Agilent Technologies 6571A, Service Manual

Page: 77 / 161

Principles Of Operation 77
4
Principles of Operation
Introduction
Figure 4-1 (at the end of this chapter) is a block diagram showing the major circuits within the power supply. The supply’s
interface and control circuits consist of circuits on the A1 Front Panel Board, A2 GPIB or Isolator Board, and A5 Control
Board. The power circuits are the A4 AC Input Board, A3 FET Board, A7 Snubber Board, A6 Output Filter Board, and a
few components mounted on the chassis.
Each block in Figure 4-1 identifies a schematic diagram in Chapter 6 where the circuits are shown in detail. You can refer to
the schematic to locate specific components mentioned in this description. Chapter 6 also has a cabling diagram (Figure 6-2)
showing the circuit board interconnections.
INTERFACE CONTROL CIRCUITS
A2 GPIB Board (667xA Series Only)
Circuits on the A2 GPIB board (see A2 GPIB Board schematic) provide the interface between the GPIB controller and the
power supply. All communications between the power supply and the GPIB controller are processed by the GPIB interface
and primary microprocessor circuits on the A2 board. The primary microprocessor circuits (microprocessor U114, ROM
U106, and RAM U108) ICs decode and execute all instructions and control all data transfers between the GPIB controller
and the Secondary Interface on the A5 Control Board. The primary microprocessor also processes measurement and status
data received from the Secondary Interface.
A UART (universal asynchronous receive/transmit) IC (U112) on the A2 board converts data between the primary
microprocessor’s 8-bit, parallel bus and the serial I/O port. The serial data is transferred between the primary interface and
the secondary interface via a programmed GAL (gated array logic) IC (U119) and optical isolator ICs (U110/U111). These
ICs isolate the primary interface circuits (referenced to earth ground) from the secondary interface circuits (referenced to
power supply common). The GAL IC also provides a serial I/O port to the A1 Front Panel Board to enable front panel
control of the power supply.
A serial link interface IC (U109) on the A2 GPIB Board allows up to sixteen supplies to be connected together and
programmed from one GPIB address. The first supply is the only supply connected directly to the GPIB controller and is set
to the primary GPIB address. The remaining supplies are set to secondary addresses and are linked (daisy chained) together
via the Jl/J2 phone jacks at the rear of each supply. The serial link configuration is described in the Power Supply Operating
Manual.
A digital control interface on the A2 GPIB Board provides the following power supply functions:
•
Relay link.
•
Digital 1/0.
•
Remote inhibit (INH).
•
Discrete fault indicator (FLT).
An optical isolator IC (U113) isolates the FLT output signal common from the external fault circuit common.