Information Technology Reference
In-Depth Information
though many new types are configured using configuration menus. The addresses range from
0 to 31 (although address 31 is often used by some manufacturers for self-diagnostics). Nor-
mally only 15 address can be used, unless a bus extender is used. The assigned address is the
primary address of the instrument. If the instrument is a host for other instruments, each of
the connected instruments can be assigned a unique secondary address.
12 11 10
9
8
7
6
5
4
3
2
1
24 23 22 21 20 19 18 17 16 15 14 13
Figure 22.2
GPIB connector
Handshaking lines
The handshaking signal lines are:
•
DAV
(data valid) - a talker sets this line to a LOW, after it detects a HIGH on the
NRFD
line, and when the data on its I/O are settled and valid.
•
NRFD
(not ready for data) - a listener sets this line LOW when it is not ready for data. A
HIGH
level thus indicates that it is ready for data. This line will be held LOW. The
NRFD
line only goes HIGH when all the addressed listeners are ready to accept data.
•
NDAC
(not data accepted) - the listener sets this line LOW when it has not accepted the
data. When it accepts the data, it releases the
NDAC
line. The
NDAC
line does not go
HIGH until all the listeners have accepted the data.
Every byte transferred must be handshaked, and more than one device can receive data at a
time. The three handshaking signals are active-LOW and can be pulled LOW by any device.
Figure 22.3 shows a basic handshaking operation. Ini
tially,
with the first byte, the talker puts
the data byte of the data bus and then waits for the
NRFD
line to be set HIGH. It then sets
the
DAV
line active LOW. The devices then read the data and set their
NDAC
lines HIGH,
but as long as one device as not read t
he data
it will be set to an active LOW. Once the slow-
est device has read the data it sets the
NDAC
line HIGH. The talker then knows that the data
has been read and can then put more data on the bus.
