Manual Keithley 2000 (page 242 of 256) (English)

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The bus handshake lines operate in an interlocked sequence. This method ensures reli-

able 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 informa-

tion), 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 lis-

tening 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 sig-

nal 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 E-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 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 accept-

ing 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 following paragraph.

DATA

DAV

SOURCE

VALID

NRFD

NDAC

ACCEPTOR

ALL READY

ALL ACCEPTED

Figure E-2

IEEE-488 hand-

shake sequence

IEEE-488 Bus Overview E-7

2000-900 (J - Aug 2010) BOOK.fm Page 7 Wednesday, October 12, 2011 12:29 PM

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