74181 ALU NMOS Discrete Version

74LS181 is a very common arithmetic logic unit with 16 arithmetic functions and 16 logical operation functions. It uses DIP-24/SOIC-24 package and can perform 4-bit binary and 4-bit binary operations at a time.  
This ALU first appeared in 1970 and is the successor of N8260 ALU. Its internal equivalent logic diagram is shown below:
A and B terminals are binary operand inputs,
F is the operation result output
, S is the function selection input, M is the arithmetic operation/logical operation switching input,
Cn+4 is the carry input, and C0 is the carry input .
The A=B port is the comparison signal output.
The other two (pins 17 and 15) need to cooperate with 74LS182 to form a fast carry chain. They are generally not used and can be ignored. It is
 
worth noting that, first of all, the A=B port is an open-drain output, and a pull-up resistor is required for single use. The advantage of open drain is that when multiple 74181 chips are compared together, they can be wired and all chips can get A=B valid output, and the whole is effective.
In addition, its carry input and carry output (C0, Cn+4) are both inverse codes, which are opposite to the input and output codes. When the operand input and output are positive codes, the carry is inverse code; otherwise, the carry is positive code.
 
According to the logic diagram, simulation was performed:
In order to realize the NOR gate, a MOS tube parallel design was adopted. In order to realize the NAND gate, a MOS tube series design was adopted. Please see the circuit diagram for details.
 
This design was finally verified in kind, and no logical errors were found for the time being. The physical object of this design was finally used in the production of a fully discrete 4-bit transistor CPU, and it was successfully powered on and operated: