4 Assembly Level Troubleshooting
116 Keysight 3458A Assembly-Level Repair Manual
Outguard section
The outguard circuitry consists of the main controller, GPIB circuitry, front panel
circuitry (keyboard and display), isolation circuitry, and power supply. The
outguard section controls the instrument through the isolation logic, provides the
interface between the keyboard and display in the front panel, and communicates
with instruments and controllers connected to the General Purpose Interface Bus
(GPIB). The outguard circuitry is explained as follows.
Main controller
The Main Controller consists of a microprocessor, program ROMs, storage RAMs,
calibration RAMs, and other associated circuitry. It controls the measurement
operation of the instrument, communicates with the front panel (keyboard and
display) and GPIB circuitry, performs the math operations, and calculates for the
correct measurements. The correct measurements are calculated using the
calibration constants in the calibration RAM which were stored during instrument
adjustment.
GPIB circuitry
The GPIB circuitry provides communication between the 3458A and other
instruments and controllers connected to the GPIB. Instrument control commands
are transferred to the main controller and measurement data is transferred from
the main controller through the GPIB circuitry.
Front panel circuitry
The front panel circuitry consists of keyboard and display circuitry. The main
controller sends display data to the display circuitry to be displayed on the front
panel. The keyboard circuitry interrupts the main controller when a key is pressed.
The keyboard circuitry also sends data to the main controller that determines
which key is pressed to perform the appropriate action.
Isolation logic
Since both the outguard and inguard circuitry are referenced to two different
circuit grounds, they must be isolated from each other. To maintain this isolation,
direct connections for communications cannot be used. The isolation logic is used
to provide communication between the two circuits, while keeping them isolated
from each other. The isolation logic uses fiber optic cables to transfer data
between the inguard and outguard.
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