National Instruments GPIB-PC - Manual (Page 21)

Section One Operation of the GPIB
©National Instruments Corp. 1-5 GPIB-PC User Manual
SRQ (service request)
Any device can drive the SRQ line to asynchronously request service
from the Controller.
EOI (end or identify)
The EOI line has two purposes. The Talker uses the EOI line to mark
the end of a message string. The Controller uses the EOI line to tell
devices to identify their response in a parallel poll.
Physical and Electrical Characteristics
Devices are usually connected with a cable assembly consisting of a
shielded 24-conductor cable with both a plug and receptacle connector
at each end. This design allows devices to be linked in either a linear
or a star configuration, or a combination of the two. See Figures 1.1,
1.2, and 1.3.
The standard connector is the Amphenol or Cinch Series 57
MICRORIBBON or AMP CHAMP type. An adapter cable using non-
standard cable and/or connector is used for special interconnect
applications.
The GPIB uses negative logic with standard TTL logic level. When
DAV is true, for example, it is a TTL low-level (
≤
0.8V), and when
DAV is false, it is a TTL high-level (
≥
2.0V).