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Archive for the 'Macintosh Floppy Emu' Category

Designing a Laser-Cut Enclosure

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An official laser-cut case for Floppy Emu has finally arrived! It took a lot of prototyping and fiddling with tiny parts, but I hope you’ll agree the result was worth it. The case is only 6mm larger than the board itself, accommodates boards with the extension floppy connector or the built-in connector, and features light pipes to channel the LED’s to the outside of the case. The whole thing is cut from a single sheet of 3mm acrylic, and assembles like a 3D jigsaw puzzle.

I hope to offer these cases for sale soon, but I need help determining how many to make and in what colors. If you might be interested in a case, drop me an email or leave a comment below, and mention your color preference. Cases will probably be $19, and the color will be either clear or black.

 
Case Design

I was surprised how challenging it was to make a “simple box” case. My first plan was to design a 3D printed case, but I quickly abandoned that idea once I realized how expensive it would be. 3D printing is an impressive technology, but home 3D printers don’t really have the necessary reliability or speed for consistent manufacturing, and the material cost is significant. Online 3D printing services like Shapeways are another option, but they’re even more expensive. I also doubted I had the necessary 3D modeling skills to design a workable 3D case, so the idea never got very far.

The best alternative seemed to be a laser cut case, constructed of multiple flat pieces assembled into a box shape. Adafruit’s laser-cut enclosure design tutorial was a big help, as was the web-based design tool MakerCase. Before I knew it, I’d designed a basic six-sided box of the proper dimensions, with finger joints at the edges to hold it together. But would it work? Designing a case using finger joints this way requires compensating for the kerf – the thickness of the laser cut. Assume a zero sized kerf, and the box won’t hold together. A 5mm wide tab will end up closer to 4.8mm wide after cutting, while a 5mm wide slot will end up closer to 5.2mm, and the tabs will sit loose in the slots. To compensate, the tabs in the design file should be slightly wider than the desired finished size, and the slots slightly narrower, but not too much. Overestimate the kerf, and the finished tabs will end up wider than the slots, preventing the parts from fitting together at all.

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Next I added holes for case screws, the SD card slot, and the extension cable. Easy enough. But what about the buttons? Floppy Emu has four pushbuttons that are needed to operate it, so I couldn’t just seal them up inside the case. I could have cut a big finger-sized hole in the case lid above each button, so you could reach in and press it, but that seemed ugly and awkward. I could also have left the area above the buttons entirely uncovered, but that seemed even less appealing. If I were designing a product that was *always* in a case, I could have switched to a different type of push button with a long plunger that extended outside the case. But I’m not, and that would be goofy for everyone using a Floppy Emu without a case.

button-plunger

 
The Stick

I finally concluded the only decent solution was to use some kind of stick to poke through a small hole in the top cover, and press the push button inside the case. This proved to be tricky to get right. If the stick were just a straight shaft, it would fall out if the case were turned upside down. And there was no positive force holding the bottom end of the stick onto the push button. It might wobble around or even slide off the button entirely, causing the whole stick to fall down inside the case. My solution was to add a crossbar to the stick to prevent it from falling out, turning the stick into a sword, and hoping that a tight fit between the sword and the hole would prevent it from sliding around. The light tubes used the same sword design, but modified in size to fit on top of an LED instead of a button. Voila! A finished design.

button-stick

I sent the design file off to Ponoko for manufacturing, and about a week later I received the laser cut parts in the mail. With eager anticipation I separated the parts, fit them together, and bzzzzt! I had overestimated the kerf, and the parts didn’t fit together at all. Total failure. I went back to the design file, reduced the kerf estimate by half and made a few other mods, and sent revision 2 off to Ponoko. Another week passed. Finally I got the new parts, and it worked! Sort of.

The rev 2 case fit together, and the Floppy Emu board fit inside of it, so that much was looking good. But the swords had big-time slippage problems. They were too loose, and were constantly wobbling around or slipping down inside the case. For revision 3, I made the swords a bit thicker relative to the holes, so they’d fit more tightly and have less room to wobble. I also added “feet” to the swords, to help keep them centered on the buttons and LEDs.

button-sword-with-feet

Another week went by, and when the rev 3 case parts arrived, everything looked pretty good. The button swords still wobbled a bit, but not far enough to cause problems or fall off the button. The LED swords were more problematic, and sometimes wobbled off the LED’s centers, but generally stayed close enough to continue working as light pipes. Before offering these cases for sale I’ll probably do a rev 4 design to tighten everything up a little more, but rev 3 is definitely useable. Hooray!

