Service Model
8901B
output of the Address Decoder and the next
two
input bits to determine
if
it
is
a talk
or
listen address
and
if
the instrument should respond to
it.
The result of this determination modifies the address to
the ROM in the Interface Control Logic.
The Address Readback outputs the address from the Address Switches
to
the Instrument Bus
data
lines
when Special Function
21
(HP-IB Address) is selected. The Controller reads the HP-IB address from
the Address Switches. (See
HP-IB
Address
in the
Detailed Operating Instructions
in the
Operation
and Calibration Manual.)
The HP-IB input/output
is
directly handled by a portion of the Microprocessor that resides in the
Remote Interface Assembly-the Peripheral Input/Output (PIO). The PI0
is
a device that routes the
HP-IB data to and from the CPU and the HP-IB.
It
provides a communication link between the CPU
and the Remote Interface hardware, and provides the means for interrupting the CPU. One of the
two,
eight-bit PI0 output ports connects to the HP-IB
data
lines. The other output port connects to the
Handshake Logic and Interface Control Logic.
NOTE
For
purposes of troubleshooting the Controller, the Remote Interface
Assembly may be unplugged. Provision
has
been made to allow the
instrument to work with only the
loss
of
HP-IB
operation.
Although the Remote Interface Assembly receives data and operating information from the PIO, it is
primarily through the Instrument Bus that
it
is
controlled. (Commands such as
SRQ
that need rapid
processing come from the
PIO.)
A
Select Decoder decodes the select lines when enabled by code
e=4.
The decoded select lines enable
or
disable parts of the Remote Interface Assembly. Special Function
61
provides a means of reading back HP-IB status information
to
the front-panel display.
Power
Supply
Assemblies
(A10
and
A26)
The five regulated power supplies are:
+15V, -15V, +40V, +5V,
and
-5V.
Each supply has its own
secondary winding on the Line Transformer and all supplies except the
+40V
Supply have their
own
full-wave rectifier. The latter four supplies are referenced from the
+15V
supply. Each supply is a series
regulator type. When the instrument
is
switched
to
STBY, the
+15V
supply remains on and supplies
current only to the High-Stability, Crystal Oscillator (Option
002).
In STBY, the other supplies become
referenced to
OV
and thereby shut themselves
off.
The supply switching is via the ON/STBY Relay.
The fan is also switched by the relay.
8D-52
Service Sheet BD5
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