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Classic II FPU card design


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Hi everyone,

 

I spent some hours of my lockdown time during Christmas and worked on a simple FPU card for Macintosh Classic II. I've bought this little Macintosh some years ago - it just needed a recap to bring it back to life :)

 

Yesterday, I have assembled the FPU board and I'm happy to announce that it works flawlessly and brings some sweet Floating Point performance improvements (more than 13x over no-FPU) 8-) (not that many applications actually take a good advantage of that)

classic_ii_benchmark.png

 

I have open-sourced the KiCad files so that everyone can use the project or potentially improve it or extend it:

https://github.com/dymczykm/classic_ii_fpu

(it's under a GPL license so please (=you really should) open-source any modifications on top of this original design)

 

If you would rather order the PCBs directly, I've created a PCBway project:

https://www.pcbway.com/project/shareproject/Macintosh_Classic_II_FPU_card.html

 

IMG_3241.jpg

 

IMG_3247.jpg

 

Hope this project comes useful to the Classic II owners |)

 

 

Edited by moldy
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Nice work! There's a guy on eBay that been flogging off FPU cards fro some time (with a ROM socket) but damn they're pricy for what they are. This design definitely looks far more affordable.

 

I do hope someone here can fabricate some and offer them on here as a group buy. I'm sure I'm not the only one that would like to add an FPU to their Classic II, at the right price if course.

 

How was it trying to solder a PLCC socket on the board?

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Soldering was not too bad, just a bit boring with that many pins ;) Yeah, adding a ROM socket would considerably complicate the board (potentially call for more layers) and I think most people will be fine with just a cost-efficient FPU extension.

 

Regarding the group buy - I still have 4 spare boards and sockets so I might consider just soldering those few for early adopters. I'll keep you posted :)

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4 hours ago, moldy said:

Regarding the group buy - I still have 4 spare boards and sockets so I might consider just soldering those few for early adopters. I'll keep you posted :)

 

Well you can definitely colour me interested! I have a spare FPU somewhere and I can source a crystal without much trouble. 

 

Keep us (especially me!) posted. :)

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@moldy great work!  I don't remember seeing another for sale before.  After you run out of your boards, if you'd like to have me see about having a small batch professionally manufactured (if demand here exists), I'd be happy to help do that and can send you some kind of payment for units sold.  I don't have a Classic II but this kind of gives me motivation to want one now. 

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Nice project and thank you for open sourcing it! Sign me up for a kit or board if you get em.

 

Where are you sourcing the connector, crystal, and FPU? I've seen other posts where people try to buy FPU's for other machines and get fake ones or ones that are incompatible.

 

I'll trade you one of my boards for one of yours - let me know :)

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4 hours ago, maceffects said:

@moldy great work!  I don't remember seeing another for sale before.  After you run out of your boards, if you'd like to have me see about having a small batch professionally manufactured (if demand here exists), I'd be happy to help do that and can send you some kind of payment for units sold.  I don't have a Classic II but this kind of gives me motivation to want one now. 

They're really under appreciated compact Macs – the ROM is 32-bit clean, for example! I'd be interested in making a stripped down System 7 romdisk.

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8 hours ago, rplacd said:

They're really under appreciated compact Macs – the ROM is 32-bit clean, for example! I'd be interested in making a stripped down System 7 romdisk.

I’m sure someone here has the skills to add in a larger ROM on this. That would be so awesome. 

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Thanks a lot for your responses, I will be happy to prepare few boards I have left within few weeks + we can think how to organize the boards in a larger quantity if there's interest.

 

13 hours ago, erichelgeson said:

Where are you sourcing the connector, crystal, and FPU?

@erichelgeson The connector comes from Digikey (A34313-ND), the oscillator is a standard DIP-8 sized (but with 4 pins) part (e.g. MXO45HS-2C-40M000000) which you can find pretty much everywhere. I've got the FPUs locally, it seems someone playing with Amigas was selling out their stock.

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On 1/27/2021 at 12:09 PM, tokyoracer said:

Nice work! There's a guy on eBay that been flogging off FPU cards fro some time (with a ROM socket) but damn they're pricy for what they are. This design definitely looks far more affordable.

 

I do hope someone here can fabricate some and offer them on here as a group buy. I'm sure I'm not the only one that would like to add an FPU to their Classic II, at the right price if course.

 

How was it trying to solder a PLCC socket on the board?

 

 

That has a lot to with the cost of a 4 layer board, even the connector is 10$. My design was released on this forum also.

Edited by 360alaska
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68882's? Yes I have a bucketload of them already, but UTSource have certified NoS ones in stock for $18 US a piece (FN33's), or used certified working FN40's, mix of freescale and motorola made parts with date codes ranging from 2000 to 2009 for about $2.79 US a piece.

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OK well with Motorola stuff:

 

MC = Qualified part

XC = Unqualified part


The letters after the part number (ie, 68882, 68000, 68681, 68230, 68851 etc) denote the package, then the speed rating.

Package types:

 

P = Plastic DIP

L = Ceramic DIP

FN = PLCC

FE = QFP

RC = PGA, Ceramic

RP = PGA, Plastic

So MC68882FN40 is a 40Mhz 68882 in a PLCC package and a MC68882RC33 would be a 33Mhz 68882 in a Ceramic PGA package.

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4 hours ago, LaPorta said:

Next question is would there even be benefit to putting a 40 in a Classic II?

You can see a difference in Floating Point performance between the system clock (16 MHz) and an external clock (40 MHz) in one of my screens. It's rather little. Probably using a 33 MHz version would be very very close to 40 MHz.

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11 hours ago, maceffects said:

Thanks, I remember some processors used to be really fussy about that. I’m glad this allows for flexibility. 

If you look at my screenshot from MacBench, you can see the results for two frequencies (@16M and @40M; the CPU was always at the default 16M). 40M gives a slight boost to the benchmark score. The card itself has a jumper to configure which clock to use (OSC means the oscillator on the card, SYS is using the C16M signal from the logic board which is 16M).

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