Build 4

The fourth batch of boards contained the first boards for the Program Counter / Stack Pointer and the LED Output Display. The nine boards from the first four builds can be assembled to form a Turing-complete 8-bit CPU. One additional build is planned to add the DXY Register functionality from the original breadboard NQSAP computer.

Program Counter and Stack Pointer

The Program Counter and Stack Pointer boards passed the register load test, but the Program Counter (PC) would not count correctly. The Program Increment (PI) signal was mistakenly wired to the high counter and the carry out from that counter was wired to the carry in of the low counter. After a few wires to swap the signals between the two chips, the PC passed all tests.

Output Register

Despite the simple design, the Output Register boards had one clumsy mistake. Two enable pins on the 74HCT138 were left unconnected. A simple patch wire was added to ground these pins and then the board worked as expected.

Results

This shows the NQSAP-PCB with all of the boards from the first four builds. The new PC/SP board is in the upper right and the new Output Register is upper left. The Arduino Loader is not active and the system is running a simple program that counts and saves and restores values from the stack.