Hi,
I actually bought a variable AC-DC adaptor with the right connector. Brand is NewStar. It supplies 6V,9V,12V,18V,19.5V,22V up to 3.5A and can adjust the polarity. It cost about 50 bucks.
I set the setting to 22VDAC and with the correct polarity and plugged it into my brand new Roomba 560. It blinked for 3 seconds and give "Charging Error 2".
Did some search on the web and found that this Roomba has program its charging circuit such that we can't just use an ordinary AC-DC adaptor. What happened is, as my new Roomba was not fully charged (brand new mar), the voltage of the battery is quite low (about 14.5V according to what I read on the web), putting in a 22V charging current will cause "Charging Error 2". So I have to switch my output to 18V and it will charge alright. After about 30 minutes, when the battery is sufficiently charged (I guess it's about ~17V), I can slowly switch to 20V, and no charging error, and another hour later I can switch to 22V without anycharging error.
The next day, I let the roomba sweep my house for 1 round, for about 40 minutes. Then i press dock for it to go back, it now shows "Charging Error 2" again. I think this is again due to the supplied current is 22VAC and the battery has dropped beyond 18V. So I switched the supplied voltage back to 18VAC again and there the cycle repeats.
From what I observed, it seems that roomba actually monitor the differences between the supplied and the battery voltage and will give error if the gap is too big.
So now the newstart AC-DC adaptor is being used for my notebook and I bought another 135W Newstar convertor to use with it.