Atari 7800 edge connector spacing, pad geometry, pcb thickness, place for cuts?

[+karri]

I found a small box of odd memory chips and decided to try to make my own small 7800 cart.

 

So far I did draw the connector symbol in KiCad 7.0.

 

But I wonder if someone has already made the tedious part of measuring the footprint?

The header tells what I need to know.

 

I already got the schematics done. The flash chip is some 5V PLC chip that can carry 512k.

Unfortunately my measuring tool and the chips is at another location. I have to wait a few days before I have time to dig through my stuff.

 

Lol. I already found 2 errors in my design. The common pin of the rotary encoder should be connected to GND. And the OE needs a pulldown - not a pullup.

 

The cool thing is that I could program 8 of my small games on the device and just set the rotary encoder to which game is active and put the corresponding label on the 3D printed enclosure.

 

Nice for demoing the games at some retro event. (Perhaps eJagfest in Germany this year. Rumours say that it will return in autumn this year.)

 

The rotary encoder is the most expensive part here. Something like 4€. The flash is below €3.

 

Edit: I found my measuring stuff.

PCB thickness 1.5mm

pitch 2.58mm

pad rectangular 7mm x 2mm

connector length 47.6mm

connector depth 16mm

cut depth 9.7mm

cut width 2.4mm

The center of the cut is exactly where the pads 3 and 16 would have been if the edge connector had 18 pads

 

 

[Dissy614]

I made by own footprint from measurements.  It's in EasyEDA though, so not sure the best way to get it from one to the other.

Can you import or measure from a gerber?

 

Gerber_PCB_7800 Breakout Cart_2023-02-15.zip

[+karri]

Thanks!

 

I measured from the gerbers and the pitch is 2.54mm.

Your pad is 7.5mm by1.3mm. So I assume that this is not a critical thing.

Your cut width is very narrow. Only 1.1mm. The depth is 10mm. So again I assume it is not critical.
But the cut is not centered with the missing pad pin.

Your cart width is 45.85mm. A lot narrower than the one I have.

 

I really appreciate your help!

 

I can share my schematic symbol and footprint in KiCad 7.0 format once I get them drawn.

My PCB is probably not interesting as it uses a Lynx cart connector on one end and a 7800 connector on the other. That was the cheapest solution by which I can program the cart. I already own a Raspberry Pi with a Lynx HAT for programming Lynx carts. So I can program all 8 games to the cart from the Lynx side. And play just the chosen one on the 7800 side.

 

If there is an eJagfest in Germany in autumn I may have a small number of carts with some new homebrews on them for demonstrating  my games and selling (or giving away) the carts at the end of the event.

 

PS. The only problem I see is that the Lynx wants a 2.2mm thick pcb. So I need to glue in something to make the Lynx edge connector 0.7mm thicker. Perhaps I find some tape that I could put on the Lynx edge connector component side to make it thick enough during programming.

--
Cheers,

Karri

[Dissy614]

Yup, 2.54mm center to center, 1.27mm width.

On the 7800 side connector I measured the plastic "pins" as 1.0mm so used 1.1 for the cut width.  

It was not centered, correct.  Closer to the 'outside' pads.

I did forget to mention, I used standard FR4 at 1.6mm thick.

 

There is the tiniest bit of extra room in the connector on each side.  I don't recall if I rounded down to be safe or what.  However the two slot cuts and pins align almost perfectly, and the outside pins are held aligned.

The connector I believe must be 12mm from the bottom at this width.  After that it only matters if you're going to fit it in a cartridge shell (There's a hole needed in the board for that as well)

Unfortunately I don't think I noted down any measurements above the connector.  It wasn't of use for my purposes as you can see.

 

Best of luck on your project!

[+karri]

Here is the symbol and footprint files for KiCad 7.0.

A7800.kicad_sym A7800.kicad_mod

I have not used this in a real design yet. So you are on your own if you plan to use them.

[+karri]

A small glimpse what my new cart will look like. The Lynx connector will be hidden by the enclosure.

 

I had 48 chips in the box I found. So now I know how many PCB's to order  

[Dissy614]

A perspective on the "Gerber_PCB_7800 Breakout Cart" and how it aligns with the real hardware.

I inserted the board "backwards" so it is in the same orientation as the 3d model image above, and the silkscreen can be used as a visual reference.

(This felt easier than trying to get the camera and light between the cartridge PCB and the heatsink)

The first two have the board inserted fully, and the third with the board sitting on the connector with the pads in view.

 

If you make the cut width larger, it might help to make the board width slightly wider. 

