Keithley 486 Instruction Manual page 224

APPENDME
IEEE-488 Bus Ovm'm
Handshake Lines
The bus handshake lines that operate in an interlocked
sequence. This method ensures relizbk d&a kmnsmis-
sion regardless of the transfer rate. Generally, data ham+
fer will occur at a rate determined by the slowest active
device on the bus.
One of the three handshake lines is controlled by the
source (the talker sending information), while the re
maining two lines are controlled by accepting devices
(the listener or listeners receiving the information). The
three handshake lines are:
DAV (DATA VALID) -The source controls the state of
the DAV line to indicate to any listening devices whether
or not data bus information is valid.
NRFD (Not Ready For Data) -The acceptor controls the
state of NRFD. It is used to signal to the tmnsmitting de-
vice to hold off the byte transfer sequence until the ac-
cepting device is ready.
NDAC (Not Data Accepted) - NDAC is also controlled
by the accepting device. The state of NDAC tells the
source whether or not the device has accepted the data
byte.
The complete handshake sequence for one data byte is
shown in Figure E-2. Once data is placed on the data
lines, the solnTe checks to see that NRFD is high, indicat-
ing that all active devices are ready. At the same time,
NDAC should be low from the previous byte transfer. If
these conditions are not met, the source must wait until
NDAC and NRFD have the correct status. If the source is
a contdler, NRFD and NDAC must be stable for at least
IOOnsecafter ATN is set true. Because of the possibility of
a bus hang up many controllers have time-out routines
that display messages in case the transfer sequence stops
for any reason.
Once all NDAC and NRFD are properly set, the source
sets DAV low, indicating to accepting devices that the
byte on the data lines is now valid. NRFD will then go
low, and NDAC will go high once all devices have ac-
cepted the data. Each device will release NDAC at its
own rate, but NDAC will not be released to go high until
all devices have accepted the data byte.
The sequence just desaibed is used to transfer both data,
talk and listen addresses, as well as multiline commands.
The state of the ATN line determines whether the data
bus contains data, addresses or commands as described
DATA SOURCE
iiiiv SOURCE
DATA
"AUD
NRFD
ACCEPTOR
NDAC
ACCEPTOR
DAYA
DAfA
TRANSFER TRANSFER
BEGINS ENDS
Figure E-2.
IEEE488 Handshake Sequence
The inshnnent may be given a number of special bus
commands through the IEEE-488 interface. This section
briefly describes the purpose of the bus commands
wllich are grouped into the following three categories.
1. Uniline Commands - Sent by setting the associated
bus lines true. For example, to assert REN (Remote
Enable), the REN line would be set low (true).
2. MuliilineCo mmands -General bus commands
which are sent over the data lines with the ATN
line true (low).
3. Device-dependent Commands-Special commands
whose meanings depend on device configurations;
sent with ATN high (false).
These bus commands and their general purpose are sum-
marized in Table E-l.
Uniline Commands
ATN, IFC and REN are asserted only by the controller.
SRQ is asserted by an external device. EOI may be as-
serted either by the contioller or other devices depending
on the direction of data transfer. The following is a de-
E-3