The Minigun (A mini Super Gun)

Guest post by Charlie again as he continues his quest to take over Moop's blog with his super long posts:


Then here's a video instead...


Recently I got into arcade boards. They can be found reasonably cheaply on eBay and as most use the standard JAMMA connection, setting these up so you can play them on a TV at home is pretty straight forward. Especially in Europe where SCART is still widely used and thus almost all TVs have an RGB input.

I built a very traditional supergun (device that allows you to play JAMMA boards on a TV) last year. It was time consuming but it wasn't particularly hard. It used a PC ATX power supply to supply the voltages needed (-5V, +5V and 12V), audio/video output was via SCART and Neo Geo AES pin compatible D-SUB 15 plugs for the controller inputs. Here's a couple of pictures of my first effort...

IMG-0782 IMG-0787

Once I started getting more boards, I found that some bootlegs drew a heck of a lot of current. So much so that the voltage drop over the half a meter or so of wiring between the PC supply and the board was causing the boards not to power up (due to the voltage drop across the wire). I initially tried solving them by using some wider gauge wire in parallel to feed the board. This worked but for some bootlegs I owned even this wasn't good enough. I was reading as close to 5V as I could get at the connector but measuring the voltage on distant parts of the board, the voltage had dropped to sub 5V. I needed something better.

I could have got a JAMMA power supply off eBay with a variable voltage output. This was initially tempting but I just couldn't bring myself to use one for safety reasons. On all the ones I saw the mains power was just wired into screw terminals. This is fine as it'd be inside my super gun's case. The thing that worried me was that the outputs were right next to the mains screw terminal. The boards aren't in a cabinet like a traditional arcade, they just lay on top. If the mains lead became loose there is a possibility of the board becoming live. This may sound surprising coming from the person that made his own bluetooth controlled mains plug, it wasn't a risk I wanted to live with.

So, I went looking for a variable DC/DC convertor that could transform a lot of current without needing any active cooling. After some searching I came up with With this I could power the 5V line from the 12V output of the ATX power supply and add a pot to adjust between around 5-6V so even my very power hungry boards would be happy.

With this done I felt I had a reasonable design. Then with Christmas coming around I decided I could make another one for a friend as a present. Building the first one, I wasn't really thinking about the time it took as I built it and tweaked it over a couple of weeks. Even knowing exactly how to build it now, it still took at least 10 hours to actual do it. I made a few improvements along the way but it was a tedious build due to the huge number of wires involved (>100 connections) and all the modifications needed to the case. Also, even with a lot of salvaged parts (the ATX power supply, the SCART plug etc) it also ended up being quite expensive at about £50. A lot of this was due to the case which was quite hard to find as it needed to be tall enough to fit an ATX power supply.

With that out of the way I vowed never to make another supergun again..... but of course I did. I just kept thinking I must be able to make a better one. After trying the toner transfer method for the first time, it got me thinking that I could do away with almost all the wires by just making a supergun PCB.

The design

Starting the design of a supergun PCB, I really wanted to make it as compact as I possibly could while still making it easy and cheap to assemble. One of the ways to do this was to leave out anything superfluous. The JAMMA standard has various service switches but not all need hooking up. COIN1/COIN2 are the coin inputs for adding credits. These are useful to have on the controller so I wired these to the AES controller's select button as this would otherwise not be used by JAMMA boards. SERVICE on every board I own also just gives credits so I left this out. TILT (for detecting people shaking the machine) is rarely hooked up on superguns and I suspect does nothing interesting on 99% of boards. This only leaves TEST which is useful so this got it's own microswitch on the board.

Power was pretty easy. One reason to use an ATX power supply is that it supplies the -5V that the JAMMA specification requires. However, looking at my collection of arcade boards, one 1 (out of around 10) uses the -5V line at all. They were normally just used for older types of audio amplifiers (my one board which does use it works without it but just without sound). I felt fairly comfortable with just leaving the -5V line out all together. This decision along with using the same DC-DC convertor from my original design means the supply only has to be 12V. Many laptop power supplies are 12V, compact, cheap and can supply plenty of current.

