GPIB

IEEE-488 stacking connectors

IEEE-488 is a short-range digital communications bus specification. It was created for use with automated test equipment in the late 1960s, and is still in use for that purpose. IEEE-488 was created as HP-IB (Hewlett-Packard Interface Bus), and is commonly called GPIB (General Purpose Interface Bus). It has been the subject of several standards.

Characteristics

IEEE-488 is an 8-bit, electrically parallel bus. The bus employs sixteen signal lines — eight used for bi-directional data transfer, three for handshake, and five for bus management — plus eight ground return lines.

Every device on the bus has a unique 5-bit primary address, in the range from 0 to 30 (31 total possible addresses).

The standard allows up to 15 devices to share a single physical bus of up to 20 meters total cable length. The physical topology can be linear or star (forked). Active extenders allow longer buses, with up to 31 devices theoretically possible on a logical bus.

Control and data transfer functions are logically separate; 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.

In the original protocol, transfers use an interlocked, three-wire ready–valid–accepted handshake. The maximum data rate is about one Mbyte/s. 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.

Connectors

IEEE-488

IEEE-488

Pinout
Female IEEE-488 connector
Pin 1	DIO1	Data input/output bit.
Pin 2	DIO2	Data input/output bit.
Pin 3	DIO3	Data input/output bit.
Pin 4	DIO4	Data input/output bit.
Pin 5	EOI	End-or-identify.
Pin 6	DAV	Data valid.
Pin 7	NRFD	Not ready for data.
Pin 8	NDAC	Not data accepted.
Pin 9	IFC	Interface clear.
Pin 10	SRQ	Service request.
Pin 11	ATN	Attention.
Pin 12	SHIELD
Pin 13	DIO5	Data input/output bit.
Pin 14	DIO6	Data input/output bit.
Pin 15	DIO7	Data input/output bit.
Pin 16	DIO8	Data input/output bit.
Pin 17	REN	Remote enable.
Pin 18	GND	(wire twisted with DAV)
Pin 19	GND	(wire twisted with NRFD)
Pin 20	GND	(wire twisted with NDAC)
Pin 21	GND	(wire twisted with IFC)
Pin 22	GND	(wire twisted with SRQ)
Pin 23	GND	(wire twisted with ATN)
Pin 24	Logic ground

IEEE-488 specifies a 24-pin Amphenol-designed micro ribbon connector. Micro ribbon connectors have a D-shaped metal shell, but are larger than D-subminiature connectors. They are sometimes incorrectly called "Centronics connectors", because Centronics used a similar but unrelated 36-pin connector for their printers.

One unusual feature of IEEE-488 connectors is they commonly use a "double-headed" design, with male on one side, and female on the other (at both ends of the cable). This allows stacking connectors for easy daisy-chaining. Mechanical considerations limit the number of stacked connectors to four or fewer, although a possible workaround involving physically supporting the connectors can expand this.

They are held in place by screws, either UTS (now largely obsolete) or metric M3.5×0.6 threads. By convention, metric screws are colored black, as the two threads do not mate.

IEC-60625

The IEC-60625 standard prescribes the use of 25-pin D-subminiature connectors (the same as used for the parallel port on IBM-PCs). This connector did not gain significant market acceptance against the established 24-pin connector.

Origins

In the late 1960s, Hewlett-Packard (HP) was manufacturing various automated test and measurement instruments, such as digital multimeters and logic analyzers. They developed the HP Interface Bus (HP-IB) to enable easier interconnection between instruments and controllers (computers and other instruments).

The bus was relatively easy to implement using the technology at the time, using a simple parallel electrical bus and several individual control lines. For example, the HP 59501 Power Supply Programmer and HP 59306A Relay Actuator were both relatively simple HP-IB peripherals implemented only in TTL logic, using no microprocessor.

Other manufacturers copied HP-IB, calling their implementation the General Purpose Interface Bus (GPIB), and it became a de facto standard for automated and industrial instrument control. As GPIB became popular, it was formalized by various standards organizations.

Standards

In 1975, the IEEE standardized the bus as Standard Digital Interface for Programmable Instrumentation, IEEE-488 (now IEEE-488.1). It formalized the mechanical, electrical, and basic protocol parameters of GPIB, but said nothing about the format of commands or data.

