Connecting SEGA Mega Drive controllers to the beeb

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tricky
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Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Fri Jan 12, 2018 7:50 pm

I have been looking at adding joystick support for when I take my games on the road and not really been very happy with any of the solutions.
* I wanted something cheap (as they may walk or get broken and I might want quite a few).
* be compatible with standard analogue sticks so that they could be used with the STH archive.
* be fairly easy to attach to a beeb (minimal extra circuitry).
* be "joypad/gamepad" format so that people under 40 would understand how to use them.

The best (least worst) option so far is these from China (the controller's quality is about on a par with their boxes)!
sega_joypad.jpg
Seems to work, but does feel "cheap".

I designed a circuit (in DIA) to map the controller 9 pin d-sub (not Atari compatible) to the beeb using 3 resistors per channel to generate 0v, .95v and 1.9v (or thereabouts) to feed the ADC.
wiring.png
I may swap the fire buttons later.
This diagram is wired wrongly see the one in this post further down.

I prototyped this using another "China special" (pairing resistors to get the values a thought might work).
prototype.jpg
It seemed to work OK with ADVAL.

I am still in the process of building a proper prototype, but it shouldn't work!
chip_down.jpg
The IC (CD4069UBE) has six inverters, one for each analogue channel and two for the fire buttons.

chip_up.jpg
The switch is to map both fire buttons on the first pad or one on each.

end_view.jpg
The housing is a 30 something pin D-sub cover.

I still have to wire up the 0v, 5v and LPSTB using the same trick as Warlords to get 4 fire buttons, although here, the SEGA controllers already have LS157s in them, so I don't need to add one. If the beeb used digital directions, you could use D+8 buttons, but as we have to wait for the ADC to complete, we will have to make do with either A+A, AB (1 pad), AC+AC* or ACBstart* (1 pad) (if I have them the right way around).
* These require the game to support the LPSTB hack or someone (JGH maybe?) to support them with ADVAL.
See this thread for more info on the LPSTB hack.

A similar adapter might make a good candidate for the Master Compact , but would require the pins swapping and an inverter as the buttons are 0v unpressed and 5v pressed (opposite to most controllers).

Please check that my designs are correct before building your own and connecting it to anything!
Last edited by tricky on Sun Jan 21, 2018 12:13 pm, edited 1 time in total.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Sat Jan 20, 2018 9:01 am

Unfortunately, when I finished wiring the prototype, it was wrong!
start_again.jpg

When I first check the outputs from the joypad, I measured each button as 5v unpressed, dropping to 0v when pressed, as is normal with these things (pull up, short to ground).
When I put it on the prototyping board, it measured the other way around, so I checked and re-checked, but what ever I had done wrong, I couldn't spot it!
Once I finished the prototype, everything was backwards, so I need to start again with the original design!
I will replace the diagrams in the first post once I have a working prototype.
Sorry everyone.
PS I really think I need to get into using proper PCBs as you can see from the image these take too long to build, even though the parts are cheap and easy to work with.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby Kazzie » Sat Jan 20, 2018 4:38 pm

Coincidentally, I'm waiting on parts to build an adapter to connect a pair of Mega Drive controllers to a Dragon 32 (which like the Beeb, uses an analogue input).

My plan is based on this design, which uses a CD4066 for each controller. Two transistors for each axis swing the output voltage up to 5V, down to ground, when a direction button is pressed. (This circuit is intended for Atari pinout controllers, but the basic circuit is just the same.)

If you haven't already seen them, these two pages are useful for checking for Mega Drive 3- and 6-button controllers operate: https://segaretro.org/Control_Pad_(Mega_Drive) https://segaretro.org/Six_Button_Control_Pad_(Mega_Drive).

I'm designing for spare 3-button controllers we've already got in the house, and intend to leave the multiplexer select in the Mega Drive controller high, as I'm not fussed on reading buttons A and Start. (The Dragon only expects one fire button per controller, anyway!)
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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby jgharston » Sat Jan 20, 2018 5:05 pm

tricky wrote:Unfortunately, when I finished wiring the prototype, it was wrong!
Well, you were calling Fate on your head when you posted I am still in the process of building a proper prototype, but it shouldn't work! :)

What's the LPSTB hack? I've heard of it, but never encountered any information.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Sat Jan 20, 2018 7:02 pm

Kazzie, The idea is similar, but I am just using the circuit in the controller to pull the lines up and down and then, similar resistors to centre the voltage. I added an extra resistor to pull the voltage down to the range required by the ADC on the beeb (seemingly unnecessary on the dragon). I also use one of the inverters per channel to invert down/right.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Sun Jan 21, 2018 9:36 am

"shouldn't work" #-o
jgharston wrote:...What's the LPSTB hack?...

