National Instruments GPIB-PC - Manual (Page 20)

Operation of the GPIB Section One
GPIB-PC User Manual 1-4 ©National Instruments Corp.
NDAC (not data accepted)
NDAC indicates when a device has or has not accepted a message byte.
The line is driven by all devices when receiving commands and by
Listeners when receiving data messages.
DAV (data valid)
DAV tells when the signals on the data lines are stable (valid) and can
be accepted safely by devices. The Controller drives DAV lines when
sending commands and the Talker drives DAV lines when sending data
messages.
Interface Management Lines
Five lines are used to manage the flow of information across the
interface:
• ATN,
• IFC,
• REN,
• SRQ, and
• EOI.
ATN (attention)
The Controller drives ATN true when it uses the data lines to send
commands and false when it allows a Talker to send data messages.
IFC (interface clear)
The System Controller drives the IFC line to initialize the bus and
become CIC.
REN (remote enable)
The System Controller drives the REN line, which is used to place
devices in remote or local program mode.