Position of deposit switch

David Griffith

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Feb 16, 2020, 7:15:09 AM2/16/20

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According to the PiDP-8 instructions, all momentary switches are to be installed as normally "up", that is, to activate them, you press down. However, looking at photos of actual PDP-8i panels, I see that the deposit switch is normally "down" and must be lifted to be activated.

I haven't yet installed the switches on my panel. What's going on here?

Oscar Vermeulen

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Feb 16, 2020, 7:51:53 AM2/16/20

to David Griffith, PiDP-8

David,

Yup, the DEP switch on the original PDP-8/I is upside down (compared to the other momentaries). On the PiDP, it's not (it is in line with the other momentaries). It was a "Decision" way back when I made the kit: I feared too many people would forget that subtlety during soldering. If you want the original position, though, it is simple: cut the trace from diode (right-hand side of diode when seen from the front of the PCB) to the upper switch pin. Replace with a wire to the bottom pin. Mount switch upside-down.

Kind regards,

Oscar.

On Sun, 16 Feb 2020 at 13:15, David Griffith <frot...@gmail.com> wrote:

According to the PiDP-8 instructions, all momentary switches are to be installed as normally "up", that is, to activate them, you press down. However, looking at photos of actual PDP-8i panels, I see that the deposit switch is normally "down" and must be lifted to be activated.

I haven't yet installed the switches on my panel. What's going on here?

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Jeffrey T Apczynski

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Dec 30, 2021, 12:35:51 AM12/30/21

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Hi,

I'm about to assemble the Rev 3b 2019-05-13 version of the PiDP8i and I was thinking about mounting the deposit switch in the traditional way as switching UP to deposit the data. However, I'm a bit confused on exactly what to do and I don't want to cut traces without a picture of what the end result would look like.

Does anyone have a picture of what needs to be done to accomplish this?

Otherwise, I'll assemble the circuit board as instructed on the WEB site.

Thanks

-jeff-

William Cattey

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Dec 30, 2021, 7:11:03 PM12/30/21

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It's been a very long while since I did the Deposit Mod, so I don't remember exactly what I did. But here's a picture.

It looks like I soldered the deposit switch "upside down" and then tack-soldered the diode onto the other leg of the switch.

Does this give you enough to get you going?

Steve Tockey

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Dec 30, 2021, 7:31:15 PM12/30/21

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I did the mod on a PiDP-8/I that I built for someone else. Unfortunately I don't have that kit anymore, and I didn't take a photo. What I did was, IMHO, a bit less intrusive than what Bill suggests. The center lug is ground, and that doesn't change with the switch being right side up (as Oscar designed) or upside down (as authentic) so leave it alone. What you need to do is just cut a small gap into the PC board trace that goes to the top lug. Then carefully scrape a few mm of the clear paint-like covering over that trace--on the side away from the switch. This is to expose the metal for soldering the jumper. Finally, patch a small jumper from that exposed trace to the bottom lug. I used a piece of trimmed lead from one of the earlier resistors. Putting some kind of insulation on the jumper, as Bill shows, is good if you have some. Be very gentle and quick when you solder the jumper to the exposed trace so you don't lift it from the board.

Doing it this way keeps the switch diode in its original place.

If you send me a close-up photo of the back side of the board in that area I can photo-edit it and send it back to you.

-- steve

Robert Evans

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Dec 31, 2021, 1:17:38 PM12/31/21

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I did basically what Steve Tockey described. But there is a pad at the diode next to this switch where I could solder a short jumper. Here is more detail that corresponds to the 2019 kit version.

This edited KiCAD layout shows where I scratched through the trace at the red X, and the jumper I added is shown in orange...

The view above is from the other side of the board, so left and right are reversed. Here is a photo of my board with this mod. Today I could not easily see where I had scratched through the etch to cut it. When doing that I used an ohm meter to verify I had completely cut the trace. Then I added the jumper ...

I also thought about mounting the switch historically correctly and patching the software to invert the sense of this switch. I rejected this idea since it would require maintaining a custom patch to the PiDP-8 software and re-applying this on updates.

Best regards,

Robert Evans

Steve Tockey

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Dec 31, 2021, 2:42:12 PM12/31/21

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Robert,

Yes, that looks correct. Just one thing to be careful of. From your bottom photo it looks like there might be a short between your jumper and the the pad for the lug at the bottom of the switch. I think it's possible that the switch case might be connected to ground inside the switch itself. If so, that would make it look like your Deposit switch were permanently set. You might want to make sure that at least there's an air gap between your jumper and that lug pad. Sliding a bit of insulating material in there would be a good extra measure.

Also, in case you weren't already aware, there's a nice utility called pidp8i-test. If your PiDP-8/I is booted up, use CTRL/E to drop into SIMH then type EXIT to simh. That will drop you back into the Raspbian prompt. From there, type pidp8i-test. The test looks at both lights and switches so you can find opens and shorts on your board, if there are any. You can CTRL/C out of pidp8i-test then just pidp8i start followed by pidp8i and you're back into running as before.

Best of luck with it,

-- steve

Robert Evans

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Dec 31, 2021, 3:34:33 PM12/31/21

to Steve Tockey, PiDP-8

I completed this kit over a year ago. It tests out fine. I retested today after putting the case back together.

I'm reluctant to reopen the case but I'll keep it in mind for the future.

-RE

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