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PS/2 keyboard woes

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First post, by Rincewind42

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Hi, I just put together a new Socket 7 Pentium MMX machine that has an AT-style keyboard connector. When I connect an old AT keyboard to it, it naturally works fine. I bought a few PS/2 keyboards from eBay that I'm trying to use via a PS/2 -> DIN adapter because that AT keyboard is a bit worn out.

These are my findings:

- AT keyboard -- works perfectly fine
- Kensington "Comfort Type" ergonomic PS/2 keyboard -- works perfectly fine with a simple PS/2 -> DIN adapter
- Diamond Digital PS/2 keyboard -- works, kinda... but certain key combinations result in extra characters being typed in the DOS prompt and sporadic PC speaker beeps, then continuosly holding down keys (like in games) results in lots of beeps and extra characters that I did not even press

One would think the keyboard is simply faulty, but nope. I happened to buy TWO used Diamong Digital keyboards (yeah...), and both have the exact same issues. Moreover, when I connect these keyboards to my modern PC that happens to have PS/2 connectors, they work flawlessly.

So, it's clearly not my adapter nor my motherboard as the old AT and the Kensington PS/2 with the adapter work 100% fine. As the Diamond Digital works fine with a modern PC but not with the old Pentium, I can only think that the PS/2 standard has slightly changed over time, perhaps? Or the Diamond Digital does not 100% follow the standard, and old motherboards were more sensitive to such things?

In any case, out of the 3 keyboards I bought 2 are now useless... And this worries me because if I keep buying PS/2 keyboards online without the option to test them first, apparently there's a non-zero chance that I might get one incompatible with my old Pentium board.

Have you guys encountered such issues before? More importantly, a) is there a fix for such PS/2 keyboards to make them work with older machines, b) is there a list of known good PS/2 keyboards or something that are guaranteed to work with old machines? I'd hate to shell out more $$$ on another keyboard just to find that it works fine with modern machine but not my old Pentium... Thanks!

DOS: Soyo SY-5TF | MMX 200 | 128 MB | S3 Virge DX | ESS 1868F | QWave | S2 | McFly
Win98: Gigabyte K8VM800M | Athlon64 3200+ | 512 MB | Matrox G400 | SB Live!
WinXP: Gigabyte P31-DS3L | Core 2 Duo 2.33 GHz | 2 GB | GT 430 | Audigy 4

Reply 1 of 28, by mkarcher

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Rincewind42 wrote on 2022-08-16, 11:17:
Hi, I just put together a new Socket 7 Pentium MMX machine that has an AT-style keyboard connector. When I connect an old AT key […]
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Hi, I just put together a new Socket 7 Pentium MMX machine that has an AT-style keyboard connector. When I connect an old AT keyboard to it, it naturally works fine. I bought a few PS/2 keyboards from eBay that I'm trying to use via a PS/2 -> DIN adapter because that AT keyboard is a bit worn out.

These are my findings:

- AT keyboard -- works perfectly fine
- Kensington "Comfort Type" ergonomic PS/2 keyboard -- works perfectly fine with a simple PS/2 -> DIN adapter
- Diamond Digital PS/2 keyboard -- works, kinda... but certain key combinations result in extra characters being typed in the DOS prompt and sporadic PC speaker beeps, then continuosly holding down keys (like in games) results in lots of beeps and extra characters that I did not even press

One would think the keyboard is simply faulty, but nope. I happened to buy TWO used Diamong Digital keyboards (yeah...), and both have the exact same issues. Moreover, when I connect these keyboards to my modern PC that happens to have PS/2 connectors, they work flawlessly.

So, it's clearly not my adapter nor my motherboard as the old AT and the Kensington PS/2 with the adapter work 100% fine. As the Diamond Digital works fine with a modern PC but not with the old Pentium, I can only think that the PS/2 standard has slightly changed over time, perhaps? Or the Diamond Digital does not 100% follow the standard, and old motherboards were more sensitive to such things?

