Microprocessor
The
HP-HIL microprocessor on
the
computer's
assembly provides
support
for link protocol
and
is
responsible for:
• Configuring
the
link.
• Processing
input
device
data.
•
Error
recovery.
• Initiating
commands
onto
the
link
through
the
Link Interface.
Link
Interface
Data
is
exchanged between
the
device
and
the
host
computer
through
the
link interface.
It
accepts conlmands from
the
microprocessor for
transmitting
and
receiving messages in
an
HP-
HIL frame format.
Each frame
is
a 150
J1sec
long, 15-bit fixed format. A frame
is
shown in Figure 3-6.
idle
start
state
bit
+ +
command
bit
+
I I
address
L'
opcode/data
byte
L1211
1
0
_ L 16151413121
+/-
05%
parity
idle
bit
state
+ +
1 1
0
IJ
t
stop
bit
1
f.-
~~r
u~~,c
-
----~
1
Frome
(150
uSec)
------.,
Figure 3-6. HP-HIL Frame
Structure
Frames may
be
sent
up
to
once each 154 sec allowing
data
transfer
rates
of
up
to
6500
bytes/second. HP-HIL protocol perfornls
automatic
configuration, identification,
and
polling
of
up
to
seven devices.
Error
recovery, device
and
system reset,
and
simple
output
commands
provide a simple device integration.
When
the
computer
is
turned
on,
the
link is polled for devices. Each device on
the
link has
its own ID which is sent
to
the
computer
and
stored in memory.
The
link
is
polled 60 times
per
second. Should a device have
data
to
input
to
the
computer, it sends its
data
frame
to
the
computer
for processing. As each device's franles has its own address in a 3-bit word, no more
than
7 devices can
be
used on
the
link
at
one time.
Interface
Cables
A four-wire cable connects HP-HIL devices
to
each
other
and
to
the
computer.
AMP
Shielded
Data
Link (SDL) connectors are used
and
are easily installed
and
removed fronl
the
computer
or
device.
Keyboard
cable connectors are keyed
and
are marked with
dots
to
indicate where
they
are
connected. Figure 3-7 shows
the
connector pinouts.
50
Functional Description
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