Who uses PS/2 instead of USB?

[Keytrun]

Much or NO benefit? Because I CAN plug it into PS/2 - I just dont want to accidentally unplug it while the computer is on and brick my board.

[tuxsavvy]

Much or NO benefit? Because I CAN plug it into PS/2 - I just dont want to accidentally unplug it while the computer is on and brick my board.

• If your keyboard natively (i.e. supplied cable) was USB then there would be NO benefit apart from freeing up one USB socket.
• If your motherboard does not have PS/2 plug on the rear (they call it I/O panel) then there are NO benefits. Even if you have the PS/2 socket on a PCI card. You will only free up one USB socket.
• If your keyboard natively communicates via PS/2, you may be able to get benefits (depending on the keyboard and the controller namely) WHILST it is plugged into the back of the motherboard VIA motherboard supplied PS/2 port. Though again you are really mainly freeing up a USB port regardless. Some PS/2 keyboards offer NKRO functionality (this is handy if you are a gamer who has to press lots of keys at the same time).

I never had an issue with bricking my keyboard whilst I go around plugging and unplugging both my PS/2 keyboard and/or my USB keyboard. That said I do not have an adapter that goes from USB to PS/2 neither would I see any benefits of buying one. Some operating systems (namely windows for instance) may fail to detect a keyboard plugged into PS/2 when it has already finished booting up. Therefore the best way is to turn off computer, unplug the keyboard from USB and maybe plug it into PS/2 before turning the computer on. This is probably more safer for those fearing they may brick their keyboard or when running windows, fail to have the keyboard being detected.

[Keytrun]

That helps a lot. Thanks a bunch. I wasn't sure what the deal was, and everyone had told me that PS/2 was the better choice for the NKRO. I don't think I can hold any more than 6 keys down at a time considering I only have 5 fingers....so the USB 6 KRO should be good enough. I'm currently plugged into PS/2 with a USB -> PS/2 adapter. Next time I take the keyboard out I'll just plug it back into the standard USB port. Thanks.

[Elrick]

Therefore the best way is to turn off computer, unplug the keyboard from USB and maybe plug it into PS/2 before turning the computer on. This is probably more safer for those fearing they may brick their keyboard or when running windows, fail to have the keyboard being detected.

THIS is the ONLY way to initiate a PS/2 keyboard with your motherboard.  Always switch it off before unplugging/plugging in a PS/2 style connector because somehow people think you can use a PS/2 connected keyboard like a standard usb one and that is plain wrong.  Not only will you most probably fry your PS/2 port and never again having a fully working receptacle but also may damage the pcb on the keyboard as well.

I know because when I was younger I managed to fry numerous PS/2 ports within Win 98SE and Win 2000.  All because I wanted to find out what would happen when you fiddle around with the PS/2 ports, when the PC's were on  .

YES, I was an idiot teenager looking for some fun......

[tuxsavvy]

Therefore the best way is to turn off computer, unplug the keyboard from USB and maybe plug it into PS/2 before turning the computer on. This is probably more safer for those fearing they may brick their keyboard or when running windows, fail to have the keyboard being detected.

THIS is the ONLY way to initiate a PS/2 keyboard with your motherboard.  Always switch it off before unplugging/plugging in a PS/2 style connector because somehow people think you can use a PS/2 connected keyboard like a standard usb one and that is plain wrong.  Not only will you most probably fry your PS/2 port and never again having a fully working receptacle but also may damage the pcb on the keyboard as well.

I know because when I was younger I managed to fry numerous PS/2 ports within Win 98SE and Win 2000.  All because I wanted to find out what would happen when you fiddle around with the PS/2 ports, when the PC's were on  .

YES, I was an idiot teenager looking for some fun......

First part I disagree, you can still initiate PS/2 keyboard WHILST your motherboard is turned on. I am running linux here and I never had to turn off my computer in order to plug/unplug a PS/2 keyboard under linux. That said, according to wikipedia and superuser such idea (known as hotplugging) shouldn't be done even though they may not necessarily cause damage.

Thinking of the same said issue with PS/2 hotplugging, this might also explain the reason why two Asus motherboards I have had suddenly refused to work with PS/2 keyboards (but they still froze even if I managed to get into the BIOS). I can't remember at the time did I try USB keyboards but I do remember that it was a real pain trying to fix up some issues.

[Grendel]

I wasn't sure what the deal was, and everyone had told me that PS/2 was the better choice for the NKRO. I don't think I can hold any more than 6 keys down at a time considering I only have 5 fingers....so the USB 6 KRO should be good enough. I'm currently plugged into PS/2 with a USB -> PS/2 adapter. Next time I take the keyboard out I'll just plug it back into the standard USB port.