There were no changes to the keyboard protocol that might explain these issues. Some new key codes have been added (Windows keys, Power/Sleep keys, Multimedia keys), but the general protocol stayed the same, and the keyboard controller doesn't care about the keycodes, that's just a software thing. My guess is that the Diamond keyboards fail to work because the power supply isn't good enough. Try contact cleaner on the DIN jack in the mainboard (most likely culprit) and the PS/2 jack in your adapter. Different keyboards have different sensitivity to excessive resistance in the power supply connection, which would explain why some keyboards work and other keyboards fail.

If your mainboard has/had a NiCd/NiMH setup battery, you might also have a corroded power supply trace to the keyboard connector on the mainboard itself.

Reply 2 of 28, by dionb

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"Diamond Digital" isn't a brand I'm familiar with. I'd agree that current requirements could be to blame, as generally PS/2 and DIN are completely identical at a hardware level - but there may be a more subtle compatibility issue. Do you have any more information on those keyboards? Maybe they are relabeled from a more familiar ODM which would help to understand what's going on. The part/product ID from bar code on the bottom might help. Would also be good to know which motherboard you have, maybe it has known quirks.

If you go looking for a different keyboard, I'd recommend something good like the Cherry G80 (*NOT* the terrible G81 or G83) ranges, i.e. G80-3000 or (er, rare/expensive) G80-1000. I can confirm that both the -1000 and -3000 work fine with DIN-PS/2 on every board I've ever had, even on boards that can't supply the power needed to monsters like IBM Model Ms.

Reply 3 of 28, by Rincewind42

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mkarcher wrote on 2022-08-16, 11:31:

There were no changes to the keyboard protocol that might explain these issues. Some new key codes have been added (Windows keys, Power/Sleep keys, Multimedia keys), but the general protocol stayed the same, and the keyboard controller doesn't care about the keycodes, that's just a software thing.

Thanks, that was my understanding too, more or less.

mkarcher wrote on 2022-08-16, 11:31:

My guess is that the Diamond keyboards fail to work because the power supply isn't good enough.

Very unlikely as I'm using a brand new oversized Corsair 550W ATX PSU for this machine.

mkarcher wrote on 2022-08-16, 11:31:

Try contact cleaner on the DIN jack in the mainboard (most likely culprit) and the PS/2 jack in your adapter.

I can try that on the DIN jack, I have lots of isopropyl alcohol, would that work? The PS/2 -> AT adapters are brand new, I bought three of them just in case (they were very cheap), so might try another one as well. However, the adapter works fine with the Kensington keyboard like I said, so I think it's fine.

mkarcher wrote on 2022-08-16, 11:31:

Different keyboards have different sensitivity to excessive resistance in the power supply connection, which would explain why some keyboards work and other keyboards fail.

Huh, interesting. So in your experience 100% of PS/2 keyboards *must* work on old Pentium boards (or any motherboard, for the matter), and if they don't, it's always the motherboard's fault? That makes me sad, it's a very nice Intel 430HX board, otherwise...

mkarcher wrote on 2022-08-16, 11:31:

If your mainboard has/had a NiCd/NiMH setup battery, you might also have a corroded power supply trace to the keyboard connector on the mainboard itself.

That doesn't sound good, and I'm not an electronics person, so sadly can't do much about that if that's the case... The guy I bought it from seemed to be knowledgeable about this stuff, and he replaced the original DALLAS module with the Necroware RTC thingy, that's all I know. I've no idea what was in it before.

DOS: Soyo SY-5TF | MMX 200 | 128 MB | S3 Virge DX | ESS 1868F | QWave | S2 | McFly
Win98: Gigabyte K8VM800M | Athlon64 3200+ | 512 MB | Matrox G400 | SB Live!
WinXP: Gigabyte P31-DS3L | Core 2 Duo 2.33 GHz | 2 GB | GT 430 | Audigy 4

Reply 4 of 28, by Rincewind42

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dionb wrote on 2022-08-16, 12:14:

Do you have any more information on those keyboards? Maybe they are relabeled from a more familiar ODM which would help to understand what's going on. The part/product ID from bar code on the bottom might help.

