HP 8112A Operating, Programming And Servicing Manual page 245

Servicing the Control Board
Theory of Operation
Introduction
Note
Address Decoders
The main function of the control board A2 is to convert digital
control data into analog control signals used on the main board
A l . The main board supplies power to the control board and the
microprocesor board A3 supplies the digital control data. The
standard control board is divided into the following areas:
•
Timer circuit
•
Address decoders
•
Timing range decoder
•
DAC Reference circuit
•
Digital to Analog Converters (DA Cs)
•
Byte Offset and Offset DAC
•
Parameter control
Refer to Figure 1 0.5-3. The timer circuit, U26 and associated
Timer
components, provides the Non-Maskable Interrupt signal NMI, used
by the microprocessor to produce the flashing error display when an
error condition is detected. The 555 timer is configured as an astable
multivibrator which produces an output of approximately 100 Hz
when enabled when TIRE is active (low ) .
RESET is held high for a short time at instrument switch on, to
disable the timer so that NMI cannot be transmitted before the
power supplies have settled.
If an error code is shown in the display, it is necessary to press a
key, e.g. (LCL ), to set the microprocessor to normal, then commence
troubleshooting.
Refer to Figure 10.5-3. Local address decoding is performed by U l
and U 1 6 . Both decoders share LA3 - LA5 from the microprocessor
board as address inputs, WS5 enables decoder Ul and WS4 enables
decoder U16. The decoder outputs enable the various devices on the
latched-data bus LDO-LD7.
Servicing the Control Board 1 0.5·1
1 0 . 5