Rev 3 also includes two alternate versions of the base and left side pieces: one for boards with a built-in floppy connector, and one for boards using the extension connector. That makes eight total side pieces, of which you’ll use six.

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Sword Assembly

The only aspect of the design I’m not thrilled with is handling of the swords during assembly. How do you get those little buggers in there and aligned correctly, before you put the top on the case? You can’t just balance the swords on the buttons and then lower the top plate onto them – the swords won’t balance by themselves. One option is to assemble everything upside down: put the top plate upside-down on the table, then place the swords into the holes in the top plate, and finally lower the inverted Floppy Emu board onto the whole assembly. That works, but it’s pretty awkward.

The best solution I’ve found is to do assembly right-side up, and use tape to temporarily hold the swords in the top plate. You assemble the bottom and side pieces normally, and place the Floppy Emu board inside. Then you loosely cover all the top plate holes with tape, and push the swords up from underneath until their top surfaces touch the tape. Now you’ll be holding a top plate with all the swords dangling down under it. Finally, you lower this whole package onto the rest of the assembly, add the case screws, and then remove the tape. It’s not the most elegant system, but it works.

button-with-tape

 
Colors

I originally planned to design the case in black, to give the final product a sleek iPhone-style appearance. But when I did a prototype in clear acrylic, my wife loved it and predicted it would be much more popular than black. One big advantage of a clear case is that it won’t need light pipes at all, since you can see the LEDs inside. The material is also a bit cheaper. But the etching on a clear case is difficult to see from some angles, and the final result with its exposed internals looks more like a science fair project than a professional product. What do you think? Which would you prefer?

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emu-top-clear

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LEGO Case for Floppy Emu

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Chris Siegle has designed an amazing Floppy Emu case made entirely out of LEGO bricks! The case features LED “light tubes” and working buttons. Step-by-step build instructions are here. Thanks Chris!

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Floppy Emu “Scratch and Dent” Sale

Sometimes a Floppy Emu board fails one of my functional tests, and I can’t find the cause of the problem. I have several boards that appear to work fine for 400K and 800K disk emulation (tested on a Mac Plus and Mac 512K), but that don’t work reliably for 1.4MB disk emulation on newer Macs. Instead of throwing these in the trash, I’ve decided to sell the “scratch and dent” boards for $15.

If you’ve got a Mac 512K, Plus, or older Mac SE that only supports 400K and 800K disks, one of these boards might work well for you. Because the boards failed some of the functional tests, there’s definitely a problem with them, so keep that in mind when deciding between these and the regular Floppy Emu boards. Scratch and dent boards are warranted for 400K and 800K operation for 30 days. The 1.4MB emulation on these boards isn’t guaranteed to work, but maybe you’ll get lucky. :-)

These Floppy Emu boards have a built-in connector, and are physically identical to those that normally sell for $89.
 
Update: The scratch and dent boards have all been sold. Thanks for the interest!

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Fresh from the Factory

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It’s been quiet here in electronics hobby land, but I do have some good news to report: as of now, all Floppy Emu boards are professionally assembled by Microsystems Development Technologies in California, USA. No more hand assembly! It’s a glorious thing to receive a big box stuffed with assembled boards, and as good as a kid opening a package on Christmas Day. Microsystems wasn’t the cheapest option I found, but they weren’t too far off. I was convinced to go with them thanks to their quick and helpful answers to my many questions, and by their nearby location in San Jose. That’s a short drive from where I live, so when the boards were finished I was able to drive down there and meet the owner in person, and discuss potential changes for future board revisions. That alone was worth the cost difference versus slightly cheaper Asian alternatives.

Microsystems took my design files and bill of materials, and handled everything from there. They made the PCBs, purchased the parts, assembled everything, programmed the chips, and ran the board self-test. That’s a huge time savings for me, and it also removed a major source of potential faults because they handled all the tricky surface-mount work.

Unfortunately, the “finished” boards from Microsystems still aren’t quite ready to sell. It takes another 15-20 minutes of labor per board for me to attach a DB-19 connector (or build a DB-19 extension cable, depending on the type of board), assemble an LCD module, adjust the LCD contrast, and run the board through real-world file copy tests on a couple of vintage Macs. I thought Microsystems wouldn’t be able to handle those steps very easily, so I asked them to skip it. After more discussion, though, it looks like they can do everything except the file copy tests without much trouble. It’ll cost me a few extra dollars, but if it saves me time and headache, it’s probably worth it.

