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Feasible to replace a 6520 PIA with a 6521?

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Some background: the 1200XL (UAV and +12V mod; otherwise stock) has a flaky PIA.  I ordered a couple to swap out with the existing one, but ran into the issues with timing discussed in this thread.  Essentially, SIO breaks if I use the two that I just received, but joystick reads on both ports are flaky with the one that the machine shipped with.

 

The two that I received have 1979 and 1981 datecodes and were marked as being 6520As with part number C012298 on them; the one that I pulled had a 1982 datecode and was marked as a 6520AP without an Atari part number on it.

 

AFAIK the replacement PIAs are pulls.  By the datecodes, I'm assuming that they came out of either a 400 or 800.  I have no reason to believe that they are counterfeit.

 

So, coming back to the original question: is a 6521 a drop-in for a 6520AP?  I'm having better luck finding the 6521 than the 6520.

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Do you need anything else in the way of Atari chips? The reason I ask this is to get you over the $20 minimum at Best Electronics.

 

Brad sells the original 6520 PIA that came with your Atari for $1.95 (see Best Custom ICs Page).

 

So if you need or can use a Pokey, that'll take you to $20 all by itself. Note as long as you don't abuse Brad's time, and organize your order in a email with "Atari Parts Order" in the subject line, he's pretty good about letting you order multiplies of most items (exception: Pokey chips are limited to one per order). So what I normally do is order a couple of PIAs, GTIAs, Antics, and Sallys while I'm at it. Just don't go wild with either the number of different items, or the quantities of each, and you shouldn't have any problems.

 

Also it pays to familiarize yourself with his ordering process ahead of time, if you've never ordered from him before.

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Posted (edited)

If you need to replace your stock PiA, then you may consider moving up to a drop-in MOTO. 68B21P.

 

Not just a suggestion, see it outperforming my PIAs on the scope (I already upgraded):

 

 

Faster and higher rises, also faster falls, and I still believe it will drive applications like XEP80 even faster than today's already faster speeds (!)

 

Also a pretty good read before that summary post, above.

 

Edited by Faicuai
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Posted (edited)
13 hours ago, mytek said:

Do you need anything else in the way of Atari chips? The reason I ask this is to get you over the $20 minimum at Best Electronics.

Nope, I'm good on that front for now.  More:

Quote

Brad sells the original 6520 PIA that came with your Atari for $1.95 (see Best Custom ICs Page).

Yep.  That's where the replacements came from; note that I do not hold Brad responsible for this state of affairs.  Continuing on:

Quote

So if you need or can use a Pokey, that'll take you to $20 all by itself. Note as long as you don't abuse Brad's time, and organize your order in a email with "Atari Parts Order" in the subject line, he's pretty good about letting you order multiplies of most items (exception: Pokey chips are limited to one per order). So what I normally do is order a couple of PIAs, GTIAs, Antics, and Sallys while I'm at it. Just don't go wild with either the number of different items, or the quantities of each, and you shouldn't have any problems.

 

Also it pays to familiarize yourself with his ordering process ahead of time, if you've never ordered from him before.

Believe me, I've ordered from him many, many times in the past :-D Very familiar with his requirements, and really don't have an issue with working within his bounds.

Edited by x=usr(1536)

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12 hours ago, Faicuai said:

If you need to replace your stock PiA, then you may consider moving up to a drop-in MOTO. 68B21P.

I'm not opposed to the idea.  Having said that:

12 hours ago, Faicuai said:

Faster and higher rises, also faster falls, and I still believe it will drive applications like XEP80 even faster than today's already faster speeds (!)

Bear in mind that the machine that needs the PIA is the 1200XL, which are known to have sensitivity with FujiNets and timing.  I'm good with fast, but being able to boot from a FujiNet is a non-negotiable requirement.  Is this something that the 68B21P can do reliably?

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4 hours ago, x=usr(1536) said:

Bear in mind that the machine that needs the PIA is the 1200XL, which are known to have sensitivity with FujiNets and timing. 

They are? Says who?

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Posted (edited)
4 hours ago, x=usr(1536) said:

I'm good with fast, but being able to boot from a FujiNet is a non-negotiable requirement.  Is this something that the 68B21P can do reliably?

Well, I am jumping myself into Fujinet once v1.6 revision is mainstream.

 

I would then be able to tell you myself ! 😁

 

(NOTE: can't find a reason at this point for not working properly with Fujinet).

 

Edited by Faicuai

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6 hours ago, DrVenkman said:

They are? Says who?

Good question, and it appears the answer is down to a misrecollection on my behalf of comments made in this thread a while back.

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6 hours ago, Faicuai said:

Well, I am jumping myself into Fujinet once v1.6 revision is mainstream.

 

I would then be able to tell you myself ! 😁

 

(NOTE: can't find a reason at this point for not working properly with Fujinet).

