grinding down chips was Re: QX10 graphics board

Tony Duell ard at p850ug1.demon.co.uk
Sat Jan 28 06:51:49 CST 2006


> 
> so by your reckoning not all of the asics are readily
> reproduceable. What about the gate array on the
> graphics board (lets pretend there wasnt a primitive
> ttl prototype to guide)? What about the 7220 itself?
> Now something like that is bound to have loads of
> docs.

I think there's enough published data on the 7220, and enough working 
samples around to be able to design a replacement without copying the 
existing IC. It would be a lot of work, sure, the 7220 is not a simple 
chip. 

As regards the other ASICs in a QX10, the problem is that some of them 
contain considerable analogue circuitry, and certainly couldn't be 
replaced by an FPGA (IIRC the mains chopper transistor is in one of the 
PSU hybrids). But I thinkit would be possible to work out a replacement.

The disk drive ASICs are harder in that I have no idea what the exact 
functions of them is. That is something I intend to look at fairly soon, 
actually. See what the pins do during a seek, etc.

In general there are 4 classes to consider : 

1) Well-documented, simple. For example a 555 timer. It should be 
possible to make an exact functional replacement from the docs alone.

2) Undocumented, simple. For example a programmed PAL. Simple enough that 
given a working example of the chip and maybe the schematic it's used in, 
you could work out a series of tests to identify the internal logic and 
make some other device that would do the same thing.

3) Documented, complicated. For example the Z80. Again you'd not need to 
actually inspect the chip. Thre are Z80-a-likes around (e.g. as programs 
for FPGAs) that seem to have been designed based on the behaviour of a 
real Z80. 

4) Undocumetned, complicated. A large ASIC, ULA, etc. Since yoy've almost 
certainly got hidden state, you can't always work out what's going on 
from outside. At this point you may well have to consdier removing the 
encapsulation.

-tony



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