What happened to your other 5 fingers ? The USB -> PS/2 adapter that comes w/ keyboards is a passive part -- the keyboard will detect what it is connected to and talk to that port natively, using USB 1.1 low speed. In these scenarios there are multiple advantages using the PS/2 port:

 - Lower latency:
  -  the PS/2 receiver is event driven (KB sends event as soon it happens) vs. USB having to poll for changes
  - a lot less CPU resources are needed for PS/2
  - most KBs ask for 8ms polling rate
  - USB 1.1 is limited to an 8ms polling rate on Windows anyway

 - NKRO -- PS/2 sends key up/down events vs. USBs key status (limited to 6 keys + 8 modifiers)

 - USB ports have a tendency to be in short supply

Disadvantages of the PS/2 port are:

 - not P&P
 - no suspend state, back-lit KBs will stay illuminated in soft-off (Decks eg. do this)

[osi]

One thing I've noticed recently while hot swapping PS/2 (bad I know) is the repeating rate. To test, boot up the machine with keyboard in PS/2, hold down a key and watch how fast it repeats. Unplug the board, then plug it back it and watch the difference.

I've seen this on both PS/2 native keyboard as well as through the USB->PS/2 adapters. A reboot will fix this. Have not seen this behavior with USB connected boards.

[Linkbane]

I wasn't sure what the deal was, and everyone had told me that PS/2 was the better choice for the NKRO. I don't think I can hold any more than 6 keys down at a time considering I only have 5 fingers....so the USB 6 KRO should be good enough. I'm currently plugged into PS/2 with a USB -> PS/2 adapter. Next time I take the keyboard out I'll just plug it back into the standard USB port.

What happened to your other 5 fingers ? The USB -> PS/2 adapter that comes w/ keyboards is a passive part -- the keyboard will detect what it is connected to and talk to that port natively, using USB 1.1 low speed. In these scenarios there are multiple advantages using the PS/2 port:

 - Lower latency:
  -  the PS/2 receiver is event driven (KB sends event as soon it happens) vs. USB having to poll for changes
  - a lot less CPU resources are needed for PS/2
  - most KBs ask for 8ms polling rate
  - USB 1.1 is limited to an 8ms polling rate on Windows anyway

 - NKRO -- PS/2 sends key up/down events vs. USBs key status (limited to 6 keys + 8 modifiers)

 - USB ports have a tendency to be in short supply

Disadvantages of the PS/2 port are:

 - not P&P
 - no suspend state, back-lit KBs will stay illuminated in soft-off (Decks eg. do this)

Grendel, I think that it might be valuable to mention that this is 2013 and not 2005. Almost every keyboard these days has NKRO over USB, or at least 10. Also all of the 'gaming' keyboards I know of have a 1ms latency. 8ms might be something, but 1ms is not noticeable by anyone, and will make no effect on the game.  Also, USB 2.0 and 3.0 is universal, nothing uses 1.1 now.
USB ports can also be expanded quite simple, unlike PS/2.

[Soarer]

Most USB keyboards are still low-speed and 6KRO.
Most NKRO USB keyboards are still low-speed, and therefore use hackish methods.
Some keyboards claiming 1ms latency are still low-speed, and therefore 8ms, so the claim is false.
Some keyboards which do have NKRO USB and full-speed, still use hackish methods.

Which leaves very few, if any, that actually do it 'right', as in 'as good as PS/2', even though it's perfectly possible.

Don't forget that PS/2 has latency too, of ~1ms for most presses and ~2ms for most releases. Add ~1ms to those figures for the 'extended' keys that the 101-key added over the 84-key AT. These are the times it takes to transmit the scancodes serially along the wire.

[Findecanor]

The USB -> PS/2 adapter that comes w/ keyboards is a passive part -- the keyboard will detect what it is connected to and talk to that port natively, using USB 1.1 low speed.

I am not sure I understand if you understand correctly... but to clarify:

A keyboard that comes with a passive adapter can talk PS/2 or USB over the same cable. PS/2 with the passive adapter through the host's PS/2 port or USB through the USB port.
Over PS/2 it will receive its power on another lead in the cable, and that allows it to detect that it should be in "PS/2 mode" and talk PS/2. A keyboard that talks only USB and which is connected with a passive USB -> PS/2 adapter will not work - it will not even try, because it will not receive power on the lead that it expects to.

There is no way that the USB protocol can be used over the host computer's PS/2 port.

Which leaves very few, if any, that actually do it 'right', as in 'as good as PS/2', even though it's perfectly possible.

Don't forget that PS/2 has latency too, of ~1ms for most presses and ~2ms for most releases. Add ~1ms to those figures for the 'extended' keys that the 101-key added over the 84-key AT. These are the times it takes to transmit the scancodes serially along the wire.

If more than one key event would have occurred (actually: been debounced) on the same millisecond, a USB keyboard would theoretically also be able to send both events in the same packet - whereas the PS/2 keyboard would be required to delay the second event.

