Im an idiot
 

Lent my battery to someon earlier to start their 200sx which has been in bits for ages, (which was a success).

Then put it back in my S12, and put it in backwards :(:(:( :trout
i.e. reversed the polarity of the terminals.

Now all of the things on fusible links work
i.e. lights up and down, fuel pump, starter etc.
none of the fuses in the fusebox are gone.
and the diagnostic function of the ECU doesn't work

So I'm pretty sure that I've fooooooked the ECU.

I've had a quick search on a couple of forums, but none of them come with definitive answers, so does anybody have any ideas, what might be wrong with it?

Cheers guys

I still have this ECU for sale.

All sorted now guys.

Cheers