Service manual

Principles Of Operation

Introduction

Figure 4-1 is a block diagram illustrating the major circuits and signals within the Electronic Load. Each block on the

diagram identifies the schematic diagram sheet where the circuits are shown in detail. Schematic diagrams for the

mainframe, consisting mainly of the GPIB Board, are on foldout pages at the end of this manual. Schematic diagrams for the

Control Board and Power Board are provided with the Service Manuals for the individual load modules. Note that the block

diagram provided in this manual and described in this chapter covers both the mainframe and load module.

The block diagram shows many of the major signals between blocks, but most of the control signals to and from the

microprocessors are not shown. Multiple signal lines are sometimes combined into one for clarity. For example, the outputs

of the OV, OP, and UNREG Status Comparators are shown on a single line.

The electronic load is functionally divided into three assemblies, the GPIB Board (which is part of the mainframe), and the

Control Board and Power Board (which are part of each module). The block diagram and this description are similarly

divided.

At the end of this chapter is a description of how the mainframe identifies data sent to and from each module.

Bias Supplies

The electronic load contains two Bias Supplies. One is on the GPIB Board, and supplies bias for the primary

microprocessor and other circuits referenced to chassis common. The other is on the Control Board, and supplies voltages

for the secondary microprocessor and other circuits referenced to the--input. Both Bias Supplies generate clear signals that

are used to ensure proper start-up when the unit is turned on.

GPIB Board Circuits

Circuits on the mainframe GPIB Board provide the interface between the electronic load and the user, generate trigger

signals, and control fan speed. The GPIB Interface is the link between the electronic load and the system controller. Almost

all communication between the electronic load and the controller is processed by the primary microprocessor, except that

Group Execute Trigger (GET) goes directly from the GPIB Interface to the Trigger-Source Select circuits.

The Primary Microprocessor interprets commands from the GPIB or from the front-panel keypad to control the load's input

current. The Primary Microprocessor also processes measurement and status data received from the input circuits via the

Secondary Microprocessor. This data may be read back to the controller over the GPIB and/or displayed on the unit's

front-panel display.

The Primary Microprocessor circuits contain an EEPROM (electrically erasable programmable memory), which stores the

load's GPIB address, model number, and other information. The EEPROM is non-volatile, allowing it to retain stored

information after power is turned off and back on.

The Trigger-Source Select circuit selects one of four trigger-source signals to be applied to the Trigger Generator. LSNK is