Keithley 2002 User Manual page 305

EOI (End or Identify) — The EOI is usually used to mark the
end of a multi-byte data transfer sequence.
SRQ (Service Request) — This line is used by devices when
they require service from the controller.
Handshake lines
The bus handshake lines operate in an interlocked sequence.
This method ensures reliable data transmission regardless of
the transfer rate. Generally, data transfer 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 remaining two
lines are controlled by accepting devices (the listener or lis-
teners 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 transmitting device
to hold off the byte transfer sequence until the accepting
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 D-2. Once data is placed on the data lines,
the source checks to see that NRFD is high, indicating 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 controller, NRFD and
NDAC must be stable for at least 100nsec after ATN is set
true. Because of the possibility of a bus hang up, many con-
trollers 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 accepted 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 described 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 con-
tains data, addresses or commands as described in the fol-
lowing paragraph.
DATA
DAV
VALID
NRFD
NDAC
Figure D-2
IEEE-488 handshake sequence
Bus commands
The instrument may be given a number of special bus com-
mands through the IEEE-488 interface. This section briefly
describes the purpose of the bus commands which 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. Multiline Commands — General bus commands which
are sent over the data lines with the ATN line true (low).
3. Common Commands — Commands that are common to
all devices on the bus; sent with ATN high (false).
4. SCPI Commands — Commands that are particular to
each device on the bus; sent with ATN (false).
These bus commands and their general purpose are summa-
rized in Table D-1.
IEEE-488 Bus Overview
SOURCE
SOURCE
ALL READY
ACCEPTOR
ALL ACCEPTED
ACCEPTOR
D-3