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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 12/13/2020 at 19:200 Comments

Thanks to @Al Williams recent writeup a few questions came up about the Internet connection, "does this have ethernet? Or does it use PPP over that serial line". Well basically, all of the above.

The physical data connection to the board is RS-232-C running at 9600 baud (8-N-1) with RTS/CTS flow control. There's a couple of options from here to get to the Internet. The classical method is via a serial line protocol like SLIP or PPP to a dialup modem. This requires a TCP/IP stack on the machine to handle the rest of the layer-2 and layer-3 network protocol. This would involve porting a stack like uIP and is still some way off in terms of development.

An easier way to connect is via an IoT Wifi/Ethernet-to-UART module. Shown below is the Novasaur with one of these modules to support an Ethernet network connection (also shown with HDMI).

These modules are a bit of a cheat though. They not only adapt the physical Wifi/Ethernet interface but also contain a micro-controller to handle the TCP/IP connections. The payload is pulled out of the protocol and then sent over the RS-232 like a simple UART serial connection.

In fact, the current serial terminal program can already display protocols such as HTTP. The (blurry) image below shows a browser connecting to the Novasaur and asking for a web page. The HTTP protocol is just echoed to the screen, but a client program could interpret this and serve up a web page in response.

A web server is also some way off. The good news is the 8080 CPU is partially tested and running. There's still a lot more to test and plenty of bugs to chase down over the next few weeks. After that a simple monitor program can be added and the work to bring up CP/M can begin.

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