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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.