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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 P I 0 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 P I 0 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 O V 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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