Logic Operator Helps 8-Bit Programmers Calculate, Logically
SB181 takes hex input and displays logic operations in hex and binary.
Building functional hardware with gates or 7400-series logic chips takes a particular skill. And even with that skill, doing binary math in your head is not always easy. So, Simon Boak created SB161: a Logic Operator based on the 74LS181 ALU chip. It is an 8-bit logical calculator that operates without a CPU or even a microcontroller.
At SB161's core are two 74LS181 chips. The '181 is a 4-bit arithmetic logic unit (ALU) capable of performing 16 arithmetic operations and 16 logic operations on two variables.
Input comes from 16 key switches connected to a 74C923 encoder chip and a slide switch. A slide switch determines which register Logic Operator stores the input value. The A and B registers are both using a 74HC273 latch.
TIL311 ooze retro output displays with their red glowing hexadecimal values from the '181. While the TIL311 is a dazzling display, it is also power hungry. SB161 consumes about nine watts, even with a switching regulator!
Below the hexadecimal displays are a row of LEDs for binary output. Discrete LEDs in each key of the keypad indicate the currently selected function. A function register and decoder determines which LED to illuminate.
The main chassis consists of aluminum, retro-inspired oak-laminated MDF, and some black labeling for this design. All of which accentuate those glowing red LED displays.
Over time, Boak cobbled together the parts list. Some used in the SB161 may no longer be readily available. (Check out this article for some TIL311 alternatives.) Which means no kit is available. However, on unimplementedtrap.com, you can find the schematics for the SB161's three PCBs. The links are on the bottom.
