "listeners" without having to participate in the data transfer. It is possible for multiple controllers to share the same bus, but only one can be the "Controller In Charge" at a time.<ref>{{Cite book | title = NI-488.2 User Manual | date = February 2005 | publisher = National Instruments Corporation | id = NI P/N 370428C-01 | page = A-1 | url = http://www.ni.com/pdf/manuals/370428c.pdf | access-date = 2010-02-16 | archive-url = https://web.archive.org/web/20081202204121/http://www.ni.com/pdf/manuals/370428c.pdf | archive-date = 2008-12-02 | url-status = dead }}</ref>

In the original protocol, transfers use an interlocked, three-wire ''ready–valid–accepted'' handshake.<ref>{{Cite book | title = NI-488.2 User Manual | date = February 2005 | publisher = National Instruments Corporation | id = NI P/N 370428C-01 | page = A-3 | chapter = Handshake Lines | chapter-url = http://www.ni.com/pdf/manuals/370428c.pdf | access-date = 2010-02-16 }}</ref> The maximum data rate is about one megabyte per second. The later HS-488 extension relaxes the handshake requirements, allowing up to 8 Mbyte/s. The slowest participating device determines the speed of the bus.<ref>{{Cite web | title = Using HS488 to Improve GPIB System Performance | publisher = National Instruments Corporation | date = 30 March 2009 | url = http://zone.ni.com/devzone/cda/tut/p/id/4552 | access-date = 2010-02-16 }}</ref>