HP 9000 Series 300 Tutorials Manual page 24

The timing diagram in Figure 1-2 shows how handshake lines are used to
complete a data item transfer. The discussion which follows is based on the
contents of Figure 1-2.
Figure 1-2. HP-IB Handshake Sequence
All handshake lines are electrically connected in a "wired-OR" configuration
which means that any device can pull the line low (active or asserted) at any
time, and more than one device may pull the line low simultaneously or later in
a given handshake cycle. The line then remains low until every device that was
previously pulling the line low has released the line, allowing it to float to its
high state. At the start of the handshake cycle (point A), the handshake lines
are in the following states:
• DAV is false (high), meaning that the current talker has not yet placed
valid data on the bus.
• NRFD is true (low), meaning that one or more listeners is not yet ready
to accept data from the bus.
• NDAC is true (low), meaning that bus data has not yet been accepted
by every listener on the bus.
1-12
Interfacing Concepts