binary encoding circuit

Binary numbers are used in digital circuits, while decimal numbers are used in daily life. In order to input decimal numbers into digital
circuits, a binary decimal encoding circuit must be used to convert the decimal numbers into binary encoding signals and BCD codes.
    The picture is a binary encoding circuit composed of NAND gates. The ten button switches s[~slo in the circuit are self-locking buttons.
When pressed once, the station will be closed, and when pressed again, the station will be disconnected. When they are all pressed, all the input terminals of the four NAND gates are high level,
and the four output terminals of the NAND gates 1 to 4 output "0000". When the SL switch ("0" key) is pressed, the circuit status does not change, and the output is still
"0000", which is the BCD code of decimal number 0. When the S2 switch ("1" key) is pressed, the input terminals of NAND gates 2, 3, and 4 do not change,
the input terminals are still high level, and their output terminals are all low level, so the circuit output is BCD code "OO01". When the s8 switch ("7"
key) is pressed, each of the NAND gates l, 2, and 3 has an input terminal at low level, and the encoder outputs the BCD code "OII1".
    As can be seen from the above, which numeric key is pressed, the corresponding BCD code will be obtained at the corresponding output end of the encoding circuit , thus realizing the encoding process from decimal number to binary number.