Principles of operation
The following information is provided to support the troubleshooting tests and procedures
covered in this section of the manual. Refer to the following block diagrams:
Block Diagrams:
Figure 2-1 — Power supply block diagram
Figure 2-2 — Digital circuitry block diagram
Figure 2-3 — Analog circuitry block diagram
Display board
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 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 U144 (+5VD) and
U101 (+37V).
Troubleshooting 2-5
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