[Coco] Aussie 2 meg installation - results

[Robert Gault]

Bob Devries wrote:
> Well, I installed the 2 Meg board as per instructions (Well, almost).
> 
> Firstly, I just connected it as I would expect it to work, that is the 
> DAT board is in, the memory board is in with 2 1MB SIMMS. 4 wires go 
> from the DAT to the mem board, and one from the mem board to GIME pin 2, 
> and one from the mem board to Pin 40 of IC5.
> 
> This did not give a workable screen, however, when the plug on the mem 
> board was removed, BASIC came up as usual.
> 
> I then inserted a 150 ohm resistor in series with the wire from pin 2 of 
> the GIME (the resistor as close as I can get to the GIME), and fired up 
> again.
> Hey presto, it comes up right now.
> 
> Run the supplied MEMTEST program.
> 
> This goes through until it gets to block 47 ($2F), and quits - BAD MEMORY.
> If I now pull the plug off the mem board, and run the test again, it 
> does the same; i.e. gets to block 47 and quits.
> 
> A couple of things come to mind:
> - The Coco3 I am using may be flaky.
> - The 2 Meg kit may be faulty. I have no way of testing this.
> - the wires between the DAT and MEM board are too long and/or need some 
> shielding. (the wire from GIME pin 2 needed to be run from the bottom of 
> the motherboard. Maybe I should run that in shielded cable?
> - The SIMMS may be faulty.
> 
> Any comments from the hardware gurus?
> 
> -- 
> Regards, Bob Devries, Dalby, Queensland, Australia

Assuming this memory test program is doing what I think it should, you 
don't even have 512K active memory.

What does "pulling the plug from the memory board" mean? Does that 
disconnect the four wires going to the DAT or something else? If the 
memory board is disconnected, does that mean there is still 512K 
conventional memory installed in the Coco3?

  If the simms are faulty, swapping them might show something of 
interest. What happens if you try running with just one simm?
  If the original memory is still installed, I'd remove it as that is 
what is required with all/most add-on memory boards. Also caps 65&65 are 
typically cut. These load the CAS/RAS lines.
  Your choice of 150 ohms for the resistor is based on the NTSC 
schematics but you don't have an equivalent circuit. It seems you need a 
high impedance input to the memory board to prevent degradation of the 
master clock signal. However, without a scope on GIME pin#2 you can't 
tell whether 150 ohms a large enough value. Still as the Coco boots, 150 
ohms probably is OK.