Model NO: K201M
Serial No: G6140030510
Ratinv: 5V/50mA

Interestingly, the small print below that says "MITSUBISHI ELECTRIC AUSTRALIA".

dionb wrote on 2022-08-16, 12:14:

Would also be good to know which motherboard you have, maybe it has known quirks.

I have a Soyo SY-5TF board (Socket 7, i430HX chipset).

dionb wrote on 2022-08-16, 12:14:

If you go looking for a different keyboard, I'd recommend something good like the Cherry G80 (*NOT* the terrible G81 or G83) ranges, i.e. G80-3000 or (er, rare/expensive) G80-1000. I can confirm that both the -1000 and -3000 work fine with DIN-PS/2 on every board I've ever had, even on boards that can't supply the power needed to monsters like IBM Model Ms.

Thanks for the tips, that certainly helps. Certain keyboards needing more power and thus being incompatible with some boards is news to me, good to know.

DOS: Soyo SY-5TF | MMX 200 | 128 MB | S3 Virge DX | ESS 1868F | QWave | S2 | McFly
Win98: Gigabyte K8VM800M | Athlon64 3200+ | 512 MB | Matrox G400 | SB Live!
WinXP: Gigabyte P31-DS3L | Core 2 Duo 2.33 GHz | 2 GB | GT 430 | Audigy 4

Reply 5 of 28, by timw4mail

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Rincewind42 wrote on 2022-08-16, 12:25:
mkarcher wrote on 2022-08-16, 11:31:

My guess is that the Diamond keyboards fail to work because the power supply isn't good enough.

Very unlikely as I'm using a brand new oversized Corsair 550W ATX PSU for this machine.

Not the power supply as a whole, but the amount of power that is allowed to go to the AT connector.

Also, when it comes to powering older machines, the 5V rail is far more important than the 12V rail that modern PSUs are optimised for. For Socket 7 this is probably fine anyway, but just something to keep in mind.

Reply 6 of 28, by dionb

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Rincewind42 wrote on 2022-08-16, 12:33:
Model NO: K201M Serial No: G6140030510 Ratinv: 5V/50mA […]
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dionb wrote on 2022-08-16, 12:14:

Do you have any more information on those keyboards? Maybe they are relabeled from a more familiar ODM which would help to understand what's going on. The part/product ID from bar code on the bottom might help.

Model NO: K201M
Serial No: G6140030510
Ratinv: 5V/50mA

Interestingly, the small print below that says "MITSUBISHI ELECTRIC AUSTRALIA".

Mitsubishi - now that is a more familiar brand 😉

Looking up the K201M, I don't find any good pics, but I do see that it is a keyboard + mouse combo. That might be very relevant here if they share the same PS/2 connector. The Mini-DIN PS/2 connector has 6 pins, two of which are Not Connected for a single link (keyboard or mouse) but used for the second clock and data signals if it's a dual PS/2 device. The AT-DIN connector only has 5 pins, so can only support a single device. I'm suspecting that for some reason the keyboard is the second device on the connector, which explains why it's not working when hooked up to the DIN->PS/2 adapter.

"Simple" way to test: DIN pins 1 and 2 should be connected to PS/2 pins 5 and 1 respectively at the moment. If my hunch is correct, if you connect DIN 1 and 2 to resp PS/2 pins 6 and 2 it should work. In that case, the issue is very specific to this one keyboard you have bought two of, as it's a) two combined devices on a single PS/2 port (rare) and b) put keyboard as device 2, not 1 (never seen that before...)

I have a Soyo SY-5TF board (Socket 7, i430HX chipset).

