4 Theory of Operation
126 Keysight 34420A Service Guide
Earth-Referenced Logic
Chapter 8, "Earth Referenced Logic Schematic" provides all rear panel input/
output capability. Microprocessor U700 handles GPIB (IEEE-488) control through
bus interface chip U701 and bus receiver/driver chips U702 and U703. The
RS-232 interface is also controlled through U700. RS-232 transceiver chip U706
provides the required level shifting to approximate ± 9 volt logic levels through
on-chip charge-pump power supplies using capacitors C708 and C709.
Communication between the earth referenced logic interface circuits and the
floating measurement logic is accomplished through an optically isolated
bi-directional serial interface. Isolator U506 couples data from U700 to
microprocessor U501. Isolator U704 couples data from U501 to microprocessor
U700.
Shown on the Chapter 8, "Configuration Data Shift Registers Schematic" is the
Chart Recorder Output. U700 programs DAC U610 to output a voltage in the
range of± 3Vdc, in direct proportion to the measured input voltage on a given
range. A full 3 Vdc output represents a 120% full scale input voltage.
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