Electronic interfaces/IEEE-488

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1 IEEE-488
2 References
3 References

IEEE-488

The IEEE-488 is a short-range, digital communications bus specification. The IEEE-488 bus was developed to connect and control programmable instruments, and to provide a standard interface for communication between instruments from different sources, and is still widely used for test and measurement equipment today. Hewlett-Packard originally developed the interfacing technique, and called it HP-IB. The interface quickly gained popularity in the computer industry. Because the interface was so versatile, the IEEE committee renamed it GPIB (General Purpose interface Bus). In 1975 the bus was standardized by the Institute of Electrical and Electronics Engineers as the IEEE Standard Digital Interface for Programmable Instrumentation, IEEE-488-1975 (now 488.1). IEEE-488.1 formalized the mechanical, electrical, and basic protocol parameters of GPIB, but said nothing about the format of commands or data. The IEEE-488.2 standard, Codes, Formats, Protocols, and Common Commands for IEEE-488.1 (June 1987), 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 syntax, 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. A standard for device commands, SCPI, was introduced in the 1990s. Due to the late introduction, it has not been universally implemented.

IEEE-488 allows up to 15 devices to share a single 8-bit parallel electrical bus by daisy chaining connections. The slowest device participates in control and data transfer handshakes to determine the speed of the transaction. The maximum data rate is about one Mbyte/s in the original standard, and about 8 Mbyte/s with later extensions.

The IEEE-488 connector has 24 pins. The bus employs 16 signal lines — eight bi-directional used for data transfer, three for handshake, and five for bus management — plus eight ground return lines.

Cables and connectors

			IEEE-488 connector configuration
Pin #	Signal Names	Signal Description	Pin #	Signal Names	Signal Description
1	DIO1	Data Input/Output Bit 1	13	DIO5	Data Input/Output Bit 5
2	DIO2	Data Input/Output Bit 2	14	DIO6	Data Input/Output Bit 6
3	DIO3	Data Input/Output Bit 3	15	DIO7	Data Input/Output Bit 7
4	DIO4	Data Input/Output Bit 4	16	DIO8	Data Input/Output Bit 8
5	EIO	End-Or-Identify	17	REN	Remote Enable
6	DAV	Data Valid	18	Shield Ground	(DAV)
7	NRFD	Not Ready For Data	19	Shield Ground	(NRFD)
8	NDAC	Not Data Accepted	20	Shield Ground	(NDAC)
9	IFC	Interface Clear	21	Shield Ground	(IFC)
10	SRQ	Service Request	22	Shield Ground	(SRQ)
11	ATN	Attention	23	Shield Ground	(ATN)
12	Shield	Chassis Ground	24	Single GND	Single Ground

The IEEE-488 standard allows up to 15 devices to be interconnected on one bus. Each device is assigned a unique primary address, ranging from 0-30, by setting the address switches on the device. A secondary address may also be specified,ranging from 0-30. See the device documentation for more information on how to set the device primary and optional secondary address. You can link devices in either a linear, star or combination configuration using a shielded 24-conductor cable. The standard IEEE-488 cable has both a plug and receptacle connector on both ends. This connector is the Amphenol CHAMP or Cinch Series 57 MICRO RIBBON type. Special adapters and non-standard cables are available for special interconnect applications. The IEEE-488 bus specifies a maximum total cable length of 20 meters with no more than 20 devices connected to the bus and at least two-thirds of the devices powered on. A maximum separation of 4 meters between devices and an average separation of 2 meters over the full bus should be followed. Bus extenders and expanders are available to overcome these system limits.

Data

There are 3 types of devices that can be connected to the IEEE-488 bus (Listeners, Talkers, and Controllers). Some devices include more than one of these functions. The standard allows a maximum of 15 devices to be connected on the same bus. A minimum system consists of one Controller and one Talker or Listener device (i.e., an HP 700 with an IEEE-488 interface and a voltmeter).
It is possible to have several Controllers on the bus but only one may be active at any given time. The Active Controller may pass control to another controller which in turn can pass it back or on to another controller. A Listener is a device that can receive data from the bus when instructed by the controller and a Talker transmits data on to the bus when instructed. The Controller can set up a talker and a group of listeners so that it is possible to send data between groups of devices as well.
The IEEE-488 interface system consists of 16 signal lines and 8 ground lines. The 16 signal lines are divided into 3 groups (8 data lines, 3 handshake lines, and 5 interface management lines).

Data Lines: The lines DIO1 through DIO8 are used to transfer addresses, control information and data. The formats for addresses and control bytes are defined by the IEEE 488 standard. Data formats are undefined and may be ASCII (with or without parity) or binary. DIO1 is the Least Significant Bit (note that this will correspond to bit 0 on most computers).
Handshake Lines: The three handshake lines (NRFD, NDAC, DAV) control the transfer of message bytes among the devices and form the method for acknowledging the transfer of data. This handshaking process guarantees that the bytes on the data lines are sent and received without any transmission errors and is one of the unique features of the IEEE-488 bus.
The NRFD (Not Ready for Data) handshake line is asserted by a Listener to indicate it is not yet ready for the next data or control byte. Note that the Controller will not see NRFD released (i.e., ready for data) until all devices have released it. The NDAC (Not Data Accepted) handshake line is asserted by a Listener to indicate it has not yet accepted the data or control byte on the data lines. Note that the Controller will not see NDAC released (i.e., data accepted) until all devices have released it. The DAV (Data Valid) handshake line is asserted by the Talker to indicate that a data or control byte has been placed on the data lines and has had the minimum specified stabilizing time. The byte can now be safely accepted by the devices.
Interface Management Lines
The five interface management lines (ATN, EOI, IFC, REN, SRQ) manage the flow of control and data bytes across the interface.
The ATN (Attention) signal is asserted by the Controller to indicate that it is placing an address or control byte on the data bus. ATN is released to allow the assigned Talker to place status or data on the data bus. The Controller regains control by reasserting ATN; this is normally done synchronously with the handshake to avoid confusion between control and data bytes. The EOI (End or Identify) signal has two uses. A Talker may assert EOI simultaneously with the last byte of data to indicate end-of-data. The Controller may assert EOI along with ATN to initiate a parallel poll. Although many devices do not use parallel poll, all devices should use EOI to end transfers (many currently available ones do not). The IFC (Interface Clear) signal is asserted only by the System Controller in order to initialize all device interfaces to a known state. After releasing IFC, the System Controller is the Active Controller. The REN (Remote Enable) signal is asserted only by the System Controller. Its assertion does not place devices into remote control mode; REN only enables a device to go into remote mode when addressed to listen. When in remote mode, a device should ignore its local front panel controls. The SRQ (Service Request) line is like an interrupt: it may be asserted by any device to request the Controller to take some action. The Controller must determine which device is asserting SRQ by conducting a serial poll. The requesting device releases SRQ when it is polled.

Laboratory use

The IEE-488 interface is still a standard feature of laboratory instruments built by Agilent (formerly Hewlett-Packard). Few, if any, other laboratory equipment providers have adopted the interface.