Standard, decent and generally reliable board & chipset. Not likely to be the issue, unless physically damaged.

Thanks for the tips, that certainly helps. Certain keyboards needing more power and thus being incompatible with some boards is news to me, good to know.

It happens, but it's pretty rare. The Model M is infamous, as it draws 10x more power than most contemporary 1980s designs. Even so, I've only ever had problems with it on one or two motherboards, and one KVM switch.

Reply 8 of 28, by Rincewind42

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dionb wrote on 2022-08-16, 13:39:

Looking up the K201M, I don't find any good pics, but I do see that it is a keyboard + mouse combo.

For reference, this is what I bought. I don't think it's a KB+M combo, there's just a single standard PS/2 connector coming out of it.
https://www.ebay.com.au/itm/225000261258

dionb wrote on 2022-08-16, 13:39:

That might be very relevant here if they share the same PS/2 connector. The Mini-DIN PS/2 connector has 6 pins, two of which are Not Connected for a single link (keyboard or mouse) but used for the second clock and data signals if it's a dual PS/2 device. The AT-DIN connector only has 5 pins, so can only support a single device. I'm suspecting that for some reason the keyboard is the second device on the connector, which explains why it's not working when hooked up to the DIN->PS/2 adapter.

Yeah, so like I said it's "working" in the sense that if I keep pressing single keys with 100-200ms pauses between the keypresses then all keys works fine. It's just that when I start typing normally, there are *extra* characters inserted sometimes (but quite deterministically, it's not random), and sometimes various key combinations get triggered (e.g. in DOS Navigator I just start typing normally, then suddenly dialog boxes start appearing because some Alt/Ctrl+<other key> combos got triggered). This is always accompanied by frequent PC speaker beeps. Same in games when I just use the cursor keys; it kinda works, but there's tons of beeps and other keys get sporadically triggered too.

I'm guessing if the clock and data signals were on the wrong pins no keystrokes would be registered whatsoever, right?

DOS: Soyo SY-5TF | MMX 200 | 128 MB | S3 Virge DX | ESS 1868F | QWave | S2 | McFly
Win98: Gigabyte K8VM800M | Athlon64 3200+ | 512 MB | Matrox G400 | SB Live!
WinXP: Gigabyte P31-DS3L | Core 2 Duo 2.33 GHz | 2 GB | GT 430 | Audigy 4

Reply 9 of 28, by mkarcher

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Rincewind42 wrote on 2022-08-16, 12:25:
mkarcher wrote on 2022-08-16, 11:31:

My guess is that the Diamond keyboards fail to work because the power supply isn't good enough.

Very unlikely as I'm using a brand new oversized Corsair 550W ATX PSU for this machine.

I'm sorry. I meant the power arriving at the keyboard. I didn't mean to imply the PSU itself is bad, but instead the wiring from the power supply to the keyboard. That wiring consists of the traces on the mainboard, the DIN socket on the mainboard, the DIN plug at your converter, the internal wires in your converter, the PS/2 socket in your converter, the PS/2 plug of your keyboard, the cable of your keyboard and possibly a connection between the keyboard cable and the keyboard PCB inside the keyboard.

As the problem happens with two copies of the problematic keyboard, I'm going to exclude all parts of the keyboard, which leaves the mainboard traces, the DIN plug and your adapter as most likely culprits.

Rincewind42 wrote on 2022-08-16, 12:25:
mkarcher wrote on 2022-08-16, 11:31:

Try contact cleaner on the DIN jack in the mainboard (most likely culprit) and the PS/2 jack in your adapter.

I can try that on the DIN jack, I have lots of isopropyl alcohol, would that work? The PS/2 -> AT adapters are brand new, I bought three of them just in case (they were very cheap), so might try another one as well. However, the adapter works fine with the Kensington keyboard like I said, so I think it's fine.

