HP 7470A Interfacing And Programming Manual page 193

a PC board. While the ATN line is true, all devices must listen to the
data lines. When the ATN line is false, only devices that have been
addressed will actively send or receive data. All others ignore the data
lines.
Several listeners can be active simultaneously but only one talker can
be active at a time. Whenever a talk address is put on the data lines
(while ATN is true), all other talkers will be automatically unaddressed.
Information is transmitted on the data lines under sequential control of
the three handshake lines (DAV, NRFD, and NDAC). N0 step in the
sequence can be initiated until the previous step is completed. Informa­
tion transfer can proceed as fast as devices can respond, but no faster
than allowed by the slowest device presently addressed as active. This
permits several devices to receive the same message byte concurrently.
The ATN line is one of the five bus management lines. When ATN is
true, addresses and universal commands are transmitted on only seven
of the data lines using the ASCII code. When ATN is false, any code of
eight bits or less understood by both talker and listener(s) may be used.
The IFC (interface clear) line places the interface system in a known
quiescent state.
The REN (remote enable) line is used with the Remote, Local, and
Clear Lockout/ Set Local messages to select either local or remote con­
trol of each device.
Any active device can set the SRQ (service request) line true via the
Require Service Message. This indicates to the controller that some
device on the bus wants attention, such as a counter that has just com­
pleted a time-interval measurement and wants to transmit the reading
to a printer.
The EOI (end or identify) line is used by a device to indicate the end of
a multiplebyte transfer sequence. When a controller sets both the ATN
and EOI lines true, each device capable of a parallel poll indicates its
current status on the DIO line assigned to it.
In the interest of cost-effectiveness, it is not necessary for every device
to be capable of responding to all the lines. Each can be designed to
respond only to those lines that are pertinent to its function on the bus.
The operation of the interface is generally controlled by one device
equipped to act as controller. The interface transmits a group of com­
mands to direct the other instruments on the bus in carrying out their
functions of talking and listening.
The controller has two ways of sending interface messages. Multi-line
messages, which cannot exist concurrently with other multi-line
AN HP-IB OVERVIEW A-5