6-10
The dual-line VFD (vacuum fluorescent display) mod-
ule has a top line consisting of 20 alpha-numeric char-
acters and annunciators. It is used to display patterns
as well as text. The bottom line of the display is a 32-
character alphanumeric, which is used to display text
only.
Each character is organized as a 5
×
7 matrix of dots or
pixels, and includes a long under-bar segment used as
a cursor.
The display uses a common multiplexing scheme in
which each character is refreshed in sequence. Dual
UCN5812’s are the dot drivers while a UCN5818 is the
grid driver. The dot drivers and grid drivers are serial-
ly transmitted from PD3 and PC1, respectively. The
clock for the dot drivers is from PD4 of the microcon-
troller, while the clock for the grid driver is taken from
PC0 of the microcontroller.
The VFD requires both +60VDC and +5VAC for the fil-
aments. These VFD voltages are supplied by U117,
which is located on the digital board.
The front panel keys are organized into a row-column
matrix to minimize the number of microcontroller pe-
ripheral lines required to read the keyboard. A key is
read by strobing the columns and reading all rows for
each strobed column. Keypress data is interpreted by
the display microcontroller and sent back to the main
microprocessor using proprietary encoding schemes.
Figure 6-8
Display board block diagram
From
Digital
Board
+60V
Filament
5VAC
VFD
5812
Dot
Drivers
Grid
Drivers
5818
To/From
Digital
Board
4MHz
RX Switch Matrix
Data
Reset
Microcontroller
68HC705
Switch Pad
Matrix
TX Data
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