Black Box IC095A, Руководство пользователя

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CHAPTER 5: IEEE 488 Primer
5.4.3 R
EMOTE
E
NABLE
(REN)
When the System Controller sends the REN command, bus devices will respond to
remote operation. Generally, the REN command should be issued before any bus
programming is attempted. Only the System Controller has control of the Remote
Enable line.
5.4.4 E
ND OR
I
DENTIFY
(EOI)
The EOI line is used to signal the last byte of a multibyte data transfer. The device
that is sending the data asserts EOI during the transfer of the last data byte. The
EOI signal is not always necessary, as the end of the data may be indicated by some
special character such as carriage return.
The Active Controller also uses EOI to perform a Parallel Poll by simultaneously
asserting EOI and ATN.
5.4.5 S
ERVICE
R
EQUEST
(SRQ)
When a device desires the immediate attention of the Active Controller it asserts
SRQ. It is then the Controller’s responsibility to determine which device requested
service. This is accomplished with a Serial Poll or a Parallel Poll.
5.5 Handshake Lines
The GPIB uses three handshake lines in an “I’m ready—Here’s the data—I’ve got
it” sequence. This handshake protocol assures reliable data transfer, at the rate
determined by the slowest Listener. One line is controlled by the Talker, while the
other two are shared by all Active Listeners. The handshake lines, like the other
IEEE 488 lines, are active low.
5.5.1 D
ATA
V
ALID
(DAV)
The DAV line is controlled by the Talker. The Talker verifies that NDAC is asserted
(active low), which indicates that all Listeners have accepted the previous data byte
transferred. The Talker then outputs data on the bus and waits until NRFD is
unasserted (high), which indicates that all Addressed Listeners are ready to accept
the information. When NRFD and NDAC are in the proper state, the Talker asserts
DAV (active low) to indicate that the data on the bus is valid.