Panasonic KX-FT21BX Service Manual page 86

KX-FT21LA
Before we begin mass production, several hundred trial units are produced at the plant, various tests are applied and any
malfunctions are analyzed. (In past experiences, digital IC (especially SRAM and ROM) malfunctions are extremely rare after
installation in the product.)
This may be repaired by replacing the IC, (ASIC etc.). However, the real cause may not have been an IC malfunction but a
soldering fault instead.
Soldering faults which are difficult to detect with the naked eye are common, particularly for an ASIC and RA (Resistor Array).
But if you have an oscilloscope, you can easily determine the problem site or IC malfunction by checking the main signal lines.
Even if you don't have such a measuring instrument, by checking each main signal line and resoldering it, in many cases the
problem will be resolved.
An explanation of the main signals (for booting up the unit) is below.
Don't exchange ICs or stop repairing until checking the signal lines.
An IC malfunction rarely occurs. (By understanding the necessary signals for booting up the unit, the "Not
Boot up" display is not a serious problem.)
What are the main signals for booting up the unit?
Please refer to the Block diagram.
The ASIC (including the CPU) (IC4) controls all the other digital ICs. When the power is turned on, the ASIC (CPU) retrieves the
operation code stored in the ROM (IC2), then follows the instructions for controlling each IC. All ICs have some inner registers
that are assigned to a certain address.
It is the address bus by which the ASIC (CPU) designates the location inside each IC. And the data bus reads or writes the data
in order to transmit the instructions from the ASIC (CPU) to the ICs.
These signal lines are all controlled by voltages of 5V (H) or 0V (L).
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