Associated Research HYPOTULTRA III 7620, Operation And Service Manual, Page: 84 / 148

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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
asynchronous 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 that 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.
EOI (end or identify) This line has two purposes- the Talker uses this line to mark the end of a
message string, and the Controller uses it to tell devices to identify their response in a parallel
poll.
8.5. GPIB Connector
Connection is usually accomplished with a 24-conductor cable with a plug on one end and a
connector at the other end. Devices may be connected in a linear, star or a combination
configuration.
The standard connector is the Amphenol or Cinch Series 57 Microribbon or AMP CHAMP
type. The GPIB uses negative logic with standard transistor-transistor logic (TTL) levels.
When DAV is true, for example, it is a TTL low level (
≤
0/8 V), and when DAV is false, it is a
TTL high level (
≥
2.0 V).
Restrictions and Limitations on the GPIB