Theory of Operation
Theory of Operation
3Ć10
The GPIB circuitry is composed of GPIB controller U8, with transceivers U9
and U10 buffering signals to and from the GPIB on the option board.
Refer to schematic A2Ă 2. The RSĆ232 portion of the board connects to the
rear panel through port J2. Signals travel from the processor board through
the J1 connector to the U4 dual asynchronous receiver/transmitter (DUART).
The DUART sends data to the U5 driver/receiver.U5 converts signals from
logic levels on the DUART side to RSĆ232 levels at the 9Ćpin connector.From
U5, information goes out port P2.
The Centronics portion of the board connects to the rear panel through the
9Ćpin connector J3. Data travels from the processor board through the J1
connector to register U6 where it is sent out J3. U4 manages control signals.
U12 buffers printer status information to be read through U4.
Refer to schematic A3Ă 1. The 9Ćpin VGA video connector routes auxiliary
HSYNC, VSYNC, and VIDEO signals from main board connector J703 to the
rear panel.
The printer power converter usesswitching regulatorU1 toconvert +15 V
from the power supply to+8 Voutput atrear panelconnector J2.This
provides power for the Option 3P thermal printer.
The CPU system sends instructions to and receives information from the
FrontĆPanel Processor on the FrontĆPanelBoard. The FrontĆPanel Processor
reads the frontĆpanel switches and ports, and reports any change in their
settings to the processor system. The FrontĆPanelProcessor also turns the
LEDs on and off.
The FrontĆPanel Processor reads the frontĆpanelmenu switches and sends
any changes in menu selections to the CPU system. The Front Panel
Processor does not read the ON/STBY button; its signal passes through the
FrontĆPanel Board and the Main Board to the A20 Low VoltagePower
Supply.
The front panel also generates the probe compensation signal.

Pots, FPP, and Calibrator A4Ă 1

The frontĆpanel processor monitors the frontĆpanel controls. It consists of a
singleĆchip microprocessor (U101) with builtĆin RAM, ROM, AĆtoĆD converter
(for digitizing the potentiometer wiper voltages), a programmable timer (for
generating the output of the probe compensator signal), and a serial
communications interface (for data transfer to and from the CPU).
The knob scanner,working with the AĆtoĆD converter internal to the
frontĆpanel processor,produces digital values for the wiper voltages of the
frontĆpanel knobs. Analog multiplexers U420 and U421 select one of 12
possible pot inputs to read. Although there are only six knobs on the front
panel, three are continuousĆrotation potentiometers made up of two wipers,
separated by 180 degrees, which contact a single resistive arc.
A4 Front Panel