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TM 11-6625-2975-40
MICROPROCESSOR (CPU) OPERATION.
Refer to FO-2 while reading the text in this paragraph.
The microprocessor, also referred to as the central processing unit (cpu), controls all internal signal
generator operations and is contained in cpu assembly A2A1.
The cpu is interconnected with front panel assembly A1, rf modulator assembly A3, and TDM board
A2A3 by a parallel control bus. The control bus is capable of handling 8-bit addresses, 8-bit data,
and I/O read and write pulses. The 8-bit addresses and the I/O read/write pulses are transmitted only
from the cpu while the 8-bit data can be generated by the CPU or, on request, obtained from A1 A1,
A2A3, or A3A3, depending on I/O port addressed.
The 8-bit address selects the correct I/O port. The I/O read pulse strobes data out of the port over the
control bus to the CPU . The I/O write pulse strobes data from the CPU into the correct I/O port via the
control/data bus.
KEYBOARD OPERATION
Every 4 milliseconds the CPU is internally interrupted, and when these interruptions occur, address,
data, and 1/0 write pulses from the cpu are supplied over the control bus to the 1/0 port on the front
panel assembly A1. The I/O port supplies this data to a keyboard matrix, which is scanned to
determine if any keys have been pressed. If the scanner detects any key has been pressed, that data
is returned to the I/O port on the A1 card. Upon request from the cpu, this information is returned to
the CPU over the control bus. The cpu stores this information in memory, but does not act on it until it
receives the same information 12 times in 12 consecutive scanner cycles. On the twelfth cycle, the
cpu recognizes the key entry as valid and proceeds to act on the data stored in memory. In order to
ensure the data is valid and not just key bounce, 12 consecutive cycles are required. Since the key-
board is arranged in a matrix, the data contains the key position in the matrix.
When rf frequency, VOR radial, DDM, or percent modulation entries are made with the keyboard, the
cpu recognizes and stores the data until the DATA ENTRY-ENTER key is pressed. When the cpu
recognizes the ENTER key data, the stored information is processed. If a mistake is made when
entering data, or if an invalid entry is made producing a blinking display, the operator uses the DATA
ENTRY-CLEAR key to clear the CPU memory.
When key activity is not detected by the scanner, the scanner continues to perform its normal routine.
In its normal routine, the scanner:
1. Monitors the keyboard
2. Requests a frequency count from A3 to make sure the frequency displayed is accurate
3. Requests the status of synthesizer phase lock from A3 to make sure the selected rf output
frequency remains phase-locked to the reference frequency
4. Requests the rf level calibration from A3 to make sure the rf level is within specified limits.
At every scanner loop the same information is requested. If the information is not available when
requested, it is obtained during subsequent loops.

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