Saturday, April 8, 2017

Using a FTDI USB-Serial connection with the Z80_mini

The Z80_mini is another minimalist, Grant Searle-style Z80-based single board computer.

Its creator, Bernd Ulmann, was generous enough to make the Eagle PCB files available on his website. I went ahead and had a set (of 3) made with OSH Park.

You can tell that the Z80_mini is a pre-Arduino design: (a) the power supply connector is the type used in PC-era floppy/hard drives, (b) there is a MAX232 and a 10-pin connector for a traditional RS232 interface.

I was able to make a little tweak to use an FTDI-style serial interface along with power routing from the FTDI connector.

The theory behind this is illustrated in the following picture:

As you can see, Pins 11, 12 and 15 of the MAX232 matches the similar signal pins of the typical FTDI header.

So if you were to insert a 6-pin header from pins 10 to 15 of the MAX232 location, you will essentially have an FTDI interface. The following pictures shows how this will look.

The FTDI cable plugs in as shown in the following picture. The GND line is the black one on the right.

If you want to power the Z80_mini board from the FTDI connector, all you need to do is to wire a link from pin 13 of the MAX232 (or pin 3 from the FTDI header) to the nearest VCC signal, for example, on pin 14 of the 74xx14 chip.

The only caveats with this arrangement are:
a. You're not using the RTS line. In fact, this is not connected from the 16550 at all
b. You'll want to make sure that you use low power chips to not overload your FTDI adapter

Otherwise, voila!

I really like the Z80_mini design and will write another blog about what else I've done with it.

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