[Coco] Help with Nitros9 on SDC?

[Don Barber]

Rocky, this is *amazingly* helpful, thank you so much. I have not tested 
on another CoCo as I didn't have access to one before today.

For you and everyone else on the list: this morning I bought another 
batch of Coco gear, which included another SDC.

I get identical behavior with the new SDC. So its likely not the SDC!

Also included in this morning's batch is a stock coco3, so now I have a 
second one. Once I get a 512k upgrade then I'll be able to test on that 
to confirm its something funky with my coco3.

But I have high hopes for using a pull-up resister on IC5/IC7. I'll try 
it later today or this weekend. Thanks so much!

Don

On 1/14/22 9:28 AM, Rocky Hill via Coco wrote:
>   Hi Don,
> Maybe I missed it, but did you try the SDC on  another CoCo?I read through the posts but I didn't see any mention of that.
> Assuming you didn't try it on another CoCo, here is my story.....
>
> I had a very similar issue with my SDC randomly locking up my CoCo 3and occasionally displaying random characters. It was frustratingand it made me yell obscenities at my CoCo 3.
> Most of the lockups I experienced where while launching the SDC explorer,Launching Nitros 9 EOU, or trying to run one of sockmasters demos.Occasionally my CoCo3 would crash when playing a game loaded from the SDC butRegular carts seemed to work just fine.
>
> I wasn't totally surprised this was happening to me because I expected things to act funny as this was while I was testing a recreation of the CoCo 3 board.
> Relative to my new CoCo3 board, most of the evidence pointed to a faulty SDC butI knew the SDC was fine or at least fine enough because it didn't crash when usingit on an original CoCo 2 or CoCo 3.
> Through trial and error I did eventually figure it out andthe SDC no longer crashes when using it on the new CoCo 3 board. The culprit turned out to be a slight difference in the replacement PIAI was using for IC5. The number on the chip for IC5 is "LSC81001P"but it's "6821" compatible with some extra magic to make it safer when readingkeyboard presses.
> WDC makes a drop in replacement for the 6821 and it is what I decided to useinstead of sourcing older PIA's. The part number is W65C21N6TPG-14.
> I know that this is a bit of a stretch in your case but it might be worth a thinkso here are the two things that fixed the SDC's behavior on my new board.
> 1- I replaced the modern PIA with an older 6821. This of coarse was the easiestthing to do but it meant I had to use an older PIA.
> 2- I added a 4.7k pull-up resistor to the SELA line of the SC77526 DAC/ADCchip. This can be added to either pin 11 of IC7 or pin 39 of IC5.I didn't deduce this fix, I simply remembered that the CoCo 2 has a pull upresistor on the SELA line and since I had already zeroed in on thePIA as the culprit, I figured someone had a similar issue when designingthe CoCo 2 so I copied it and it worked.
> I added option two to the latest board revision because it's a much better fixthat allows the board to work with older PIA's and the new replacement one.
> Again, I know our situations are different and I'm not saying you should add a pull-up resistor but consider that the PIA might be misbehaving and at thevery least I think it might be worth a look around the PIA to make surethere isn't anything out of the ordinary.
> That said, adding a pull up resistor with two clips is a really easy test thatwill not hurt anything. 4.7k, 10k, 47k any of those values should work.
>
> I hope you get it figured out soon.
>
> Pedro
>