HP 437B Service Manual page 33

HP 437B
Service Sheet BD1
8-16
Note
SERVICE
—_— heer rere ee ges
The ROM contains permanent instructions that are interpreted and executed
by the microprocessor. The RAM is used by the microprocessor to store
various data such as front panel conditions and the Sensor Data tables. A
battery supplies power to the RAM when the Power Meter is not connected
to the mains power line. This prevents loss of stored data.
When the Power Meter is turned on, the reset circuitry holds the
microprocessor in the reset state for 25 milliseconds to allow the power
supply to reach a steady +5V. If the +5V power supply drops below +4.5V
the reset circuitry will issue a signal to halt the microprocessor.
The analog PIA is an input/output device that allows the microprocessor to
control the components in the Analog Section. Data from the data bus is
transferred by the analog PIA to the Analog Section. This data is sent to the
Analog Section via eleven data lines and two control lines (STRAMP and
NADS).
The service test buffer is a digital multiplexer. Data lines AAO-AA2,
STRAMP, NADS, and NWR control the multiplexer and select one of 8 data
lines ADO-AD7 from the analog PIA. During self-test, the service test buffer
verifies the data and control lines from the Digital Section to the Analog
Section.