User manual

Table Of Contents

VETTA Theory of Operation………………Line 6 confidential ……………………………. Page 16 of 18

temperature (GRAPHICLCD_TEMP line). This voltage is digitized by the U.I. ADC U10

(page 2 of 3) and read by the main PCB microprocessor. The microprocessor code uses this

temperature reading, plus two other parameters, to determine the required LCD contrast

through the following process:

1) According to the VETTA version an initial value is selected which optimize the viewing

angle as follows:

- Bottom for VETTA Combo

- Front for VETTA Rack (not available)

- Top for VETTA Head

2) The front panel temperature reading is scaled by a function and added to this initial value.

This automatically corrects for the LCD contrast change with temperature.

3) The U.I. user adjustable LCD contrast setting is scaled by a function and added

The scaling functions, initial values and contrast values are different for the graphic and

alphanumeric LCD, but the temperature reading is common to both.

The following diagram represents the firmware flowchart for one of the LCD

The result of this computation is used to control the duty cycle of two100Hz pulse generators

implemented by the main PCB microprocessor U27 and the two U.I. PCB flip-flops U1-4 and

U1-B. Each of these flip-flops output is then rectified by Q18, R4 and C24 for the graphic LCD

and R1, R3 and C23 for the character LCD. This produces two negative voltages proportional

to the generator’s duty cycle. These voltages are then sent to the LCDs contrast adjustment pin.

While the Vetta is in test mode (power the unit with the REVERB switch depressed), the

software does not perform the temperature compensation algorithm. For this reason, when the

Vetta is running in test mode, and the unit had had a chance to significantly warm up, the

contrast of the LCD will be significantly off.

U.I. potentiometers:

The 10 U.I. potentiometers (6 on the left side, and 4 on the right side) provide a voltage from 0

to 5V. These voltages are digitized by the U.I. ADC U10. The resulting serial stream is read by

the Main PCB microprocessor U27.

Initial value for Combo

Initial value for Head

Initial value for Rack

FRONT PANEL TEMPERATURE READING

U.I. CONTRAST SETING

VETTA

VERSION

LCD CONTRAST Pulse With Modulator

Function

LCD Bias Voltage