RS232 & GPIB INTERFACE
43
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.
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.
12.4
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.
Note:
A bus extender which is available from numerous manufacturers is
available to overcome these limitations.
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