Appendix C: Introduction to the GPIB

The IEEE-488 Standard specifies the voltage levels, handshake requirements, timing, hardware details, pinout and connector dimensions for a 16 line, bit parallel bus. Many instruments may be connected in series to communicate over the same cable. Because the bits are passed in parallel, the GPIB is faster than the RS232.

The controller (generally your computer) coordinates data transfer on the bus by designating all participating instruments (including itself) as either a talker or a listener. Listeners can receive data placed on the bus by the Talker. Devices can have the capacity to operate in either mode. The address of each device is set by switches in the device and must be between 0 and 30.

Bus Description

Byte Transfer Control Group. This consists of 3 negative logic lines that implement the GPIB handshaking. The NRFD (Not Ready For Data) line is held low by any designated listener who is not ready to accept data. When every listener is ready, the line goes high and the talker may release data to the bus. After data is on the bus, the talker pulls the DAV (Data Valid) line down. At this point, each listener retrieves the data. Before and during the retrieval of the data, the listener holds the NDAC (No Data Accepted) line down. When every listener has received the data, the NDAC line goes high, allowing the talker to release the DAV line high. Finally, the listener pulls down the NDAC line until another transfer is initiated.

Data Bus: There are eight data lines which use negative logic and pass the bits of each byte in parallel.

General Interface Lines: These five lines operate independently of the handshake lines and use negative logic.

1)The EOI (End or Identify) line is used by the talker to designate the end of message.

2)The SRQ (Service Request) line is used by any device to ask for service. The controller can serial poll each device (each device returns an 8 bit status byte) to determine who needs attention. It can also do a parallel poll using the EOI and ATN lines where each device is assigned a single data line.

3)The ATN (Attention) line makes both talkers and listeners accept information and passes control of the DAV line to the controller. This line is used by the controller to identify talkers and listeners through their addresses.

4)The REN (Remote Enable) line changes the status of an instrument from local to remote.

5)The IFC (Interface Clear) line clears the bus of all data and activity.

Though GPIB is a very powerful interface, strict protocol must be observed for it to operate successfully.

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