In my experience, IPA wouldn't do much about corrosion inside the DIN jack, if that is the cause of your issue. Repeatedly plugging and unplugging the connector, and possibly just wiggling the connector might scratch away corrosion of the +5V or GND contacts, providing a more stable supply to the keyboard.

Rincewind42 wrote on 2022-08-16, 12:25:
mkarcher wrote on 2022-08-16, 11:31:

Different keyboards have different sensitivity to excessive resistance in the power supply connection, which would explain why some keyboards work and other keyboards fail.
If your mainboard has/had a NiCd/NiMH setup battery, you might also have a corroded power supply trace to the keyboard connector on the mainboard itself.

Huh, interesting. So in your experience 100% of PS/2 keyboards *must* work on old Pentium boards (or any motherboard, for the matter), and if they don't, it's always the motherboard's fault? That makes me sad, it's a very nice Intel 430HX board, otherwise...

I know the SY-5TF, I have one myself, although it currently isn't included in any system. I just checked it: As the board used a Dallas chip with an integrated battery, it doesn't have the dreaded Nickel battery that starts to leak and eat traces. I expect any PS/2 keyboard to work on any system with a DIN (AT-Style) or Mini-DIN (PS/2-style) keyboard socket starting at the IBM AT.

If my theory about the power supply to the keyboard being weak due to bad contacts is true, you should be able to observe varying brightness of the NumLock-LED when your wiggle the DIN or the PS/2 plug. Another way to confirm power supply issues is testing whether you get more issues with that keyboard the more LEDs (Num, Caps, Scroll) are lighting up.

Reply 10 of 28, by dionb

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Rincewind42 wrote on 2022-08-16, 14:06:

[...]

For reference, this is what I bought. I don't think it's a KB+M combo, there's just a single standard PS/2 connector coming out of it.
https://www.ebay.com.au/itm/225000261258

The thing about the combo is that it would also work over a single PS/2 connector. But no, they don't look much like a combo.

Yeah, so like I said it's "working" in the sense that if I keep pressing single keys with 100-200ms pauses between the keypresses then all keys works fine. It's just that when I start typing normally, there are *extra* characters inserted sometimes (but quite deterministically, it's not random), and sometimes various key combinations get triggered (e.g. in DOS Navigator I just start typing normally, then suddenly dialog boxes start appearing because some Alt/Ctrl+<other key> combos got triggered). This is always accompanied by frequent PC speaker beeps. Same in games when I just use the cursor keys; it kinda works, but there's tons of beeps and other keys get sporadically triggered too.

I'm guessing if the clock and data signals were on the wrong pins no keystrokes would be registered whatsoever, right?

Yes, sorry, was barking up the wrong tree - looks like I didn't read your description accurately enough. This is exactly behaviour you'd expect if the signals are right on the edge of spec and so clocks are out of sync and keypresses are interpreted incorrectly.

Reply 11 of 28, by Rincewind42

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dionb wrote on 2022-08-16, 15:20:

Yes, sorry, was barking up the wrong tree - looks like I didn't read your description accurately enough. This is exactly behaviour you'd expect if the signals are right on the edge of spec and so clocks are out of sync and keypresses are interpreted incorrectly.

No problem, and I think yours is the winning answer. I've done some further testing and these are my findings:

- Both Mitshubishi keyboards work perfectly in the BIOS
- Trying a different PS/2 -> AT adapter doesn't make any difference whatsoever
- The keyboard actually works 100% fine when only typing at the DOS prompt (!)
- The CapsLock/NumLock/ScrollLock LEDs are working just fine; I can turn on all three at the same time and there's no blinking or dimming or anything
- The keyboard works 100% fine in TimeQuest, Spellcasting 101 (so probably all Legend Entertainment titles)
- In Space Quest 3 or Quest for Glory 1, I'm getting lots of beeps and extra random keypresses when typing anything. Same deal in lots of other programs, e.g. DOS Navigator.
- If I type sloooooowly, pausing for about half a second between keypresses and making sure not to trigger keyrepeat, there are no beeps or extra characters being typed in any program I tried

So yeah, I think what's happening is that DOS itself and some games use the standard DOS interrupts to interact with the keyboard -- that works fine and the DOS-level keyboard services are apparently robustly coded. But lots of other games and programs that just bang the hardware directly run into problems because they keyboard doesn't 100% follow the standard and those keyboard routines are probably sloppily coded that just happen to work with most PS/2 keyboards out of sheer luck...

