[Coco] Help with Nitros9 on SDC?

[L. Curtis Boyle]

On Jan 26, 2022, at 5:01 PM, Don Barber <don at dgb3.net> wrote:
> 
> Hi folks, wanted to send an update. I narrowed it down!
> 
> Basically, I got another coco3, and bought a 512k sram update and 6309 for it.
> 
> When I put the 512k sram into my existing coco, EOU6 boots up great!
> When I put the old 512k card into my new coco, I get the same behavior!
> 
> So clearly the old 512k card is faulty. This is despite passing every single ram test I throw at it.
> 
> The old ram upgrade is a Tandy AXX-7117. I can't find any reference to it online, but its identical to the picture in https://colorcomputerarchive.com/repo/Documents/Manuals/Hardware/512K%20Expansion%20RAM%20Card%20%28Tandy%29.pdf which is part no 26-3334.
> 
> As the 6809 EOU boots fine on the old RAM upgrade, I suspect its something specific that EOU is doing when using 6309 instructions...perhaps the TFM instruction? Maybe the old RAM just doesn't switch fast enough? No idea, just thinking out loud.
> 
> So...curious if anyone has any advice on testing the old ram card and determining whats going on there? The chips on it are MT 1259-15. There are also several capacitors and one resistor. Nothing looks blown or anything. The card does get warm. Not a big deal to simply buy a new 512k upgrade but willing to try to a few things to get this one working too.
> 
> Thanks,
> 
> Don
> 
   It could very well be RAM speed. 6309 EOU runs in 6309 native mode, which does run faster including a cycle out of accessing the RAM. I do remember that 150 ns RAM chips were hit or miss on the Coco 3 - some would work ok, others would have lock-ups, junk on the screen (especially when using hardware scrolling,etc.) . Once RAM upgrade manufacturers upgraded to 120 ns most of those problems went away. I know we had to take a few minor speedups out in the final versions of the original NitrOS-9 (back in the early 1990’s) because the GIME itself had difficulties with faster access to it’s registers while in the middle of the vector page RAM and/or GIME I/O access.