NQSAP-PCB TTL computer The Not Quite Simple As Possible Computer - PCB version

NQSAP-PCB is the second iteration of the NQSAP breadboard TTL computer.

Highlights of the design are:

  • Modular backplane and bus connectors, providing flexible arrangement of the computing boards to easily modify and extend the computer’s functionality.
  • Arduino-based loader/debugger with full access to all data and microcode control lines to load programs and run interactive diagnostics
  • 74LS181 ALU provides arithmetic and logic operations including add, subtract, AND, OR, and XOR
  • Four flag registers with hardware-based conditional jumps
  • Double-buffered Instruction Register to mitigate issues with control EEPROM output glitching
  • Instruction set and addressing modes similar to the 6502.

Most of this design was prototyped in original NQSAP breadboard build. Some of the enhancements from that original design are:

  • Loader design with shift registers now allows access to all microcode control lines
  • Microcode ROM count reduced from 4 to 3 by combining some signals
  • ALU shift register design replaced with a simple offset register for the right shift operation
  • Maximum number of microcode steps increased from 8 to 16
  • RAM can now optionally address four different 256-byte blocks, allowing for separate program, data, and stack areas.

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