3.9
IEEE
Interface Option
The
IEEE
interface option available for the
DRC
instruments fully com-
plies with the
IEEE
standard 488-1978 and incorporates the functional, electrical
and mechanical specifications of the standard.
3.9.1
General
IEEE
Specifications and Operation
The following discussion covers the general operation of the IEEE-488
interface. For
a
more detailed description of signal level and interaction,
refer to the
IEEE
Std. 488-1978 publication
"IEEE
Standard Digital Interface
for Programmable Instrumentation".
All
instruments on the interface bus must be able to perform the inter-
face functions of
TALKER,
LISTENER, or
CONTROLLER.
A
TALKER
transmits data
onto the bus to other devices.
A
LISTENER
receives
data from other devices
through the bus.
nates to the devices on the bus which function to perform.
Some devices perform both functions. The
CONTROLLER
desig-
The
IEEE
works on
a
party line basis with
all
devices on the bus con-
nected in parallel.
All
the
active
circuitry of the bus
is
contained within
the individual devices with the cable connecting
all
the devices in parallel
to allow the transfer of data between
all
devices on the bus.
There
are
16 signal lines contained on the bus:
A)
8
Data
Lines
B)
C)
3 Byte Transfer Control Lines
5
General Interface Management Lines
The data lines consist of
8
signal lines that carry data in
a
bit
parallel, byte
serial
format. These lines carry universal commands, addresses,
program data, measurement data, and status to
all
the devices on the bus.
The
controller designates the functions of the units on the bus by setting the
ATN
line low
(true) and sending talk or listen addresses on the
DATA
lines.
When the
ATN
line
is
low,
all
devices listen to the
DATA
lines. When the
ATN
line goes high (false), then the devices addressed to send or receive data
perform their functions while
all
others ignore the
DATA
lines.
Transfer
of
the information on the data lines
is
accomplished through
the use of three signal lines:
DAV
(Data
Valid),
NRFD
(Not Ready For
Data),
and
NDAC
(Not
Data
Accepted).
handshake mode. The two signal lines, NRFD, and
NDAC
are
each connected in
a
logical
AND
to
all
devices connected to the bus.
The
DAV
is
sent by the
talker and received by listeners while the
NRFD
and
NDAC
are
sent by listen-
ers
back to the talker.
These signals operate in an interlocking
The General Interface Management Lines manage the
bus
and control
the orderly flow
of
commands on the bus. The IFC (Interface
Clear)
message
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