Firstly, sorry for the use of the word "hack", I must be getting down with the kids!"
Instead of using the LPSTB input to trigger capturing the CRTC beam position for a light pen, you set it as output and use it to swap between two sets of input.
Here it is used to activate the select input on the MeagDrive controller which swaps between the two fire buttons being B+C and A+Start (if I have that bit correct) and on the six button controller three of the directions with the XYZ buttons.
On Warlords it triggers either a set of NAND gates or an LS157 (select between two sets of four inputs - I only use 2) to give each of the four players a fire button.
The pull up on the LPSTB seems to be strong enough to hold the default buttons at B+C, but another pull up could be added if required to make sure that when it is set to input the correct defaults are used.
This wasn't my idea, but I have since found that I think it is used for the beeb controllers with the key pad, here swapping the keys connected to the user port.
Since the six button version will swap the directions out, I need to do it between ADC conversions; on Warlords, I just do the ADC on interrupt and the check the buttons when my emulation look is ready to start a new frame.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Sun Jan 21, 2018 9:42 am

For the beeb, I would need to check if that chip works with 1.9v outputs.
For the Dragon32, there is someone making a 2-Atari to beeb converter, maybe he could change his design slightly for the Dragon.
The dragon seems to have its analogue directions inverted compared to the beeb and be 5v instead of 1.9v on the analogue channels, so it might not be easily bodgeable.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Sun Jan 21, 2018 11:56 am

The second prototype works, although I haven't tried the extra fire buttons yet!
The 6.8K resistors should probably be a little lower, or the 20K ones a little higher as the midpoint is over 1/2 voltage, &8C00 instead of &8000 when measured through the ADC.
I checked the datasheet and it seems that the voltage should be between 2.25v and 2.75v, so the beeb already seems to be using it out of spec'!
mk2_circuit.png
N.B. I connected the centre pin of the switch to e and then !e to c (the one between !b and !a) as it seems bad form to leave unused inputs floating.
I went for a slightly quicker to build option where the three resistors for an analogue channel terminate in mid air.
mk2_complete_chip.jpg
This made it easier to solder and fit onto the small piece of board that I had cut (should be bigger next time!).
mk2_complete_copper.jpg
I bought some new solder from RS for this attempt, as the stuff I was using from China never gave a nice shiny joint and didn't "stick" very well, no matter how much extra flux I used.
bad_solder.jpg
I only connected the pins I needed in the beeb plug, as it seems from the circuit diagram that all similar pins are shorted on the beeb end.
beeb_end.jpg

I will try and make a more sensible circuit diagram and maybe a PCB layout, but until then, if you try building this, I would recommend adding the chip socket last, but mark out where it will be so that the area doesn't get too congested with wires.

Please feel free to take and adapt this design for your own purposes, but if you are going to sell them, please send me one and credit stardot (unless the mod's disagree!) and myself.
Last edited by tricky on Wed Jan 24, 2018 7:04 pm, edited 3 times in total.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Sun Jan 21, 2018 12:10 pm

The cable I used which I found very easy to work with for this was £4.15 for 10m from ebay (UK) and the mega drive controllers were £1.60+15p p&p each from ebay (China). I have no connection to these auctions and as I said before, the controllers feel very "cheap", but then they are :lol:

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby jgharston » Sun Jan 21, 2018 12:52 pm

tricky wrote:"shouldn't work" #-o
jgharston wrote:...What's the LPSTB hack?...

Firstly, sorry for the use of the word "hack", I must be getting down with the kids!"
Instead of using the LPSTB input to trigger capturing the CRTC beam position for a light pen, you set it as output and use it to swap between two sets of input.
How do you set it as an output? I've checked through the 6845 documentation and there's nothing about it being usable as an output.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Sun Jan 21, 2018 1:38 pm

It actually connects to CB2 on the system via, driving it doesn't seem to upset the 6845, but that is when I have full control.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Mon Jan 22, 2018 11:32 pm

In case you aren't board with this, here is what I made in KiCad, I don't know if it is any good, you judge :roll:
Image
The three pin header is where the switch attaches to swap between 1 joystick 2/4 buttons and 2 joysticks 1/2 buttons each.
layout.png
I'm not very happy with the routing, but it doesn't have any vias (it probably should have!).
The beeb DB15 could be left off and a cable soldered to the pads with a plug on the other end (what I would do).
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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby myelin » Tue Jan 23, 2018 1:01 am

The PCB routing looks fine, for the signal traces at least.

