[Coco] Help with Nitros9 on SDC?

[Don Barber]

Quick update: adding a pull-up resister to the SELA line didn't change 
anything. Didn't notice anything unusual 'around' the PIA either. Ug. 
Will report back when I upgrade my new-to-me coco to 512k.

Don

On 1/15/22 12:11 PM, Don Barber wrote:
> 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
>>