5-4
5.5.1 Display board
The following information provides some basic circuit the-
ory that can be used as an aid to troubleshoot the display and
keyboard. Figure 5-2 shows a block diagram of the display
board.
Display microcontroller
U902 is the display microcontroller that controls the VFD
(vacuum fluorescent display) and interprets key data. The
microcontroller has four peripheral I/O ports that are used
for the various control and read functions.
Display data is serially transmitted to the microcontroller
from the digital board via the TXB line to the microcontrol-
ler PD0 terminal. In a similar manner, key data is serially
sent back to the digital board through the RXB line via PD1.
The 4MHz clock for the microcontroller is generated on the
digital board.
Vacuum fluorescent display
DS901 is the VFD (vacuum fluorescent display) module,
which can display up to 49 characters. Each character is or-
ganized as a 5
×
7 matrix of dots or pixels and includes a long
under-bar segment to act as a cursor.
Figure 5-2
Display board block diagram
Vacuum Fluorescent Display
DS901
Dot Drivers
U901, U905
Grid Drivers
U903, U904
Controller
U902
Key Matrix
S901 - S931
Reset
Tx
Rx
4MHz
Clock
The display uses a common multiplexing scheme with each
character refreshed in sequence. U903 and U904 are the grid
drivers, while U901 and U905 are the dot drivers. Note that
dot driver and grid driver data is serially transmitted from the
microcontroller (PD3 and PC1).
The VFD requires both +60VDC and 5VAC for the fila-
ments. These VFD voltages are supplied by T601, which is
located on the digital board.
Key matrix
The front panel keys (S901-S931) are organized into a row-
column matrix to minimize the number of microcontroller
peripheral lines required to read the keyboard. A key is read
by strobing the columns and reading all rows for each
strobed column. Key down data is interpreted by the display
microcontroller and sent back to the main microprocessor
using proprietary encoding schemes.
5.5.2 Power supply
The following information provides some basic circuit the-
ory that can be used as an aide to troubleshoot the power sup-
ply. Figure 5-3 shows a block diagram of the power supply.