The rest of the design mapped quite nicely to the PCB concept. PCB mounted DSUB and SCART connectors are common. How to connect the JAMMA edge connector was slightly more difficult. However, I noted that two standard 1.6mm PCBs stacked on top of each other was almost exactly the tag separation on the rear of the JAMMA connector. I could make a small board that sat on top that could then run the signals for the top back of the connector down onto the bottom board. Both boards I designed to be single sided (for ease of creating at home) although because they sit on top of each other it does effectively make it partially double sided.

The design was quick to knock up in Eagle (maybe too quick as we'll see later) but when I came to lay it out I hit my first snag. The free version of Eagle is restricted by the size of the board it will let you lay out. The longest dimension is 10cm. This wasn't long enough to connect to every pin of the JAMMA connector. In fact, it was 3 pins short. Luckily of these three, one is unconnected and the last two are ground which exists on the other side of the connector anyway. It would have been nice to connect these additional grounds but I didn't see much harm in leaving them floating (assuming the grounds are connected on the arcade board itself). So I designed the board to the Eagle limitations and just added terminals at that end of the board if in the future I wanted to wire the extra grounds in with some short jumper wires.

Laying out the board itself was straight forward but time consuming as I wanted to keep the layout single sided (with no jumpers) as it needed to be simple to etch at home using hobbyist processes like the toner transfer method. As such I needed to keep the traces fat too. This led to another compromise. Because the traces had to be fat and single sided fitting two controller ports was just too difficult. It's something I'd like to fix in a future design if I was just going to get it professionally manufactured via PCBcart or similar.


Construction wasn't particularly interesting and was quite straight forward. It was only my second attempt at the toner transfer method but the traces were fat enough that I could fix most problems by hand with just an etch resist pen. It wasn't perfect but I was fairly pleased with the end result.


Soldering the various connectors was quick and painless. The only difficult bit was lining up the top and bottom boards to make the double sided portion. Although because the via pads were quite big the positioning wasn't that critical and any slight alignment problems could be fixed by just adjusting some of the drilling locations.

Once the components were all added, it was ready to test!



Once everything was assembled I grabbed my male to male SCART cable and immediately realised I had made a mistake. I knew I'd made a mistake as I'd made the exact same one with my friend's supergun. Male to male SCART cables don't map all the pins through one to one. Most do but some are crossed over, so for example the audio output pins on one side go to the audio input pins on the other. And the same for the video signal. D'oh! So, out came the scalpel the offending traces were cut and replaced by bodge wires.

Tried again and this time the game sprang to life, everything was great and then I noticed the colours were off. A quick check on the Neo Geo test menu showed red and blue were reversed! Two mistakes on the same board was quite embarrassing. I know exactly how it happened though as when I tried to fix it, I made the exact same mistake. I initially wired up the RGB pins on the SCART from memory and then checked them before doing the board layout. I saw the mistake at this stage and thought I had corrected it. Everything was connected via named labels in the schematic so I just renamed the label on the pin (say from RED to BLUE). Eagle asked "Do you want to connect RED to BLUE?". Thinking this would then cause all the pins to connect I said no but assumed the rename had worked. Nope, I'd just done nothing. A second, "D'oh!".

So back out came the soldering iron and another set of bodge wires causing the final board to be a messier than I would have liked but everything I have thrown at it has worked so I'm quite pleased with the results. Everything works as well as my big traditional super gun but now it's in a very tiny portable package.


Make your own

I made this as my contribution to the super gun / JAMMA comunity so I've fixed up my board layout (the swapped channel problem etc) and I'm offering it as a download here: Minigun - Final (EAGLE file format). I hope people will build there own, make improvements and share these improved versions with the community.

Nb: The current design has the SCART socket mounts overlapping as the connector I used had a different mounting to the stock one in EAGLE. Also the single top layer trace is the 5V line to the controller port. I don't use this in my home-brew controllers so I left it unconnected but if you did want to use AES controllers you may have to solder a jumper wire between these points.