One bummer is that I’m still seeing a few boards that consistently fail my file copy tests, and can’t be sold. This happened sometimes with the old hand-assembled boards, and I never did find the cause, but I suspected it was related to my lousy hand-soldering job. But since it’s still happening with the professionally assembled boards, it’s probably some kind of design flaw. Ugh. For the time being I’m just setting these boards aside in the reject bin, but eventually when I’m sufficiently motivated I’ll see if I can figure out what’s wrong.

TL;DNR – While it doesn’t solve every problem, having professionals source the parts and assemble the boards is very nearly the best thing since sliced bread. I’m happy to give my soldering iron a well-deserved rest.

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Floppy Emu Backlight

I’m something of an anti-backlight guy, and I intentionally designed Floppy Emu with the LCD screen’s backlight disabled. Without the backlight, the text is crisp and the contrast is excellent. With the backlight, the text looks more washed out, and imperfections in the LCD glass become visible. Nevertheless, some people really want a backlight, so this post will show you how to hack your Floppy Emu to turn the backlight on.

The LCD already has four backlight LEDs built-in to the edges of the display, and all you need to do to enable them is solder a resistor or a piece of wire to the right pins. The procedure is slightly different, depending on which version of the LCD you have.

For Floppy Emus with serial numbers 51 and higher, connect the holes labeled LIGHT and GND at the top-right of the LCD with a low-value resistor, or a plain piece of wire. If you use a resistor, I recommend something in the range of 10 to 50 Ohms (lower values will give a brighter light). Because the LIGHT and GND pins are so close together, you’ll probably need to orient the resistor vertically, as shown in the photo. For the brightest backlight, use a plain piece of wire instead of a resistor. This won’t damage the LCD, because it already has a small backlight resistor built-in.

For Floppy Emus with serial numbers 1-50, the LCD design is slightly different. Connect a resistor between the pins labeled LED and VCC at the top of the board. You’ll probably find that there’s already a cut-off pin at those two spots, so you can solder your resistor to those pins. I recommend a resistor in the range of 47 to 100 Ohms. Don’t use plain wire here – these LCDs do not have any built-in resistors, and using them without any resistance may damage the LCD.

Some of the LCDs have a white backlight, and some have blue. It’s a surprise!

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Troubleshooting Damaged Chips

There’s a big difference between building one of something, and building a hundred. When building one, the challenge is simply to get the thing working at all. When building a hundred, the focus changes to issues like how fast you can do the build, and how reliably. Little problems that only crop up rarely start to become headaches. And if you’re like me, you start to get obsessed with achieving 100% reliability without sacrificing build speed or cost.

With Floppy Emu now past the 100 units mark, I can start to get some meaningful data from the assembly process. Thus far slightly more than 90% of the units I’ve built passed all my tests, and were able to be sold. Even for a hand-built piece of hardware, that’s not great. Getting closer to 100% yield will require troubleshooting what went wrong, and making sure it doesn’t happen again, but that’s easier said than done.

Releasing the Magic Smoke

The most common failure I’ve seen is something I call “burnout”, and has affected about 4% of the units. After anywhere from one minute to a few hours of working normally, the Floppy Emu stops functioning, and both the 3.3V regulator and the CPLD become hot to the touch. The AVR, SD card, and LCD still seem to operate normally, but floppy emulation or anything else involving the CPLD no longer work. After some experimentation, I discovered that if the CPLD is removed and replaced using a hot air gun, the Floppy Emu can be returned to normal functioning and the problem does not reappear.

Hot chips imply a short circuit somewhere. Measuring the current draw is tricky, because Floppy Emu is normally connected directly to the Macintosh which powers it, and there’s no place to insert an ammeter inline and measure the current. I finally broke down and built a simple bench test rig, where the Floppy Emu is powered from an external power supply and no Macintosh is involved. This only provides a way to measure the current draw of the whole board, and not individual chips, but it’s better than nothing.

What I found is that a normal board idling on the main menu screen draws about 124 mA. Removing the CPLD with the hot air gun lowers this to 41 mA, implying that the CPLD and the incremental 3.3V regulator current are about 83 mA combined. That’s a bit more than the CPLD datasheet says is typical, but the actual supply current depends on how the CPLD is configured, so it’s within the realm of possibility. The CPLD current likely increases when the device is active and floppy emulation is happening, but I don’t have any way to measure that with the existing bench test rig.