I guess there's not much else to do than to call it a loss and buy some other PS/2 keyboards off eBay. Hopefully the Microsoft ones are more reliable. I'll pass on the Cherry mechanical keyboards, they would cost me as much as my whole Pentium machine, I think that would be a bit crazy 😀

Thanks for the help everybody!

DOS: Soyo SY-5TF | MMX 200 | 128 MB | S3 Virge DX | ESS 1868F | QWave | S2 | McFly
Win98: Gigabyte K8VM800M | Athlon64 3200+ | 512 MB | Matrox G400 | SB Live!
WinXP: Gigabyte P31-DS3L | Core 2 Duo 2.33 GHz | 2 GB | GT 430 | Audigy 4

Reply 12 of 28, by Solo761

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I had similar issue with one of my Socket 7 motherboards. Some cheapo PS/2 keyboard I had, relatively modern, about 5 years old, worked fine. I found two older PS/2 keyboards from 15-20 years ago and it was no go with them.

One appeared to work fine when powering on (LEDs turning on), but when you pressed any key it would just blink it's LEDs, unless I turned caps lock LED off on boot, then it worked fine, until I tried to press any of the keys that turn LEDs on.
The other one was just blinking with it's LEDs. Didn't recognize any key press.

Turned out it was motherboard's fault (btw. it was QDI Titanium IB+), it had transistor to controll voltage on AT keyboard (and I guess PS/2 mouse port) pins and it was faulty. Without anything connected it held at 5V, but when keyboard was connected it was sub 4V. And when you actually started to use the keyboard it went below 3V.
Modern keyboard was least power hungry so even below 3V was enough for it to work. For these others it wasn't, first one managed to keep a bit above 3V when LEDs were off so that's why it worked if caps lock was off. But turning any LEDs on would dip it below 3V and it couldn't recover.

After I replaced that transistor all of these keyboards work without issues.

Reply 13 of 28, by dionb

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Rincewind42 wrote on 2022-08-17, 02:13:

[...]

I guess there's not much else to do than to call it a loss and buy some other PS/2 keyboards off eBay. Hopefully the Microsoft ones are more reliable. I'll pass on the Cherry mechanical keyboards, they would cost me as much as my whole Pentium machine, I think that would be a bit crazy 😀

Tip: eBay isn't the only place to buy stuff, and for on-the-spur-of-the-moment buys it's almost certainly the most expensive (simple economics: biggest platform, so limited supply is exposed to highest demand). I have quite a few, both Cherry branded and others (Compaq used quite a few G80-11800 designs). Never spent more than EUR 20 on one, usually got them as part of a bigger lot of random old crap, which is 1/10 of what I would have paid on eBay. Not sure what alternatives to eBay there are Down Under, but the more local, the higher the chance of finding little gems for acceptable prices. Also saves exorbitant shipping costs. Trawling garage sales would be very effective if you have the time.

Btw, in the mechanical keyboard community Cherry boards aren't considered particularly good. There are far better switches out there, and the build quality of the G80-1000 is nothing special and G80-3000 is downright bad. But they sold widely and are a lot better than some random el-cheapo rubber dome - plus they look the retro part if you go for the old beige ones. Better stuff is something of a rabbit hole though, which is why I didn't suggest looking for boards with complicated Alps switches, or NMB Hi-Tek space invaders 😉

Reply 14 of 28, by Rincewind42

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dionb wrote on 2022-08-17, 09:49:

Trawling garage sales would be very effective if you have the time.