One thing I'd change would be to fatten up the +5V traces (make them 20 mil / 0.02" at least), and to pour a big ground plane over the entire board, using a ton of vias to 'staple' the front and back planes together nicely.

If the ground plane option doesn't work well, I'd just fatten up the GND traces as much as you can.

Avoiding vias is only really necessary if you're doing a high speed design (tens of MHz) or if you're making the board by hand and have to individually drill and coat them. Nothing wrong in most cases with using them liberally to make the layout easier :)
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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby jgharston » Tue Jan 23, 2018 2:58 am

tricky wrote:It actually connects to CB2 on the system via, driving it doesn't seem to upset the 6845, but that is when I have full control.

Ah, that rings a bell. So the LPSTB can generate an interupt so code can then go and read the CRTC registers. It was there in the back of my mind, but searching through the schematic I couldn't find it.

The Econet technical documentation mentions: generating a delayed interupt by setting the system VIA counter, which outputs to CB2 connected to LPSTB, but the 6845 isn't adversly affected.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby cmorley » Tue Jan 23, 2018 7:42 am

tricky wrote:I'm not very happy with the routing, but it doesn't have any vias (it probably should have!).
The beeb DB15 could be left off and a cable soldered to the pads with a plug on the other end (what I would do).


Hi Tricky,
No vias is a reasonable goal. Easier with through hole because you get a via for free at each pin :)
Your traces look a bit thin... 10 mil trace & 10 mil clearance is common minimum for the cheap chinese PCB manufacturers (they will be able to make that). Maybe turn the 4 resitors top & bottom 90 degrees and pack them 0.1" then they will fit in the width of the IC but will present a big gap east-west to route tracks... that might route better. Also you might like to unroute your power and ground nets (tweak your signal routes) then pour the 5v & gnd. I find this makes routing easier.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Tue Jan 23, 2018 8:05 am

JGH, thanks for the reassurance and thanks to the others who's advice is very welcome as I might actually get to order one of these designs one day.
I would like about a dozen of these for myself, but doing them as I have for the prototype is really out of the question.
Thanks again all of you.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby daveejhitchins » Tue Jan 23, 2018 8:42 am

@tricky

You need to check all your traces, that go between or near pads - in particular between R8 and R12 - - - You may meet your clearance rules (what are they?) but try and increase gaps to give the maximum clearance possible within the space you have - Your tracks look as though they could be a little thicker too, in places. Looks good overall =D>

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby 1024MAK » Tue Jan 23, 2018 10:56 am

To add to the good advice about PCBs that others have already given, it is sometimes possible to reorder or relocate things like resistors to make the routing easier and more straightforward. In the past, I have designed and built my own designs of PCB manually (with no help from any computer). I would spend plenty of time working out the layout of components that would result in the easiest track layout.

I agree with Dave about the spaces between tracks and component pads. Generally you should put a reasonable distance between them, so that you minimise the chance of a solder bridge. Also if you can, have a maximum of one track going between adjacent closely spaced pads. For some of the resistors you may be able to increase the spacing between them.
If designing a board where amateurs will be soldering them, try to use component pads that are a bit bigger (amateurs find soldering to the really small round pads more difficult).

The power supply tracks (+5V, VREF, analogue ground/0V/GND) should be nice and wide, with the 0V/GND track being as wide as possible. For this board, I would not worry too much about a ground pour/ground plane.

Good luck :D

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby cmorley » Tue Jan 23, 2018 11:14 am

1024MAK wrote:For this board, I would not worry too much about a ground pour/ground plane.


Electrically irrelevant for this project but it makes routing easier because you don't need to lay traces for power and ground. Just do signals, pour and move signals or stitch with traces and vias so all power nets are connected (i.e. no island that needs connectivity). This is easier in my opinion than having a serpentine and forking ground/5v net.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Tue Jan 23, 2018 12:22 pm

Thanks guys, I might try and do one that I actually get produced.
This was just something that I tried last night - it is "by hand" if that means no auto routing, other than picking the best line between two point - that is no push the button and sit back while lines appear everywhere.
I just used the defaults for tracks and pads, I will change them using the suggestions above if I go for production (and maybe just for fun too, although I should really get back to Centipede ;)).

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby cmorley » Tue Jan 23, 2018 12:42 pm

tricky wrote:Thanks guys, I might try and do one that I actually get produced.

