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A project log for Novasaur CP/M TTL Retrocomputer

Retrocomputer built from TTL logic running CP/M with no CPU or ALU

alastair-hewittAlastair Hewitt 05/11/2020 at 02:430 Comments

Just as productivity picks up I decide to wander off on some tangents. Such are the joys of a no-deadline hobby project :)

First up was the reset button. This was super handy when the hardware and software were still in an embryonic state, but the final board will not need one. The user works within the virtual machine and the design prevents the user from changing the underlying state. This means there is no way for the user to crash the computer. The user could crash the operating system, but the hardware abstraction layer will continue to run and the keyboard will still work. A handy ctrl-alt-del can be used to force a reboot.

The reset button has now be changed to a power switch. The regulator has a shutdown pin that the power switch uses shut the power off to the board. One issue is the size of the toggle button: The switch is too long to fit in front of the mounting hole where the old reset button was positioned. A slight reshuffle and the power switch can be placed next to the mounting hole, freeing up all the space left by the old reset button.

This same change was made on the other side of the board to free up enough space to fit two banks of super-capacitors. These provide power to the CMOS memory chip at around 10 days per farad. The plan is to use 2F capacitors for up to 3 weeks of backup, but there's room to fit 3F capacitors for up to 1 month of backup.

The primary reason for the power backup is to give the impression of persistent storage. Most of the banked RAM will be used to create a single RAM disk for a CP/M operating system. This disk will be the primary persistent storage as long as the machine is used on a regular-ish basis. An FTDI serial cable (RS232-to-USB) can be used to back things up for longer term storage.

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