Introduction; Overall Functional Description; Main Assemblies - Fluke 8508A Service Manual

Introduction

This chapter provides a board-level theory of operation for the 8508A Reference
Multimeter. Functional block diagrams supplement the text when appropriate. In addition
to providing a conceptual overview of the Multimeter's operation, the descriptions are
helpful for troubleshooting and isolating problems to a board level.
Reference Designators are used throughout this chapter to identify the major Printed
Circuit Assemblies (PCAs) used in the Multimeter. These reference designators also
appear in the List of Repaceable Parts (Chapter 5) to simplify the process of purchasing
replacement modules and PCAs.

Overall Functional Description

Main Assemblies

Refer to the functional block diagram shown in Figure 2-1 while reading this description.
In addition to identifying all of the assemblies contained in the Multimuter, this diagram
provides a functional view of the data flow between the assemblies. Assembly
interconnections and cable identification is detailed in chapter 4, Maintenance.
The following list identifies the six major assemblies contained in the Multimeter and
includes a description of the basic functions each assembly performs:
A1 DC PCA
A2 AC PCA
A3 Ohms PCA
A4 Digital PCA
A5 Bezel Assembly
A6 Rear Input Assembly
(optional)
Normal operator interaction with the Multimeter begins at the front panel (A5) which
includes a keyboard for entering measurement configurations (functions, ranges, etc.) and
a display for verifying the selected configurations and for displaying measurement
results. Both the keyboard switches and the display are included on the A5A1 Display
PCA. A rubber keypad, whose keys include a conductive backing, provides switching
information to A5A1. A controller on the A4 Digital PCA provides the addressing,
strobing, and storage of data coming from and going to the A5A1 Display PCA. The data
stored on the A4 Digital PCA includes both the manually entered configuration data for
use by the display and other assemblies, as well as, the resulting measurement data for
use by the display as measurement results.
Before being used by the Multimeter, the stored configuration data is converted on the
A4 Digital PCA from a parallel to serial format. Similarily, the A4 Digital PCA receives
serial measurement data from the instrument and converts it to a parallel format for use as
display data. A rear panel IEEE-488 (GPIB) bus is intergrated into the format conversion
process.
Optical isolation; measurement configuration; V dc, I dc and I
ac measurements
True-rms measurement
Resistance measurements: 2-wire, 4-wire, high-voltage,
ratio, etc.
Out-guard power supplies, control of IEEE (GPIB) and serial
data (measurement and control)
Front panel user interface, display and keyboard, includes
the A5A1 Display PCA.
Rear input connections, includes the A6A1 Rear Input PCA.
Theory of Operation
Introduction
2
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