Build 2

The second batch of boards contained a new Loader/Debugger as well as the first boards for the microcode ROMs, Instruction Register/Ring Counter, and RAM/MAR. These boards were chosen for the second build because they form the core of the control circuitry. The addition of a program counter would allow a fully-functional, although somewhat boring, computer. The next build will include the ALU and flags to round out the functionality.

Loader plus Clock and Reset

The new Loader functioned as designed. The Arduino software from the V1 board was simplified to use the new shift register scheme.

Microcode ROMs

The LED portion of the Microcode module was assembled early to help verify operation of the Loader and other modules. The actual ROMs were added later and their operation was checked with the Loader/debugger.

Instruction Register and Ring Counter

The dream of a perfect build ended with the IR-RC board. This one had two minor problems that will be fixed with a new board update.

The first problem was the T-EXT LED wired to Vcc instead of ground. The temporary fix was to install the LED backwards, causing it to display the inverted signal.

The second problem is that the Ring Counter requires an inverted N signal from the bus. The Loader was providing some inverted signals over the module interconnects, but this one was not included. As a temporary fix, the load input to the ring counter was wired to an unused bus line where it can be manually set or wired to an external inverter. The next version of the board will add a hex inverter to produce the needed signals locally.

RAM and MAR

The RAM and MAR module worked as designed and was verified by the self test routines on the Loader/Debugger. The timing of this test will need to be adjusted because the Loader’s new shift register design is slower that the previous parallel register design. Most of the tests have timing delays built-in to allow visual observation of the test, so these will be shortened appropriately.

Results