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8.3. Functions
A GPIB device can be a Listener, Talker and/or Controller. A Talker sends data
messages to one or more Listeners, which receive data. A Controller manages the
information flow on the GPIB by sending commands to all devices. The GPIB bus is
much like a computer bus except a computer has circuit cards connected via a backplane
and the GPIB has stand alone devices connected via a cable.
8.4. Signals and Lines
The GPIB consists of 16 signal lines and 8 ground-return or shield drain lines. The 16
signal lines are grouped into 8 data lines, 3 handshake lines and 5 interface management
lines.
Data Lines: The eight data lines, DI01 through DI08 carry data and command messages.
The 7-bit ASCII or ISO code set is used and the eighth bit DI08 is unused.
Handshake Lines: The transfer of message bytes between devices is done via three
asynchronously control lines. Referred to as three-wire interlocked handshake. This
guarantees that message bytes on the data lines are sent and received without
transmission error.
NRFD (not ready for data) indicates when a device is ready or not ready to receive a
message byte.
NDAC (not data accepted) indicates when a device has or has not accepted a message
byte.
DAV (data valid) tells when the signals on the data lines are stable (valid) and can be
accepted safely by devices.
Interface Management Lines: Five lines are used to manage the flow of information
across the interface.
ATN (attention) ATN is driven true by the controller when it uses the data lines to send
commands, and drivers ATN false when a Talker can send data messages.
IFC (interface clear) IFC is driven by the system controller to initialize the bus and
become CIC.
REN (remote enable) The REN line is driven by the controller which is used to place
devices in remote or local program mode.
SRQ (service request) The SRQ line can be driven by any device to asynchronously
request service from the Controller.
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