Implementing CP/M on the eZ80 MDS board

DRAFT 21sep05

This page will describe my experiences with getting a CP/M machine of out the Zilog eZ81 MDS development board. It's nothing more than my practical supplement to what's on www.vegeneering.com. I'm not a hardware person, so on the downside: beware of mistakes. On the upside: if I can do it, then anyone can...

Enabling simultaneous use of Serial and SPI

The MDS uses pin PB6 as the chip enable signal for the MAX232 serial chip. But PB6 is also part of the SPI. The solution suggested on the eZ80 Yahoo group is to cut the chip select and tie it down to ground, so the MAX232 is always active.

For this, a wire leads from U6, pin1 to GND to permanently enable the serial port. The gray wire runs from the bottom left of the board to the upper right, where pin 1 of U6 is linked to it, and disconnected from the board. Unsoldering the pin is a bit difficult – see the Yahoo newsgroup conversation on this in September 2005.

The MMC card reader

I removed an MMC connector from a 9-in-1 USB card reader – cheaper than buying the connector separately, I thought. The connector fits to perf board nicely, I used part of a Eurocard with a DB9 solder pad that connects the connector pins neatly with solder terminals:

Linking the MMC with the eZ80 MDS board

I used a normal 60-pin flat cable connector. All pins required for the MMC adapter are on JP2 of the MDS, between pins 9 and 18.

There is one thing that caught me off guard. JP2 arranges the pins in the format of:

2 1
4 3
6 5
8 7
...etc

That's the mirror image of what you expect from a flat cable connector, you'd expect

1 2
3 4
5 6
7 8
...etc

So, check which pin is which number before soldering them on to the MMC connector. The point is, the flat cable runs the wires as

10
9
12
11
14
13

Instead of

9
10
11
12
13
14

Very annoying, count carefully.

Schematic

Below is the connection between JP 2 on the MDS board and the MMC connector. Note the two 10K resistors. Apologies for the draft...

The source for this, obviously, is the original schematic on www.vegeneering.com. That one bases itself on the eZ80 module connectors rather than the JP2 connector on the MDS board, hence my additional schematic.

Good luck! o<dot>vermeulen<at>altis<dot>ch.