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Reference and Sources:



1Data I/O 1 
2Data I/O 2 
3Data I/O 3 
4Data I/O 4 
5EOIEnd Or Indentify has two uses. EOI is asserted on the last byte of a data transfer. This signals all devices that no more data should be expected on the transfer.
6DAVData Valid is a handshake line indicating that the active talker has placed data on the data lines.
7NRFDNot Ready For Data is a handshake line indicating that one or more active listeners is not ready for more data. Note the active talker should then wait before sending any more data on the bus.
8NDACNot Data Accepted is a handshake line indocating that one or more active listeners has not accepted the current data byte. Note the active talker should leave the current byte asserted on the data lines until it has been accepted ot timed out.
9IFCInterface Clear is under the exclusive control of the system controller. When it is active high all devices on the bus are returned to an idle state and the bus is cleared
10SRQService Request can be set by a devive on the interface to indicate it is in need of service. SRQ could be set at the completion of a task. E.g. finished doing a measurement, or when an error as occurred.
11ATNAttention is used primarily to differentiate between command mode and data mode. When ATN is TRUE (I.E. Active high) information on the bus is a command and when ATN is FALSE (Active LOW) the information on the bus is data.
13Data I/O 5 
14Data I/O 6 
15Data I/O 7 
16Data I/O 8 
17RENRemote Enable may be set by the system controller to allow other devices to operate in remote mode.
18P/O Twisted pair with 6 
19P/O Twisted pair with 7 
20P/O Twisted pair with 8 
21P/O Twisted pair with 9 
22P/O Twisted pair with 10 
23P/O Twisted pair with 11 

The IEEE 488 ia a common parallel interface standard that was originally developed by the Hewlett-Packard Corporation in 1974. Hewlett-Packard called their interface the HPIB (Hewlett-Packard Interface Bus). This interface became so popular that it was adopted by the IEEE (Institute of Electrical and Electronics Engineers) in 1975 and designated the IEEE-488. In common practice, it is referred to as the GPIB interface (General Purpose Interface Bus). In other words, IEEE-488, GPIB, and HPIB all refer to the same parallel interface standard.

The primary purpose of the GPIB is to interface laboratory instruments with each other and with computers. In order for a computer to be connected to an instrument using the GPIB interface, both devices must have a GPIB interface card installed. In addition, the computer must also be running a GPIB controller program to control the flow of data to and from the various devices connected to the GPIB.

The GPIB standard will allow up to 15 devices on the same cable (daisy-chained).

Like most parallel interfaces, the GPIB is limited to rather short distances, each instrument in the chain must be no more than 2 meters apart and the total length of cable cannot exceed 20 meters.

A GPIB interface is capable of transferring data at rates up to 1 MB/second.

GPIB defines three types of devices:

  1. Controller: typically the microcomputer in the system, manages the GPIB bus. It determines which devices will be talkers and listeners and connects (or disconnects) them accordingly.
  2. Talker: is a device that is capable of sending data or instructions to other devices. A voltmeter would be a talker since it can send voltage readings to another device.
  3. Listener: is a device that can receive data or instructions. Printers and plotters are examples of listeners.
Some devices, such as voltmeters and computers, can be both talkers and listeners. They can both send and receive data or instructions.

Each instrument on the GPIB bus is assigned its own device number. The control software addresses each device by number and assigns it either talker or listener status.

There are 24 lines in the GPIB interface bus and cable. Eight of these lines carry data, five lines are for device control, three are handshaking lines, and eight are signal grounds.

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