I don't think that any USB keyboard takes advantage of this, though. It is so much less complex for the controller to send only at most one event per packet.

[Grendel]

Grendel, I think that it might be valuable to mention that this is 2013 and not 2005. Almost every keyboard these days has NKRO over USB, or at least 10. Also all of the 'gaming' keyboards I know of have a 1ms latency. 8ms might be something, but 1ms is not noticeable by anyone, and will make no effect on the game.  Also, USB 2.0 and 3.0 is universal, nothing uses 1.1 now.
USB ports can also be expanded quite simple, unlike PS/2.

I would like to add to Soarers answer above that we are talking about PS/2 vs. USB and I was implying that you have a keyboard that actually can do both. I don't know of a single keyboard w/ a USB 2.0 interface that also can do PS/2 on that port (if anyone does, please, speak up !) If you read "NKRO via PS/2, else 6KRO" in an ad it means that the USB link is 1.1 low speed. This includes quite a few keyboard models from this and last year. It also implies that if you need low latency (gaming) you will get noticeable better performance by using the PS/2 port instead of the USB with these boards.

The USB -> PS/2 adapter that comes w/ keyboards is a passive part -- the keyboard will detect what it is connected to and talk to that port natively, using USB 1.1 low speed.

I am not sure I understand if you understand correctly... but to clarify:

A keyboard that comes with a passive adapter can talk PS/2 or USB over the same cable. PS/2 with the passive adapter through the host's PS/2 port or USB through the USB port.
Over PS/2 it will receive its power on another lead in the cable, and that allows it to detect that it should be in "PS/2 mode" and talk PS/2. A keyboard that talks only USB and which is connected with a passive USB -> PS/2 adapter will not work - it will not even try, because it will not receive power on the lead that it expects to.

There is no way that the USB protocol can be used over the host computer's PS/2 port.

To quote you -- I am not sure I understand if you understand correctly Both connectors have exactly 4 leads -- 5V and GND (both), and +Clk and +Data (PS/2) or D- and D+ (USB). A passive adapter will just connect 5V w/ 5V, GND w/ GND, D+ w/ +Clk, and D- w/ +Data. It's up to the MCU in the keyboard to figure out if the two data signals are connected to a PS/2 or USB port. reference

If more than one key event would have occurred (actually: been debounced) on the same millisecond, a USB keyboard would theoretically also be able to send both events in the same packet - whereas the PS/2 keyboard would be required to delay the second event.

I don't think that any USB keyboard takes advantage of this, though. It is so much less complex for the controller to send only at most one event per packet.

A keyboard transfers its state via USB, not key up/down events. All keys that are down when an USB report is generated will be in that report, provided the report can hold however many keys are active.

[Linkbane]

Grendel, I think that it might be valuable to mention that this is 2013 and not 2005. Almost every keyboard these days has NKRO over USB, or at least 10. Also all of the 'gaming' keyboards I know of have a 1ms latency. 8ms might be something, but 1ms is not noticeable by anyone, and will make no effect on the game.  Also, USB 2.0 and 3.0 is universal, nothing uses 1.1 now.
USB ports can also be expanded quite simple, unlike PS/2.

I would like to add to Soarers answer above that we are talking about PS/2 vs. USB and I was implying that you have a keyboard that actually can do both. I don't know of a single keyboard w/ a USB 2.0 interface that also can do PS/2 on that port (if anyone does, please, speak up !) If you read "NKRO via PS/2, else 6KRO" in an ad it means that the USB link is 1.1 low speed. This includes quite a few keyboard models from this and last year. It also implies that if you need low latency (gaming) you will get noticeable better performance by using the PS/2 port inste

Every keyboard obviously has NKRO over PS/2. If any gamer actually got a benefit from using PS/2 boards, some probably would. We obviously don't see any. The TK, Shine 3, Filco, Noppoo, etc. all have NKRO over USB, among countless others. It's not unique, and I personally expect it from any board that I buy.
And you must be a wizard if you can notice the difference between a 1ms response and a slightly less than 1ms. You are Grendel, after all.

[Soarer]

I'm pretty sure Filco sticks with the standard 6KRO for their 'boards.

Grendel was agreeing about the difference between 8ms and 1ms, I thought   The reason that no keyboards with full-speed also support PS/2 is that there is no point! Full-speed is faster, even if only by a negligible amount.

[Soarer]

A keyboard transfers its state via USB, not key up/down events. All keys that are down when an USB report is generated will be in that report, provided the report can hold however many keys are active.

True, but internally the output from an individual key debounce routine is generally events. It's then up to the firmware writer to decide whether to queue those and present them sequentially, or copy them directly into the output. I've no idea which style is more common! For batch debouncing, the output is new full state, and it tends to bunch key changes together anyway. Batch debounce is probably still far too common, even in 'gamer' keyboards.

