488 CONTROLLER
88
8.5 Handshake Lines
The GPIB uses three handshake lines in an “I’m ready—Here’s the data—I’ve got it” sequence. This hand-
shake 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.
8.5.1 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.
8.5.2 N
OT
R
EADY
F
OR
D
ATA
(NRFD)
This line is used by the Listeners to inform the Talker when they are ready to accept new data. The Talker
must wait for each Listener to unassert this line (high), which they will do at their own rate when they are
ready for more data. This assures that all devices that are to accept the information are ready to receive it.
8.5.3 N
OT
D
ATA
A
CCEPTED
(NDAC)
The NDAC line is also controlled by the Listeners. This line indicates to the Talker that each device
addressed to listen has accepted the information. Each device releases NDAC (high) at its own rate,
but the NDAC will not go high until the slowest Listener has accepted the data byte.
Figure 8-2. IEEE bus handshaking.
1st Data Byte
2nd Data Byte
Valid
Valid
Not
Valid
Not
Valid
DAV
Source
DIO-8
(composite)
NFRD
Acceptor
NDAC
Acceptor
All
Ready
None
Ready
All
Ready
None
Ready
None
Accept
All
Accept
None
Accept
All
Accept
Содержание 488 Controller
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Страница 95: ...93 CHAPTER 9 Theory of Operation Board Layout Figure 9 2 The interface converter s serial I O board...
Страница 102: ...488 CONTROLLER 100 How To Read the Chart Hexadecimal Decimal Equivalent Equivalent ASCII Character Address or Command...