Bus Line Signals And Operations; Gpib Handshaking - Keithley 3321 Operator's Manual

SECTION 4
GPIB Interface
DlOl
D102
D103
D104
EOI
DAV
NRFD
NDAC
IFC
SRQ
ATN
SHIELD
Fiaure 4-l.
IEEE-488
Interface
Connector
Receptacle Side
D105
D106
D107
D108
REN
DAV GROUND
NRFD GROUND
NDAC GROUND
IFC GROUND
SRQ GROUND
ATN GROUND
LOGIC GROUND
4.1.2 Bus Line Signals and Operations
The GPIB bus line consists of 24 lines, including 8 data
lines, 8 control lines and 8 signal/system ground lines.
Data Bus (DIOl to DI08) - These are the data input/
output lines which are also used to input and output both
address and command information. The type of data pre-
sent on these lines are identified by means of the ATN
line. DIOl is the least significant bit (LSB).
Handshake Bus (DAV, NRFD, NDAC) - These three
lines are handshake lines used to ensure reliable data
transfer.
l
DAV (DAta Valid) -
This line indicates that the data
on the DIO lines sent from a talker or the controller is
valid.
l
NRFD (Not Ready For Data) - This line indicates the
condition of readiness of listeners to accept data on the
DIO lines.
l
NDAC (Not Data Accepted) -This line indicates the
condition of acceptance of data by listeners.
Control Bus (ATN, REN, IFC, SRQ, EOI):
l
ATN (ATteNtion) - This line is an output line from
the controller which indicates whether the signals on
the DIO bus are data signals or commands.
l
REN (Remote ENable) - This output line from the
controller switches devices between remote control
and local control.
l
IFC (InterFace Clear) -The output line from the con-
troller clears the interface of devices.
l
SRQ (Service ReQuest) - This control line is used to
call the controller from a talker or a listener. The con-
troller detects this signal and executes a serial or paral-
lel poll operation.
l
EOI (End Or Identify) - This is used to indicate the
end of a multiple byte transfer sequence or, in conjunc-
tion with ATN, to execute a parallel poll.
4.1.3 GPIB Handshaking
GPIB handshaking is performed by checking the status of
all the listeners and inhibiting the next data transfer until
all listeners have received the data. Handshaking allows
the slowest device on the bus to perform data transfer re-
liably. The handshaking operations are executed by the
following status signals:
NRFD = High level. All listeners are ready for accepting
data.
DAV = Low level. A talker is outputting valid data to the
data bus.
NDAC = High level. All listeners have completed data re-
ception.
The handshaking timing diagram is shown in Figure 4-2.
4-2
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