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Channel In terf ace
7942 and 7946
HANDSHAKE OR DATA-BYTE
GENERAL INTERFACE
DATA BUS
TRANSFER CONTROL BUS
MANAGEMENT BUS
A
(8
SIGNAL~NES)
(3 SIGNAL~INES)
<....,..
I
V
~
---
I
..
..-
\
..
..-
'-./
..
--
..
..-
..
..-.
..
..-.
J
J
J
.......
II
fI
II
\/
~
>'
~
>'
DEViCE A
DEVICE 8
DEVICE C
ABLE TO
ABLE TO
ABLE TO
TALK.LlSTEN
TALK AND
LISTEN
AND CONTROL
LISTEN
ONLY
(5 SIGNA, LINES)
I
1
I
~
I
r
.~
\
J
~
' 1
II
DEVICE D
ABLE TO
TALK
ONLY
...
...
DI01-L THROUGH DI08-L -
DATA INPUT jOUTPUT LINES
DAV-L -DATA VALID
NRFD-L -NOT READY FOR DATA
NDAC-L -NOT DATA ACCEPTED
IFC-L -INTERFACE CLEAR
ATN-L -ATIENTION
SRQ-L -SERViCE REQUEST
REN-L -REMOTE ENABLE
EOI-L -END OR IDENTIFY
Figure 2- 2. Hewlett-Packard Interface Bus Signal Lines
The Hewlett-Packard Interface Bus (HP-IB) has
certain rules which must be followed for successful
installation of the disc drive. Cabling is limited to
1 metre per HP- IB load. Typically the Central
Processing Unit (CPU) is 7 equivalent loads and the
disc drive is 1 equivalent load.
The CPU adheres to an HP standard which allows
7 metres of HP-IB cable between the CPU and the
nearest device connected to it and 1 metre of cable
between each additional device. The maximum
configuration is eight devices (not including CPU)
per HP-IB channel or a maximum of 15 metres or
15 equivalent loads.
The eight Data I/O (DIO) lines are reserved for the
transfer of commands, data, and other messages in
a byte-serial, bit-parallel manner. Data and mes-
sage transfers are asynchronous, coordinated by
three handshake lines: Data Valid (DAV -L), Not
Ready For Data (NRFD-L), and Not Data Accep-
ted (NDAC-L). The other five lines are for bus
management.
Information is transmitted on the data lines under
sequential control of the three handshake lines
(DAV-L, NRFD-L and NDAC-L). No step in the
sequence can be initiated until the previous step
has been completed. Information transfer can
proceed as fast as devices can respond, but no faster
than allowed by the slowest device presently ad-
dressed. This permits several devices to receive the
same message byte concurrently.
Devices connected to the bus may be talkers, lis-
teners, or controllers (refer to table 2- 3). The
Controller-In -Charge (CIC) dictates the role of
each of the other devices by setting the Attention
(A TN - L) line low and sending talk or listen ad-
dresses on the data lines. Addresses are set for each
device at the time of system configuration. While
the A TN - L line is low, all devices must listen to
the data lines. When the A TN - L line is high,
devices that have been addressed will send or
receive data; all others ignore the data lines.
Several listeners can be active simultaneously but
only one talker can be active at a time. Whenever
a talk address is pu t on the data lines (while A TN -
L is low), all other talkers will be automatically
unaddressed.
The Interface Clear (IFC-L) line places the inter-
face system in a known quiescent state. The
Remote Enable (REN - L) line is used to select be-
tween two alternate sources of device program-
ming data such as the front panel or the HP-IB.
The End Or Identify (EOI-L) line is used to indi-
cate the end of a multiple-byte transfer sequence.
In addition, when a controller-in-charge sets both
the ATN-L and EOI-L lines low, each device
capable of a parallel poll responds on the DIO line
assigned to it.
2-11
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