Z80 System
Part 2 - Extending it a bit

At this point the system was doing something but it was quite a bit limited missing communication with the external world. The first obivious extension was to add at least a TTL to RS232 levels converter and at least one input port to allow to read one bit. At that point the unused bit 7 of the display was used as TX line and another bit as RX line.

Adding RS232
MAX232 level converter and DB9 connector

The first ( mad ) idea was to do something entierely done in software, so a serial communication without any UART but by pure timed SW. That always been one of those situations "it should be possible so let's try to do it", you know .. a proof of concept let's say.

Well yes .. it is possible .. it gets a bit like coding on the VCS .. you change one byte of code and you have to recompute all again. I managed to create a "small console" that way with commands like edit/modify/display memory ALMOST fitting in 256 bytes of code. It was working but not really super useful.

The mini console
The mini console running

So what next ? What could I do ? What I really want to do with this system ? While thinking a bit of this & that many ideas were coming out but many of them ended up in "it would be nice to have some kind of OS".

Can't remember well how but it happened I got in touch with someone that began to talk me about CP/M. Now I only heard about CP/M I never used or even seen it before but someway the idea that it was possible to have something capable to run quite some interesting applications was very appealing to me so I decided to try "the next big step", implement it on my machine.

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