Thanks for sharing your experience. Time is definitely an issue, and to some extent I'm willing to trade my time for higher prices, but of course hundreds for a simple keyboard is just too much. I found a guy here who has lots of cool retro gear for very reasonable prices, but sadly he didn't have any PS/2 keyboards last time I went there. Btw, I just managed to score a 10-pack of Cherry PS/2 mouses for $95, couldn't pass that one up, that should last me a lifetime 😀

dionb wrote on 2022-08-17, 09:49:

Btw, in the mechanical keyboard community Cherry boards aren't considered particularly good. [...] But they sold widely and are a lot better than some random el-cheapo rubber dome - plus they look the retro part if you go for the old beige ones.

Yeah I prefer the look of the old beige keyboards for sure... although the Cherry mouses I just bought are black 🤷 I think I had a single mechanical keyboard in the 90s, but couldn't get rid of it fast enough... Never liked all the noise that they generate, and pretty much all my friends hated them too. Never understood the appeal, to be honest, and I can type perfectly fine and fast on laptop style keyboards, which is my preference actually (the shorter the travel/action, the better, IMO). But yeah, they surely look nice 😀

DOS: Soyo SY-5TF | MMX 200 | 128 MB | S3 Virge DX | ESS 1868F | QWave | S2 | McFly
Win98: Gigabyte K8VM800M | Athlon64 3200+ | 512 MB | Matrox G400 | SB Live!
WinXP: Gigabyte P31-DS3L | Core 2 Duo 2.33 GHz | 2 GB | GT 430 | Audigy 4

Reply 15 of 28, by scruit

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A thought from when I had a similar issue, and the theory I settled on was:

There are XT keyboards, PS/2 keyboards and USB keyboards. 3 different connectors.

So: Why can't you plug a USB keyboard into a ps/2 adapter, then into an XT adapter? Because of the keyboard communication protocol (electrical signals) being different between the 3.

- There are USB keyboards that can work with ps/2 adapters because, although they natively use USB protocol, they can detect and communicate on ps/2 protocol.
- There are ps/2 keyboards that can work with USB adapters because, although they natively use ps/2 protocol, they can detect and communicate on USB protocol
- There are ps/2 keyboards that can work with XT adapters because, although they natively use ps/2 protocol, they can detect and communicate on XT protocol

If they keyboard does not natively detect/support the protocol then you will need a converter (changes the signals), not just an adapter (changes physical pin layout only). There's a bunch of chatter on the internet about using arduino/teensy etc for plugging old xt mechanical keyboards into modern USB-only PCs. They have to intercept the signals and modify them for USB.

Example of a convertor (not an endorsement): https://www.hagstromelectronics.com/ke-xtusb- … ncoder-shp.html

Reply 16 of 28, by dionb

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scruit wrote on 2022-08-17, 11:25:

A thought from when I had a similar issue, and the theory I settled on was:

There are XT keyboards, PS/2 keyboards and USB keyboards. 3 different connectors.

3 connectors and 3 protocols:

XT, AT/PS-2 and USB.

>95% of keyboards with DIN connector use the AT protocol, not XT. AT and PS/2 are basically the same, XT is completely different, unidirectional rather than bidirectional. You can recognize XT keyboards by the lack of indicator LEDs. Early AT keyboards had a selector switch for XT/AT.

So: Why can't you plug a USB keyboard into a ps/2 adapter, then into an XT adapter? Because of the keyboard communication protocol (electrical signals) being different between the 3.

- There are USB keyboards that can work with ps/2 adapters because, although they natively use USB protocol, they can detect and communicate on ps/2 protocol.

Mice yes, keyboards... not really. USB-PS/2 adapters are active, not passive.

- There are ps/2 keyboards that can work with USB adapters because, although they natively use ps/2 protocol, they can detect and communicate on USB protocol

No. PS/2 keyboards don't talk USB at all. Adapters that let this work are active protocol converters.