[Linkbane]

I'm pretty sure Filco sticks with the standard 6KRO for their 'boards.

Grendel was agreeing about the difference between 8ms and 1ms, I thought   The reason that no keyboards with full-speed also support PS/2 is that there is no point! Full-speed is faster, even if only by a negligible amount.

 
So, what does support mean? I know that USB can be converted into PS/2, but I assume it's something deeper?

[Soarer]

I'm pretty sure Filco sticks with the standard 6KRO for their 'boards.

Grendel was agreeing about the difference between 8ms and 1ms, I thought   The reason that no keyboards with full-speed also support PS/2 is that there is no point! Full-speed is faster, even if only by a negligible amount.

 
So, what does support mean? I know that USB can be converted into PS/2, but I assume it's something deeper?

Support, as in the usual sense with computers! For a USB keyboard to work on PS/2 with one of those passive dongles, it has to support PS/2 as well.

[Linkbane]

I'm pretty sure Filco sticks with the standard 6KRO for their 'boards.

Grendel was agreeing about the difference between 8ms and 1ms, I thought   The reason that no keyboards with full-speed also support PS/2 is that there is no point! Full-speed is faster, even if only by a negligible amount.

 
So, what does support mean? I know that USB can be converted into PS/2, but I assume it's something deeper?

Support, as in the usual sense with computers! For a USB keyboard to work on PS/2 with one of those passive dongles, it has to support PS/2 as well.

Huh. I haven't ever tried the PS/2 on my TK because I don't have a computer with PS/2 on it, but I'll give it a shot and see how it goes. Thanks for the advice.

[Soarer]

I'm pretty sure Filco sticks with the standard 6KRO for their 'boards.

Grendel was agreeing about the difference between 8ms and 1ms, I thought   The reason that no keyboards with full-speed also support PS/2 is that there is no point! Full-speed is faster, even if only by a negligible amount.

 
So, what does support mean? I know that USB can be converted into PS/2, but I assume it's something deeper?

Support, as in the usual sense with computers! For a USB keyboard to work on PS/2 with one of those passive dongles, it has to support PS/2 as well.

Huh. I haven't ever tried the PS/2 on my TK because I don't have a computer with PS/2 on it, but I'll give it a shot and see how it goes. Thanks for the advice.

It won't work - it doesn't say PS/2 in the TK's specs.

For details of how a dual-mode interface is typically implemented, see Freescale's USB and PS/2 Multimedia Keyboard Interface documentation, pages 11-12.

[Linkbane]

I'm pretty sure Filco sticks with the standard 6KRO for their 'boards.

Grendel was agreeing about the difference between 8ms and 1ms, I thought   The reason that no keyboards with full-speed also support PS/2 is that there is no point! Full-speed is faster, even if only by a negligible amount.

 
So, what does support mean? I know that USB can be converted into PS/2, but I assume it's something deeper?

Support, as in the usual sense with computers! For a USB keyboard to work on PS/2 with one of those passive dongles, it has to support PS/2 as well.

Huh. I haven't ever tried the PS/2 on my TK because I don't have a computer with PS/2 on it, but I'll give it a shot and see how it goes. Thanks for the advice.

It won't work - it doesn't say PS/2 in the TK's specs.

For details of how a dual-mode interface is typically implemented, see Freescale's USB and PS/2 Multimedia Keyboard Interface documentation, pages 11-12.

 
Much sad. I guess it would be redundant, but it would be a nice feature nonetheless (mostly for entering system BIOS for me). 

[Grendel]

Every keyboard obviously has NKRO over PS/2. If any gamer actually got a benefit from using PS/2 boards, some probably would. We obviously don't see any. The TK, Shine 3, Filco, Noppoo, etc. all have NKRO over USB, among countless others. It's not unique, and I personally expect it from any board that I buy.
And you must be a wizard if you can notice the difference between a 1ms response and a slightly less than 1ms. You are Grendel, after all.

Like Soarer suspected, I was talking about 1ms vs. 8ms (or more -- CMS QF XT eg. requests 10ms intervals !) 1ms via PS/2 or USB 2.0 is noticeable faster than 8-10ms on USB 1.1 for someone who takes his or her gaming seriously. My Rampage IV Gene and Z87E-ITX boards have PS/2 ports. The GA-H87N-WIFI I just used in a build has a PS/2 port. The Maximus VI Hero has a PS/2 port. Etc., etc. Seems they are there for a reason.

And no, not every PS/2 board has NKRO. You need anti ghosting diodes for that feature and not every board has them. IIRC IBM Model M's are 2KRO.

For details of how a dual-mode interface is typically implemented, see Freescale's USB and PS/2 Multimedia Keyboard Interface documentation, pages 11-12.

Thanks, I was about to dig around the net for some example code.