iMac G5 won't boot, external interrupt error

I started my iMac G5 and did get the chime but it automatically goes into OF mode. The fans run loudly and it continously scrolls an "external interrupt" error with a bunch of jargon that goes by so fast I can't read it.

I tried resetting SMU, none of the bootup key sequences work, I can't boot from disk and it won't go into target disk mode. I've searched online "external interrupt" which has been an issue for others, but haven't found an answer to the problem yet.

I just tried resetting SMU again. This time I got the chime again, but now have a black screen. After about 30 seconds of nothing the fans start running and the screen remains black. Help please!