HP 9000 Series 300 Tutorials Manual page 23

NDAC
Note
Not Data ACcepted: One or more listeners has not yet accepted
the data currently on the bus.
The HP-IB interface uses negative (ground-true) logic for
handshake, data, and bus management lines. This means that
when the voltage on a line is at a logic LOW level, the line is
asserted (true). When a logic HIGH voltage level is present on
the line, the line is not asserted (false).
In general, software documentation refers to handshake and
other lines by their name acronym such as DAV, NRFD,
NDAC, etc. When discussing these same signal lines in
hardware documents, it is customary to refer to ground-true
(low-true) logic lines by their name acronym with a bar across
the top such as DAV, NRFD, NDAC, etc. In this document,
both versions are used. The overbar is usually present when
discussing hardware operation, but usually absent when
software is being treated. In this tutorial, only the name is
significant. Signal names are synonymous, with or without
the over bar unless specifically noted otherwise; the overbar is
used for the convenience of those readers whose experience is
oriented more toward hardware than software.
Interfacing Concepts 1-11