In 1987, IEEE introduced Standard Codes, Formats, Protocols, and Common Commands, IEEE-488.2, re-designating the previous specification as IEEE-488.1. IEEE-488.2 provided for basic syntax and format conventions, as well as device-independent commands, data structures, error protocols, and the like. IEEE-488.2 built on -488.1 without superseding it; equipment can conform to -488.1 without following -488.2.

While IEEE-488.1 defined the hardware, and IEEE-488.2 defined the protocol, there was still no standard for instrument-specific commands. Commands to control the same class of instrument (e.g., multimeters) would vary between manufacturers and even models.

The Air Force and later Hewlett-Packard recognized this problem. In 1989, HP developed their TML language which was the forerunner to SCPI Standard Commands for Programmable Instrumentation. SCPI was introduced as an industry standard in 1990. SCPI added standard generic commands, and a series of instrument classes with corresponding class-specific commands. SCPI mandated the IEEE-488.2 syntax, but allowed other (non-IEEE-488.1) physical transports.

The IEC developed their own standards in parallel with the IEEE, with IEC-60625-1 and IEC-60625-2, later replaced by IEC-60488.

National Instruments introduced a backwards-compatible extension to IEEE-488.1, originally known as HS-488. It increased the maximum data rate to 8 Mbyte/s, although the rate decreases as more devices are connected to the bus. This was incorporated into the standard in 2003 (IEEE-488.1-2003), over HP's objections .

In 2004, the IEEE and IEC combined their respective standards into a "Dual Logo" IEEE/IEC standard IEC-60488-1, Standard for Higher Performance Protocol for the Standard Digital Interface for Programmable Instrumentation - Part 1: General, replaces IEEE-488.1/IEC-60625-1, and IEC-60488-2, Part 2: Codes, Formats, Protocols and Common Commands, replaces IEEE-488.2/IEC-60625-2.

Use as a computer interface

HP's designers did not specifically plan for IEEE-488 to be a peripheral interface for general-purpose computers; the focus was on instrumentation. But when HP's early microcomputers needed an interface for peripherals (disk drives, tape drives, printers, plotters, etc.), HP-IB was readily available and easily adapted to the purpose.

HP computer products which used HP-IB included the HP 9800 series, the HP 2100 series, and the HP 3000 series. Some of HP's advanced pocket calculators of the 1980s, such as the HP-41 and HP-71B series, also had IEEE-488 capabilities, via an optional HP-IL/HP-IB interface module.

Other manufacturers adopted GPIB for their computers as well, such as with the Tektronix 405x line.

The Commodore PET (introduced 1977) range of personal computers connected their peripherals using the IEEE-488 bus, but with a non-standard card edge connector. Commodore's following 8-bit machines, including the VIC-20, C-64, and C-128, utilized an unrelated, proprietary serial interface, using a round DIN connector, for which they retained the IEEE-488 programming interface and terminology, however.

Eventually, faster, more complete standards such as SCSI superseded IEEE-488 for peripheral access.

Advantages and disadvantages

Advantages

• Simple hardware interface
• Ease of connecting multiple device to a single host
• Allows mixing of slow and fast devices
• Well-established and mature, widely supported

Disadvantages

• Mechanically bulky connectors and cables
• Limited speed and expansion
• Lack of command protocol standards (before SCPI)
• Implementation options (e.g. end of transmission handling) can complicate interoperability in pre-IEEE-488.2 devices
• No mandatory galvanic isolation between bus and devices
• High cost (compared to RS-232/USB/Firewire/Ethernet)
• Limited availability (again compared to RS-232/USB/Firewire/Ethernet)

See also

• Standard Commands for Programmable Instruments (SCPI)
• LAN eXtensions for Instrumentation (LXI)
• Virtual Instrument Software Architecture (VISA)
• HP series 80
• Rocky Mountain BASIC

External links

• IEC-60488-1: Higher performance protocol for the standard digital interface for programmable instrumentation. Vol. Part 1: General. International Electrotechnical Commission. 15 July 2004.
• IEC-60488-2: Standard digital interface for programmable instrumentation. Vol. Part 2: Codes, formats, protocols and common commands. International Electrotechnical Commission. 7 May 2004.