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SECTION
4
GPIB Interface
NRJ'D = High level: All listeners are ready for accepting
data.
DAV = Low level: A talker has valid data on the data bus.
NDAC = High level: All listeners have completed data
W2C?pti0*.
The handshaking
timing
diagram is in Figure 4-2. The
various timing points indicate the following:
When all listeners
have completed
receiving
data, NDAC goes high, indicating
to the talker
that data reception has been completed.
@
The talker sets DAV high, indicating
to the lis-
tener that data on the bus is no longer valid.
0
The listener checks to see whether DAV is high
and sets NDAC low, completing the handshake.
Indicates that all listeners have completed data
input, and the bus is ready to transfer the next
data byte.
Valid Data
Figure 4-2.
Handshake Timing Diagram
0
Indicates that all listeners are waiting for data.
The talker places the data byte to be sent on the
data lines. Output may have already occurred.
0
The talker checks NRFD, and, if high, DAV is set
low to indicate to the listener that data is valid.
When DAV goes low, the listener reads data, and
NRFD is set low, indicating
to the talker that data
processing
is in progr&s.
Each listener
sets
NDAC high at the completion
of data input. The
NDAC
logic level is the result of ORing the
NDAC signals from each listener.
4.1.5
Data Transfer
Example
Figure4-3
shows a a data transfer example using the
three-line handshake process. In this example, the char-
acter string "ABC" is sent, followed
by the <CR> <LF>
delimiter.
4.1.6
Basic Talker Functions
l
Only one talker may exist on the GPIB at a time.
l
Data is sent to listeners when the controller ATN line is
high (false).
l
Source handshaking
is performed automatically.
l
A service request (SRQ) is sent to the controller
by
other devices.
. The talker function is available with both the local and
remote modes.
l
The talker function is canceled by any of the following:
When the talk address of another device is received.
When the device is addressed as a listener.
When the untalk (UNT) command is received.
When the interface clear (IFC) command is received.
4.1.7
Basic Listener
Functions
l
Two or more listeners may exist on the GI'IB at any
time.
l
Data is received from a talker when the controller
ATN signal is high.
l
The acceptor handshake is performed automatically.
l
The listener function
is canceled by any of the follow-
ing.
When the device is addressed to talk
When the unlisten (UNL) command is received
When the IFC commandis
received
4-3
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