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Old 10 May 2011, 22:30   #30
Stedy
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Join Date: Jan 2008
Location: United Kingdom
Age: 40
Posts: 602
Hi,

Before I continue, I just want to say that I am trying to provide practical advice based on a number of years of experience.

The captured design, will not work as it stands but it can do with some effort.

This design needs decoupling capacitors, the original schematics list 33 capacitors ranging from 100nF to 47uF. As a minimum they must be added to the design. I would also add some 10nF capacitors around the logic and the CPU for higher frequency decoupling.

Have you checked that the TI PAL parts can be successfully programmed with JEDEC files from another Vendor?
If they can not, can you find a device fitter that supports these TI parts so you can re-compile.

Also do we know what speed grade the original PALS were? Can't see the full part numbers here: http://amiga.resource.cx/photos/phot...res=hi&lang=en

In 2011 we should not be designing PAL/GALs unless there is a very good reason, newer, smaller, lower power parts are available.

I would use something like this:
http://uk.farnell.com/xilinx/xc9572-...-5v/dp/1193231
In circuit programmable and PTH PLCC sockets are available.

Reading the documents, there appear to be noise issues. I can see how this could happen.

Going to a 4 layer PCB will help a lot. The power planes and the option of controlled impedances will help but there is more you can do.

There are two types of engineer, those who have signal integrity problems and those who know they WILL have signal integrity problems. The mid 80s PAL/GAL devices were notorious for noise problems, if not handled properly. Rise fall times of 1-4ns can wreak havoc if not dealt with.

The fixes are not too difficult.

Attached is an example of an un-terminated clock signal, not too pretty.

As a baseline, provision for a series resistor, on all signal lines that go off card. So this includes the data-bus and control strobes back to the 68000 socket. Some critical signals, to be defined, may require provision for a small (few pF) capacitor, which with the resistor, forms a low pass filter to remove noise.



If you require any further advice, I am here to help.
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