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Idea for slightly easier ExtSel for 1200XL

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This is my very first time doing something like this, so be gentle, but constructive criticism is encouraged.


I kinda boned the ExtSel mod on my 1200XL. It just seems so drastic to cut the pins off a perfectly good 74HC08. Soldering things directly to chip is also kinda unaesthetic. Also, I kinda suck at it.


So I'm thinking something like this (borrowing entirely from Bob's Wooley's PBI instructions):




The existing 74LS08 gets plugged into U1, which is then plugged into socket U17 on a 1200XL motherboard.

An unspoiled 74HC08 gets plugged straight into U2.

R1 is a 2.7K resistor, just like in Bob's instructions.

J1 could be right angle or straight headers.


It's really simple, and I'm sure I could use a few lessons on PCB design. Also, I don't know how much work it cuts down. You still need to:


1. Cut the trace and solder a wire to the pad above U11 (connects to J1 pin 6)

2. Solder a wire to U14 pin 16 (connects to J1 pin 1)

3. Solder a wire to +5v (connects to J1 pin 3)

4. Remove Q4 and Y1 from the motherboard


But then you have J1 pin 2 for CasInh and J1 pins 4 and 5 for ExtSel (VBXE and Rapidus). A larger J1 could be added if more ExtSel and CasInh lines are needed.


Waddya think, sirs?

Edited by Smokeless Joe
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I don't have the mod instructions available but - to eliminate cutting off pins, couldn't you equally just do the same to a socket and use between chip and existing socket? So long as not too many pins are to be disconnected it should work fine and cost next to nothing.

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Oo! A request!



How hard would it be to add the MPD signal to the board/design? This signal is missing on the XEGS MMU. Would be nice if we could use the same board for both the 1200XL and the XEGS.

That's a good question. I don't know how the MPD signal is normally added to an XEGS. Anyone know?

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Well, I think you exhausted the current list right there. Three EXTSEL signals each with a pin to themselves will be very nice and probably adequate. My own machine completely maxed out the signal by sending it to three different locations: VBXE, PBI connector, and Rapidus, although the latter has been evicted for the time being.

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Here's my latest design. Unless anyone spots some serious errors, I'll probably submit this to OSH Park this weekend:




U1 is the existing 74LS08 from socket U17 on a 1200XL

U2 is a 74HC08

R1 is a 2.7K resistor

J1 are pin headers

If I'm interpreting Bob's instructions correctly:

U2 pins 7 and 14 connect with U1 pins 7 and 14, respectively

U2 pin 2 connects (via R1) to U1 pin 14 (Vcc/+5V)

J1 pin 1 connects to U14 pin 16

J1 pin 2 is CasInh

J1 pins 3,4,5 are ExtSel (for VBXE, Rapidus and/or a PBI connector)

J1 pin 6 connects to the pad above U11

And I'll still need to cut the trace from U14 to U11 and remove Y1 and Q4.

Anything else?

EDIT: Huh, it's exactly like my last design only with a slightly different program. Oh well.

Edited by Smokeless Joe
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Printing out proofs revealed a couple quirks. Now I'm thinking something like this:




This fits much better on the motherboard since there's a convenient space just above U17, with the top of the board just shy of Q14. J1 pins are probably better off being straight, but they'll be clear of both Rapidus and VBXE boards (if I can figure out how to get both Rapidus and VBXE boards on the same 1200XL). Also, I had to make it a 2 layer board to get R1 tied in, so now I feel all advanced. ;)

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So do the wider holes of U1 house a counter-sunk precision socket whose legs push straight into the motherboard socket (like the VBXE adapter)?



Kind of? I don't have a good vocabulary for part names, but I got the idea from the way mytekcontrols designed his TransKey-II, using pins and a spacer like at http://www.electronicsurplus.com/augat-740-ag14d-ic-socket-40-pin-break-away.


But yes, pins that push straight into the motherboard socket U17.

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I'd strongly suggest using a 74LS08 or a 74HCT08.


The 74HC series is designed for CMOS input levels but Atari 8bit uses TTL levels.


Sure! I'm all for using whatever works better. The 74HC08 idea comes straight from Bob's PBI for 1200XL instructions at http://www.retrobits.net/atari/pbi.shtml. Are these the latest instructions?


I guess that's one advantage of the adapter board, it'll allow easy swapping of chips to see which works better.


I sent my latest design to OSH Park yesterday. $5.45 for three boards! Last time it took maybe two weeks from ordering to receiving.

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  • 3 weeks later...

I got the boards three days ago, and they sure are pretty! Here's the bare board, and an assembled board together:




Here's plugged in, but not fully booked up, to a 1200XL:




It fits better than I expected, with plenty of clearance using right angle headers for J1 instead of straight. I'm a little worried about the clearance of the 74HC08 with how tight everything can be with a full load of upgrades (U1MB, VBXE, Rapidus and Simple Stereo, maybe a TK-II) but that's a problem for another day.


I haven't actually tried hooking up anything that needs an ExtSel line. I'm experimenting with trying to get the Rapdius and VBXE properly working on any system before I try it on the 1200XL.


So... the Slightly Easier eXtsel board has an acronym that belies what it's really worth. How about Slightly Easier Extsel (SEE)?

Edited by Smokeless Joe
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I'd be interested in two. Can you post pictures of the pins on the bottom of the SEE board? How much did all the parts cost for one?


Full disclosure: I haven't actually tested these things. I've had a run of bad luck lately (insert sad music here) so I'm kinda taking break from breaking my Ataris. But I'm glad there's even mild interest. The board is sort of an answer to a question nobody asked, but it's a fun little project.


How's this? There's no chips plugged in, but you can see the pins. The long ones are from U1, where you'd plug the original 74LS08 and where the whole thing plugs into U17. I can take a picture at another angle, if you like:




You can order the bare boards here:




OSH Park requires minimum orders of 3, but 3 SEEs only cost $5.45. It takes about two weeks for me to receive orders from them, and I live in Madison, WI.


For the other parts, I used:


Jameco 45225 (74HC08): $0.35 each

Jameco 175920 (SIPP pins - gotta break these off individually): $1.75 each

Jameco 37197 (14 pin socket): $0.65 each

Jameco 103271 (right angle headers): $1.05 each

I've got a bunch of 2.7K resistors lying around, but if I needed them, I'd order Jameco 690961: 10 for $0.099.

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