NTE Electronics NTE74181

The NTE74181 is an ALU/function generator in a 24-pin DIP package, with a complexity equivalent to 75 gates on a single chip. The circuit performs 16 binary arithmetic operations on two 4-bit words, as shown in Tables 1 and 2. These operations are selected by four function select lines (S0, S1, S2, S3) and include addition, subtraction, decrement, AND, and direct transfer. For arithmetic operations, the internal carry must be enabled by applying a low voltage to the mode control input (M). The device provides a full look-ahead carry scheme for the 4 bits within the package via two cascade outputs (pin 15 and pin 17), enabling fast, synchronous carry generation. When used with the NTE74182 or NTE74S182 full look-ahead carry circuit, high-speed arithmetic operations can be performed. The typical addition times shown in the typical addition time table indicate that very little additional time is required for adding longer words when using full look-ahead carry.

Where speed is not critical, the ripple carry input (Cₙ) and ripple carry output (Cₙ₊₄) may be used. However, ripple carry delay has also been minimized, so arithmetic operations on short-length words can be performed without external circuitry.

The NTE74181 handles either active-high or active-low data, with pin names interpreted as follows:

Subtraction is accomplished via 1's complement addition, where the 1's complement of the subtrahend is generated internally. The result output is A - B - 1, requiring an end-around carry or forced carry to obtain A - B.

The NTE74181 can also be used as a comparator. The A = B output is internally decoded from the function outputs (F0, F1, F2, F3), and will go high to indicate equality (A = B) when two equal words are applied to the A and B inputs. For this comparison, the ALU must be in subtraction mode with Cₙ = H. The A = B output is open-collector, allowing wire-AND connection for comparisons of more than 4 bits. The carry output (Cₙ₊₄) can also be used to provide relative magnitude information. Likewise, the function select inputs S3, S2, S1, S0 must be set to L, H, H, L respectively to place the ALU in subtraction mode.

The circuit not only meets all designer requirements for arithmetic operations, but also provides all 16 possible functions of two Boolean variables without external circuitry. These logic functions are selected via the four function select inputs (S0, S1, S2, S3) with the mode control input (M) held high to disable the internal carry. The 16 logic functions are detailed in Tables 1 and 2, and include XOR, NAND, AND, NOR, and OR functions.

• Full look-ahead carry for high-speed long-word arithmetic operations
• Input clamp diodes minimize transmission line effects
• Darlington outputs reduce turn-off time
• Arithmetic operation modes: addition, subtraction, operand A left shift by one bit, magnitude comparison, and twelve other arithmetic operations
• Logic function modes: XOR, comparator, AND, NAND, OR, NOR, and ten other logic operations