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line returns to its low state. Finally, the NRFD line
is released by each of the devices at their own
rates, until the NRFD line finally goes high when
the slowest device is ready, and the bus is set to
repeat the sequence with the next data byte.
The sequence just described is used to transfer
both data and multiline command. The state of the
ATN line determines whether the data bus contains
data or commands.
4-3-3. Data Lines
The IEEE-488.2 bus uses the eight data lines that
allow data to be transmitted and received in a
bit-parallel, byte-serial manner. These eight lines use
the convention DI01 through DI08 instead of the
more common D0 through D7 binary terminology.
The data lines are bi-directional and, as with the
remaining bus signal lines, low is true.
4-4. INTERFACE FUNCTION CODES
The interface function codes are part of the IEEE-
488.2 standards. These codes define an instrument’s
ability to support various interface functions and
should not be confused with programming commands
found elsewhere in this manual.
Table 4-1 lists the codes for the Model 8020.
The numeric value following each one or two letter
code define Model 8020 capability as follows:
SH
- (Source Handshake Function) - The ability
for the Model 8020 to initiate the transfer of mes-
sage/data on the data bus provided by the SH
function.
AH
- (Acceptor Handshake Function) - The ability
for the Model 8020 to guarantee proper reception
of message/data on the data bus provided by the
AH function.
T
- (Talker Function) - The ability of the Model
8020 to send device-dependent data over the bus
(to another device) is provided by the T function.
Model 8020 talker capabilities exist only after the
instrument has been addressed to talk.
L
- (Listen Function) - The ability of the Model
8020 to receive device-dependent data over the bus
(from anther device) is provided by the L function.
Listener function capability of the Model 8020 exist
only after it has been addressed to listen.
RS
- (Service Request Function) - The ability of
the Model 8020 to request service from the controller
is provided by the RS function.
RL
- (Remote-Local Function) - The ability of the
Model 8020 to be placed in remote or local modes
is provided by the RL function.
PP
- (parallel Poll Function) - The ability of the
Model 8020 to respond to a parallel poll request
from the controller is provided by the PP function.
DC
- (Device Clear Function) - The ability for
the Model 8020 to be cleared (initialized) is provided
by the DC function.
DT
- (Device Trigger Function) - The ability of
the Model 8020 to have its output triggered is
provided by the DT function.
C
- (controller Function) - The Model 8020 does
not have a controller function.
TE
- (Extended Talker Capabilities) - The Model
8020 does not have extended talker capabilities.
LE
- (Extended Listener Function) - The Model
8020 does not have extended listener function.
4-5. SOFTWARE CONSIDERATIONS
The most sophisticated computer in the world would
be useless without the necessary software. This
basic requirement is also true of the IEEE-488.2
bus, which requires the use of handler routines as
described in this paragraph. Before a controller can
be used with the IEEE-488.2 interface, the user must
make certain that appropriate handler software is
present within the controller. With the IBM PC com-
puter, for example, the GPIB interface card must be
used with an additional software which contains the
necessary handler software.
Table 4-1. Model 802020 Interface Function Codes
CODE
INTERFACE FUNCTION
SH1
Source Handshake Function
AH1
Acceptor Handshake Capabilities
T6
Talker (basic talker, serial
poll,
unaddressed to talk on LAG)
L4 Listener (basic listener,
unaddressed to listen on TAG)
SR1
Service request capability
RL1
Remote/Local capability
PP2
Parallel Poll capability
DC1
Device Clear capability
DT1
Device Trigger capability
C0
No controller capability
E1
Open collector bus drivers
TE0
No Extended Talker capabilities
LE0
No Extended Listener capabilities
IEEE-488.2 Operation
Model 8020
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