I thought I would have a look at the cart ROM, specifically
this one and see if I could work out the connections for the IEEE port. I haven't done that yet but I have found some things about the cart.
It outputs a value to the I/O-2 area if it fails, this suggests that there may have been plans for a cart with a POST display of some sort, perhaps a single digit seven segment LED or similar, so you could still get some idea of what was working even if the video wasn't.
It polls the I/O-3 area during one of the IRQ routines and stops if one bit is set, I'm guessing this was for a pause switch or suchlike.
I think this cart image is for the later VIC as there only seems to be reference to two RAM chips apart from the first 1K and colour RAM, there is other code that could be for more but the message pointers reference other code and not messages.
The code is a mess, there are at least three clear screen routines two of which get called in succession. There are at least three ways it outputs "OK" to the screen. There is a basic screen output routine that can handle cursor x,y positioning, scrolling etc. but this is bypassed for a lot of the screen output. There are registers set and then the values never used before they are set again and the NMI routine trashes the X register every time because someone wrote TAX and not TXA in the code to save the registers.
Subroutines are used before the RAM at $0100 to $01FF is tested.
At one point the code tests to see if the byte at $F75D is $4C, none of the VIC ROM images I have have $4C at this address.
There is a days/hours/minutes/seconds/ticks counter that rolls over from 98/23/59/59/59 to 00/00/00/00/01. I seem to remember that the VIC kernal does something similar in that it counts one jiffy too many per day.
When a problem is discovered the code usually dead ends in a loop.
There are a few areas that seem to be messages that aren't referenced. There's a whole table of things from $ACB1 on but the only bit that seems to be used is the word "BAD" at the beginning.
That's it for now, I'm sure there's more. 8^)=
Lee.