Model 2750 Multimeter/Switch System Service Manual Troubleshooting 4-7
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 micro-
controller is generated by crystal Y401.
Display
DS401 is the display module, which can display up to 12 alphanumeric characters and
includes the various annunciators and five LED 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 U144
(+5VD) and U101 (+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 to Figure 4-2 for the following discussion on digital circuitry.
Microprocessor
U135 is a 68332 microprocessor that oversees all operating aspects of the instrument. The
MPU has a 16-bit data bus and provides a 21-bit address bus. It also has parallel and serial
ports for controlling various circuits. For example, the RXD and TXD lines are used for
the RS-232 interface.
The MPU clock frequency of 19.92MHz is controlled by crystal Y101. MPU RESET is
performed momentarily on power-up.
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