Eeprom Write-Protect Circuit; Display Board; Vf Display; Vf Display Driver - Motorola ASTRO W3 Service Manual

Theory of Operation: W9 Control Head

3.3.3.8 EEPROM Write-Protect Circuit

Transistors Q161, Q162, and associated circuitry guard against inadvertently writing into the
EEPROM U5 by inhibiting the Write Enable input. The circuitry guarantees the EEPROM is protected
during reset and when shift register U6 is not in the proper programming state.
3.3.4

Display Board

The display board contains the main operator interface points of the system, including the vacuum
fluorescent display, the status indicator LEDs, and the user keypad.
NOTE: Refer to the foldout schematic diagram of the display board for this discussion.

3.3.4.1 VF Display

The (VF) display is an 11-digit, 14-segment display that needs two separate voltages to operate, The
anode and grids use +43 volts to accelerate electrons from the cathode, and the filament uses 3.5
vac at 80 mA. These voltages are obtained from the VF voltage converter on the controller board.

3.3.4.2 VF Display Driver

VF display driver U101 receives 14-segment display data, digit enable data, and indicator state data
from the microprocessor on the controller circuit board. This data is clocked using CLOCK (pin 23)
and DATA-IN (pin 43) lines, then framed using LATCH-ENABLE (pin 24) and STROBE (pin 21) to
control brightness. The 32-bit data frame is updated every millisecond so that a coherent display is
maintained and totally updated every 11 ms.

3.3.4.3 Status LEDs

Status LEDs CR111 through CR120 are driven by the display driver as though they were decimal
points on the VF display. Level-shifting transistors are required for this since the display driver uses
43 volts for control signals.

3.3.4.4 Backlight LEDs

The same signal (VF-CONTROL) that turns the VF voltage converter on and off also operates the
backlight LEDs. Transistor Q134 supplies base current to the individual driver transistors. The driver
transistors act as current sources to the LEDs. Backlight LEDs CR131 through CR134 are
connected to thermistor RT133 via Q132. This circuit allows more current to flow through these LEDs
at room temperature and reduces current as the temperature rises.
3.3.5

Default Jumper Settings

The factory installs jumpers in each control head according to customer-ordered options. See
3-6 for the standard jumper configurations. See the diagrams on the foldout pages for other
configurations.
6881096C77-O

Table 3-6. Standard Jumper Configurations

Jumper Number
JU1
JU2
JU3
JU4
Normally
IN
OUT
IN
OUT
September 26, 2003
3-11
Table