4-6 Troubleshooting
Microcontroller
U401 is the display board microcontroller that controls the display and interprets key data.
The microcontroller uses three internal peripheral I/O ports for the various control and read
functions.
Display data is serially transmitted to the microcontroller from the digital section via the TXB
line to the microcontroller RDI terminal. In a similar manner, key data is serially sent back to
the digital section through the RXB line via TDO. The 4MHz clock for the microcontroller is
generated by crystal Y401.
Display
DS401 is the display module, which can display up to 12 alpha-numeric characters and
includes the various annunciators.
The display uses a common multiplexing scheme with each character refreshed in sequence.
U402 and U403 are the drivers for the display characters and annunciators. Note that data for
the drivers are serially transmitted from the microcontroller (MOSI and PC1).
Filament voltage for the display is derived from the power supply transformer (F1 and F2).
The display drivers require +37VDC and +5VDC, which are supplied by U304 (+5VD) and
U305 (+37V).
Key matrix
The front panel keys (S401-S430) 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.
Digital circuitry
Refer toFigure 4-2 for the following discussion on digital circuitry.
Microprocessor
U101 is a 68306 microprocessor that oversees all operating aspects of the instrument. The
MPU has a 16-bit data bus and provides an 18-bit address bus. It also has parallel and serial ports
for controlling various circuits. For example, the RXDA, TXDA, RXDB and TXDB lines are
used for the RS-232 interface.
The MPU clock frequency of 14.7456MHz is controlled by crystal Y101. MPU RESET is
performed momentarily (through C133) on power-up by the +5VD1 power supply.
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