Need to sync two power sources for retro project

[thingsis]

Hi,

I am currently designing my own SBC around a Z80 that I picked up - basically like everyone else has. Now as I don't know much about hardware I started out with some simple things like building some logic around diodes and playing with latches and...
Now the next step was to find a way to burn my EEPROM. Buying a programmer seemed not necessary as building one will teach me how these chips work and doesn't seem too difficult. Anyway, I got out my Arduino and made it write stuff to the EEPROM. Even that worked first try. Now here is what I want to do next: Use my C128 to send the data to the Arduino, which then writes it to the PROM. I have a z80 cross assembler for the C128 which I am going to use.
Yes, this might not make ANY logical sense - but it sure is FUN and the C128 is really easy to interface!
My questions is this: I need to sync the TTL levels of the Commodore and the Arduino. Do I just connect the two GND lines? Will I blow things up? How do I get these in sync? I kinda try to avoid destroying the C128... 
Once the burner is working I will post its design - the Arduino could be interfaces with any device so it should be useful even without that fun old Commodore... 

Thanks a lot!

[thingsis]

Hi,

maybe I can rephrase my question a little: Let's assume you would have two uC talking to each other. How would you make sure that both levels are compatible? By connecting both grounds together or would that fry them?

Thanks a lot!

[Pillager]

You would have to make sure, that the 2 supplies are isolated. If the 2 supplies are somehow connected to the mains, as some types of PSUs are, you could blow something up (there is something about an oscilloscope blowing up in one of Dave's videos).
If they are both isolated, you could connect the ground.
Or, to be absolutely on the safe side, you could use optical isolators. They basically take the output of one µC and drive an LED. The input of the other µC is connected to the phototransistor, which switches when the LED lights up. The two controllers are isolated and you don't have to worry about one shorting out or overloading the other.

[thingsis]

Hey Tom,

thanks a lot. That really helped. And I did end up watching Dave's video about the oscilloscope. As far as I understand I should be safe as long as I power the Arduino with a battery or a simple cell phone charger that does not have a ground connection. I don't know about the commodore, but it wouldn't even matter.
Thanks again!

Henrik

[Pillager]

Yes, when using a battery for the Arduino, you should be fine. Cell phone chargers, I don't know, haven't really looked into one before. You could do a few voltage measurements (between the 2 GNDs, etc.). If you measure a voltage, they are likely to be connected somehow.

Grüße ans Nachbarland