Fair points :-D  FWIW, I've swapped back to the original PIA and SIO is working again.  My best guess at this point is that this 1200XL (don't know about others; someone else would need to confirm) doesn't like something about the C012298 PIAs.  If had a 400 or 800 handy I could test them in contemporaneous hardware, but as I currently only have XL machines that's not a possibility right now.

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2 hours ago, x=usr(1536) said:

  My best guess at this point is that this 1200XL (don't know about others; someone else would need to confirm) doesn't like something about the C012298 PIAs.  

Whatever the issue with that 1200XL, it's not likely to be PIA-related. PIA is only used minimally for SIO operations, handling the cassette MTR control line. So unless you've got some kind of cassette loading problem, it's not likely any other problem would be connected with the PIA at all. I've got three 1200XLs, all using stock vintage Atari-numbered 6520's, and none of them have any issues with my FujiNet devices. Similarly, I've got several 800's, 400's, and a 64K-upgraded 600XL, and none of them have PIA-related issues, though in the interests of full disclaimers, I haven't dug them all out and specifically tested my FujiNets with all of them (though I have in fact used them regularly with one of my regularly-used 800's and one of my 1200XL's)

 

I also have three of @mytek's modern A8 designs - a 1088XEL, 1088XLD, and 576NUXC+, all of which I've built myself. Although these latter three are new designs for the motherboard, they all use vintage A8 chips and none of them have issues with vintage PIA chips either. And again, I have used my FujiNet devices with all three of these regularly as well. 

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PIA also handles /Command but it's not timing critical to the point where several cycles lag in state change would make a difference.

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On 5/9/2021 at 4:33 PM, DrVenkman said:

Whatever the issue with that 1200XL, it's not likely to be PIA-related. PIA is only used minimally for SIO operations, handling the cassette MTR control line. So unless you've got some kind of cassette loading problem, it's not likely any other problem would be connected with the PIA at all.

While I'm not disagreeing with what you're saying, the behaviour I'm seeing is definitely affecting SIO.  More:

On 5/9/2021 at 4:33 PM, DrVenkman said:

I've got three 1200XLs, all using stock vintage Atari-numbered 6520's, and none of them have any issues with my FujiNet devices.

And I have no problems with either of my FujiNets on the 1200XL - provided that the no-part-number PIA with the 8239 datecode is the one in the socket.  Same goes for the 1010, 1030, and 1050.  As soon as I swap it out for either of the PIAs with the 7951 and/or 8132 datecodes, SIO breaks.

On 5/9/2021 at 4:33 PM, DrVenkman said:

Similarly, I've got several 800's, 400's, and a 64K-upgraded 600XL, and none of them have PIA-related issues, though in the interests of full disclaimers, I haven't dug them all out and specifically tested my FujiNets with all of them (though I have in fact used them regularly with one of my regularly-used 800's and one of my 1200XL's)

Neither of my 600XLs nor the 800XL have PIA issues either - those three machines are 100% functional, and as far as I know the only PIA-related issue on the 1200XL is poor joystick detection.

On 5/9/2021 at 4:33 PM, DrVenkman said:

I also have three of @mytek's modern A8 designs - a 1088XEL, 1088XLD, and 576NUXC+, all of which I've built myself. Although these latter three are new designs for the motherboard, they all use vintage A8 chips and none of them have issues with vintage PIA chips either. And again, I have used my FujiNet devices with all three of these regularly as well. 

Those I'll have to take your word on; I have zero experience with them.

 

In any event, I've ordered a couple of 68B21 PIAs and will see what happens.  I'd like to get a little bit more investigation done around the general area of the PIA, so figure those are a good starting point.  If they go in and work, great, problem solved.  If not, well, at least I know there's something else that needs to be tracked down.

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3 minutes ago, x=usr(1536) said:

As soon as I swap it out for either of the PIAs with the 7951 and/or 8132 datecodes, SIO breaks.

Forget the date codes - who's the chip fab? A 7951 date code is REALLY early for a 1200XL. That's like first-generation 400/800 timeframe.

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Posted (edited)
15 minutes ago, DrVenkman said:

Forget the date codes - who's the chip fab? A 7951 date code is REALLY early for a 1200XL. That's like first-generation 400/800 timeframe.

Agreed.  The PIA that was originally removed from the 1200XL has an 8239 datecode and no Atari part number, which is way more in keeping with the 1200XL's timeframe.

 

The two that I bought were both made by Synertek, which are the ones with the 7951 and 8132 datecodes, plus the Atari part number C012298 (C012298-03 on the 8132 PIA).

 

Late edit: I should have mentioned that both of the PIAs I ordered have passed the rubbing alcohol test, so I don't have any reason to believe that they're counterfeit.

Edited by x=usr(1536)
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I'd love to put one of those questionable Synertek chips in one or two of my 1200XL's and run some tests sometime. I don't have a working tape drive around here (plenty of them that all need belt replacements though), but I do have a couple FujiNet devices, an SIO2PC device, and a pair of Happy 1050's. It would be an interesting exercise to see how they behave in different systems.

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