- There are ps/2 keyboards that can work with XT adapters because, although they natively use ps/2 protocol, they can detect and communicate on XT protocol

I've never seen PS/2 keyboards with XT protocol support. PS/2 will work with AT and vice versa over passive adapter cables as it's the same protocol.

If they keyboard does not natively detect/support the protocol then you will need a converter (changes the signals), not just an adapter (changes physical pin layout only). There's a bunch of chatter on the internet about using arduino/teensy etc for plugging old xt mechanical keyboards into modern USB-only PCs. They have to intercept the signals and modify them for USB.

Example of a convertor (not an endorsement): https://www.hagstromelectronics.com/ke-xtusb- … ncoder-shp.html

Note that this is XT, not AT.

But yes, it's pretty trivial over Arduino. I once wrote a (minimally) functional controller to connect an old Osborne Variant 1 keyboard (with a huge 20-pin connector that exposes scanlines directly) to USB. I'd need to do more work on de-bouncing if I wanted to take it further - but didn't like the feel of the keyboard enough to want to put the effort into it.

Reply 17 of 28, by dionb

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Rincewind42 wrote on 2022-08-17, 11:03:

[...]

Yeah I prefer the look of the old beige keyboards for sure... although the Cherry mouses I just bought are black 🤷 I think I had a single mechanical keyboard in the 90s, but couldn't get rid of it fast enough... Never liked all the noise that they generate, and pretty much all my friends hated them too. Never understood the appeal, to be honest, and I can type perfectly fine and fast on laptop style keyboards, which is my preference actually (the shorter the travel/action, the better, IMO). But yeah, they surely look nice 😀

In that case, look for old scissor-switch keyboards from Cherry's G84 range. They are pretty uncommon and far less sought-after than their MX-swith G80 brethren, because most mechanical devotees prefer 'real' keyboard travel to the low travel of laptop boards. But if you like that, that will work in your favour.

Note by the way that "mechanical" most definitely does not need to mean "clicky". Yes, all clicky switches are mechanical (usually IBM buckling spring. Alps white or Cherry MX blue switches) but there are a lot of non-clicky mechanicals. They can be linear (MX red is popular with gamers these days, slightly stiffer MX black was preferred for a lot of old keyboards) or tactile (the very weak MX brown is currently best-known, but Alps black used to be common and Alps salmon orange and cream on much sought-after on boards like SGI and Dell 'bigfoot' and Apple Extended keyboards).

Reply 18 of 28, by rasz_pl

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Rincewind42 I would try few things
-measure voltage at keyboard connector, static and with some kind of a scope during long keypresses generating errors
-put a scope on data lines and check frequency, should be in 10-16.7 kHz range, compare to working keyboards
-put a scope on data line and see how clean it is

scruit wrote on 2022-08-17, 11:25:

- There are ps/2 keyboards that can work with XT adapters because, although they natively use ps/2 protocol, they can detect and communicate on XT protocol

never seen one able to switch automagically

Reply 19 of 28, by dr_st

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dionb wrote on 2022-08-17, 14:22:
Mice yes, keyboards... not really. USB-PS/2 adapters are active, not passive. […]
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- There are USB keyboards that can work with ps/2 adapters because, although they natively use USB protocol, they can detect and communicate on ps/2 protocol.

Mice yes, keyboards... not really. USB-PS/2 adapters are active, not passive.

- There are ps/2 keyboards that can work with USB adapters because, although they natively use ps/2 protocol, they can detect and communicate on USB protocol

No. PS/2 keyboards don't talk USB at all. Adapters that let this work are active protocol converters.

I have a couple of keyboards that came with such a purple USB-->PS/2 adapter and work with it. It looks passive, and I don't think it works with all keyboards.

Furthermore, Startech sells such a PS/2-->USB adapter, which they claim works with the keyboards that originally shipped with a similar one. I have never seen such a keyboard myself, though.

What do you think?

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