Both of the reference carts I measured had different widths from each other and were asymmetrical.  So my footprint erred on the smaller side and relies on the cut widths to align the pads dead center with the connector.

 

[+karri]

Thanks! I realized this and made the cart wider. So it should fit the socket nicely.

 

I may have to ditch the screw hole to fit all the traces between the buses.

 

Or actually.. I could make the pcb a rectangle and 3D print my own cover.

If I make it 57mm wide it fits in a Lynx as well. I may trim the width to be 57mm for the whole cart.

This might be the 1st Atari Lynx and Atari 7800 cart  

 

I also added a few extra holes to the 8-position rotary encoder.

It is kind of expensive (€4.50) so I can also populate it with three jumpers.

Or leave it out completely. But then the 7800 will only see the 1st 64k.

 

I also checked the prices and the PCB with gold immersion would cost around €3 each in quantities of 50.

But DHL, customs and VAT will easily double the price...

My guess is that the cover costs more. I did get a link to one guy who sells covers...

 

Why do the 7800 pcb's have this strange shape?

[+karri]

After hours of editing the board is finished. Yeah!

I believe it became a bit over-engineered due to the Lynx connector.

I could have used a 32 pin PLCC socket. It only costs €1.40...

[Dissy614]

2 hours ago, karri said:

Why do the 7800 pcb's have this strange shape?

They used the same plastic cartridge shell for both 2600 and 7800 carts, but early 2600 carts had a spring-loaded locking mechanism in them.

Where 7800 cart boards have the extra 8 pins, they 2600 console connector itself had two large plastic pins that went into the shell.

That released the mechanism letting it move out of the way and expose the edge connector of the pcb.

At some point they stopped including that mechanism, probably for cost reduction, and just let the edge connector be exposed.

 

The 2600 cartridge pcb had to fit around the side pins that unlocked it.

The 7800 cartridge pcb of course has pins there, but still had to align with all the pegs in the shell on the sides that held the board in place.

 

As for why the plastic shell was made that way...  your guess is as good as mine

[Dissy614]

3 hours ago, karri said:

I may have to ditch the screw hole to fit all the traces between the buses.

That was my thinking too.  

When I made my 2600 breakout cart, I went through all the effort to include the notches properly, but forgot the screw hole cutout.

 

I also made an extremely noob mistake and completely reversed all the pin numbering! 

Fortunately I made the same mistake on both connectors, so it cancels out and still works.  But I haven't shared that layout yet as I wanted to fix it first and get another run of boards.  

As the DIP header is for connecting a ribbon cable, the shell would need additional modification anyway, and I quickly decided to break all ties to being compatible with it by the time I made the 7800 board.

 

The only 'downside' I saw to abandoning the plastic cartridge is that the shell itself is the only method provided to ensure the cartridge is inserted the right way around.

So my solution to this was the console graphic on the silkscreen, indicating the pin numbers to make sure the PCB edge connector numbers match while inserting it.

 

[Two wrongs make a right!]

Edited February 17, 2023 by Dissy614

[+karri]

5 hours ago, Dissy614 said:

[Two wrongs make a right!]

Nope. Two wrongs are still wrong - sorry...

 

But I like your zero-force EPROM connector. I do not have it on my Wizzy cart and the edge pins on my chip are almost cut due to being bent several times when I try to insert it after programming.

 

PS. The latest KiCad version 7.0 is really user friendly and it is free!

 

PPS. I just sent in the design for manufacturing 50 pcs (or actually just 48) USD 93.57 + FedEx USD 30.

(It is just the board with gold submersion for a little more reliable edge connector. The parts cost extra and I solder them in myself.)

 

The top part of the cart is obviously measured backwards. It should have been wider close to the hole. But I made it narrower instead of wider. Well, at leasts it fits any enclosure . The highes top portion needs to be narrow to fit the Lynx programmer. 

 

PPPS. In fabrication, estimated finish time 21st of February, 2023.

After that about a week for delivery. Another for customs etc. Exciting...

[+karri]

I just received the pcb's. They look nice and fit nicely in the 7800. I still don't know if they work or not though.

Black solder mask, white silkscreen and gold submersion give a nice impression.
I still need to sand away a small slope at the edges to protect the pins of the cart connector.

 

The big idea was to create cartridges for the small 7800 games I have published like

It will be nice to have my own creations visible next to my 7800. Thanks for James and Tanya for creating these covers. I also have a very nice one from Shawn, but due to the use of the name Elvira I keep it private on my own cart only.