Next, I measured a Floppy Emu in “half burnout” condition. This one actually functioned OK, but after several minutes it would grow pretty hot and stop working. Unlike the other burnout Emus I’ve had, this one would start working again if it were left to cool off for a minute. With my test rig, I measured this board’s idle current draw at 400 mA, more than three times higher than the normal board. Removing the CPLD dropped the current down to 41 mA again, so it seemed clear the trouble was related to the CPLD and not somewhere else.

So what’s going on with this burnout? It looks like something’s causing the CPLD to draw high amounts of current from the 3.3V regulator, resulting in high power dissipation and overheating in both chips. The regulator has an internal safety switch that will protect it from damage, but the CPLD apparently gets toasted. That makes sense, but what causes the high current draw in the first place? Like a good mystery detective, I came up with a few theories, which I think cover all the possibilities:

  1. The chip was defective. That’s possible, but blaming faulty chips should always be a last resort. In all the electronics projects I’ve ever built, only once have I ever encountered a problem that was conclusively linked to a faulty part.
  2. The PCB was faulty, and two closely-spaced CPLD traces were shorted together somewhere. The fact that replacing the CPLD fixed the problem seems to rule out this theory.
  3. A software error in the AVR program or the CPLD config caused two chips to simultaneously drive the same signal to different values. This seems unlikely, as almost all of the signals are unidirectional, and the only bidirectional signals are controlled by a simple mechanism that would be hard to go wrong. A software error should also affect all the hardware, not just a few units, unless it’s some rare timing-based error that only appears in very specific circumstances.
  4. The chip was damaged during assembly, due to static electricity or high heat. Possible, but I’ve never encountered a damaged AVR, and it’s the exact same package and pin count as the CPLD, and I handle it exactly the same way during assembly. Maybe the CPLD is more sensitive to mishandling somehow? Seems doubtful.
  5. I accidentally shorted two CPLD pins together with a poor soldering job. I carefully checked all the pins with a 10x magnifier, and couldn’t find any shorts. Still, this seems like the most plausible explanation.
  6. The “5V tolerant” chip isn’t very tolerant, and continuous 5V inputs eventually lead to damage. The datasheet seems clear this shouldn’t be true. Recommended operating conditions for a high input voltage are between 2.0 and 5.5 volts.
  7. “Bad” voltages from the Macintosh damage the CPLD, because it’s the only chip that’s directly connected to the Mac. I can’t rule this out, but it seems unlikely. It’s definitely possible for a vintage Mac’s 5V supply to be out of adjustment, but the CPLD is a 3.3V chip and doesn’t use the 5V directly. The Macintosh signal voltages could be out of whack, but I think that would also cause problems for the Mac itself.
  8. The Floppy Emu circuit design pushes the CPLD beyond its maximum ratings, causing damage. Maybe there’s some significant voltage overshoot or undershoot somewhere that can cause damage, or a big transient that happens at power-on. Possible, but without a specific culprit to investigate it’s hard to say.

Of these, the most likely explanations are the poor soldering job and the chip damage caused by a design that exceeds maximum ratings. Replacing the CPLD with a new one would fix both problems, since it replaces both the soldering job and the chip itself at the same time. To separate these theories, I took the “half burnout” board, removed the CPLD with the hot air gun, then resoldered the same CPLD. It still failed the same way, and drew exactly the same amount of current, demonstrating that the problem lay with the chip itself and not the soldering.

So I’ve got a few damaged CPLDs. Maybe they came defective from the factory (theory 1), maybe I damaged them during assembly (theory 4), or maybe a rare software bug causes damage (theory 3). Maybe an evil Mac is frying them with 12V logic signals (theory 7). But I’m betting on theory 8, and it’s somehow my own fault for a design that zaps the CPLD with occasional voltage overshoots, power-up transients, or other circuit gremlins that lead to failures in a small fraction of the CPLD chips.

Unfortunately there’s not a lot I can do to test this theory with the current hardware. I can only measure the current drawn by the whole board, not a single chip, and I can’t do any measurements while the board is connected to a Mac. Even if I could, an instantaneous surge in current would be met by the CPLD’s decoupling caps more than the power supply, so it might not even show up in any measurements I did. As for checking individual signals for overshoots or weird transients, it’s just not practical. The CPLD is a surface mount chip with tiny 0.5 mm pin spacing, so there’s no way to connect an oscilloscope or other probe. For now, then, there’s probably nothing to do but keep testing, and try to look for patterns in the timing and nature of future failures that might point to a more specific cause.

 

 

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