87
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.
6.2.4 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
A maximum separation of 4 m between any two devices and an average separation of 2 m over
the entire bus. A maximum total cable length of 20 m.
No more than 15 device loads connected to each bus, with no less than two-thirds powered on.
For example 1 GPIB controller and a maximum of 14 GPIB instruments.
Note:
A bus extender, which is available from numerous manufacturers, is available to