PCBs from China are so cheap there is almost no risk - apart from wasted time. e.g. 5 PCBs from Elecrow will cost you $5 + $5 P&P! Worth a shot.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby Lardo Boffin » Tue Jan 23, 2018 1:38 pm

tricky wrote:The cable I used which I found very easy to work with for this was £4.15 for 10m from ebay (UK) and the mega drive controllers were £1.60+15p p&p each from ebay (China). I have no connection to these auctions and as I said before, the controllers feel very "cheap", but then they are :lol:


How on earth can they make these for that price?!!!!! :shock:
I think I will order a few as my 3 year old has discovered Sonic the Hedgehog and is not gentle with the controllers...
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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby jgharston » Tue Jan 23, 2018 1:48 pm

Lardo Boffin wrote:How on earth can they make these for that price?!!!!! :shock:
Have you seen the cost of living in rural China? £5 can keep a peasant employed in an electronics factory in food and accommodation for more than a week. It's the equivalent purchasing value of about £150 in the UK.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby Lardo Boffin » Tue Jan 23, 2018 3:05 pm

jgharston wrote:
Lardo Boffin wrote:How on earth can they make these for that price?!!!!! :shock:
Have you seen the cost of living in rural China? £5 can keep a peasant employed in an electronics factory in food and accommodation for more than a week. It's the equivalent purchasing value of about £150 in the UK.


Fair point! 15p postage from China is also rather cheap, unless hand delivered by said Chinese person! :D

My brother lives in Shanghai and keeps telling me how cheap some stuff is even there - the phrase ‘buckets of whisky’ has been used more than once. I fear for his liver.
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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Tue Jan 23, 2018 8:12 pm

Just out of interest, I checked the tracks and they were 0.25mm, so, all but 10mils; the clearance was set to 0.2mm, so more like 6mils.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby myelin » Tue Jan 23, 2018 9:34 pm

Typical minimums for track width and spacing are 6 mils, with the recommendation that you use 7-8 or more to improve yield (because thin tracks don't always etch correctly). With one exception (a recent board with a BGA chip on it) I always use 7 mil (0.007") as the min trace/space, and all the Chinese manufacturers I've used have had no trouble with it, so I think that should be pretty safe for signal traces and anything that doesn't need to carry much current.
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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Mon Feb 05, 2018 1:45 pm

Kazzie wrote:Coincidentally, I'm waiting on parts to build an adapter to connect a pair of Mega Drive controllers to a Dragon 32 (which like the Beeb, uses an analogue input).

My plan is based on this design, which uses a CD4066 for each controller. Two transistors for each axis swing the output voltage up to 5V, down to ground, when a direction button is pressed. (This circuit is intended for Atari pinout controllers, but the basic circuit is just the same.)

...


It probably doesn't matter, depending on the internals of the 4066, but to me it seems wrong to ever short together outputs that may be at a different level.

I would have thought it would be "safer" to connect the inputs directly to 0v/5v and then put two resistors to mix the outputs.
I did try to explain my concerns, to a friend with TTL knowledge but they didn't see a problem, so there probably isn't one.

The Dragon32 way also means that the design can be built smaller with a resistor net with a common pin verses mine with pairs of pins.

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby 1024MAK » Mon Feb 05, 2018 8:22 pm

The 4066 is very different to normal logic chips. As long as the current is through each 'switch' limited to within the chips specifications, it is possible to parallel or interconnect outputs.

Datasheet here

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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Fri Feb 23, 2018 8:05 pm

I have now got designs for 4066 and 4069 based SEGA and Atari adaptes for one or two controllers, so I thought I would prototype a single 4066 based SEGA Mega drive adapter to see how well it works.
As you can see, it is a bit of a squeeze to get it in a DB15 case and that is after a couple of quick mods with the "dremel".
The PCBs that I have designed (but not yet ordered) will actually fit in the space that I have allowed for this, with all through hole components (there are four more resistors to go in yet).
The PCB will either slot between the rows of pins and be soldered to them or the DB9 end will have wires soldered to it to make an extension cable with built in adapter.
If they work, I will post the KiCad and Gerber files here and may make a few up to sell to help cover part of the costs of getting the PCBs made.
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Re: Connecting SEGA Mega Drive controllers to the beeb

Postby tricky » Sat Feb 24, 2018 8:01 pm

Well, I finished it and up/down seem fine, but right seems to be jammed on, so maybe I cooked something, I'll probably never know.
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