And on it goes. The last days i spent finding out, why the i2c interface did not work. Hardware is ok, it's a problem with the software. First i thought, all i2c eeproms have a block size of 64 bytes. But that's not that easy. Block size varies with eeprom size. And manufacturer… :-| But most 8k eeproms have blocks of 32 bytes and most 16k and 32k eeproms have blocks of 64 bytes.
Then busy polling after a block write did not work properly. I started a read cycle to detect the busy state (the eeprom does not respond if busy) but this behaviour is only defined if you start a write cycle. So i rewrote my source to do this.
Then somehow my start and stop sequences on the i2c bus sometimes failed. Now i test-read the data line to see if it is high and not pulled low by the target, for whatever reason.
Now access to the K1 bus eeproms works properly and hopefully reliably. I successfully downloaded the SIO driver code into the SIO driver eeprom and booted the CPU with driver initialization from eeproms, and not with data from the microcode. And it works! :-)
And, last not least, selecting the i2c eeprom on a K1 bus card works as planned: All eeproms are addressed with address 0b000, but only the eeprom on the currently selected extension card actually has this address. The i/o cards disable their eeprom while they are not selected, and the easiest way to do this is to set any address pin of the eeprom to '1', so that the address does not match.