6 ms·
PS/2 keyboards are used by serious gamers because the scan rate is higher than USB. See https://superuser.com/questions/16893/do-usb-or-ps-2-keyboards-respond-
by matthavener 9y ago
PS/2 keyboards are used by serious gamers because the scan rate is higher than USB.
See https://superuser.com/questions/16893/do-usb-or-ps-2-keyboards-respond-faster https://superuser.com/questions/16893/do-usb-or-ps-2-keyboar...
- gsich 9y agoIsn't PS/2 interrupt based? So there would be no scan rate. Haven't looked at your link though.
- dfox 9y agoThere are two "scan" rates: the rate at whitch keyboard matrix is scanned by whatever electronics is inside keyboard (that is probably independent of the outside interface) and the rate at which the interface is able to process input events, which for PS/2 means faster that the keyboard can produce them (as keyboard is essentially bus master on the AT/PS2 keyboard "bus") and for USB means as fast as the *HCI pools keyboard for interrupt events (old Apple's ADB works the same way and is in fact to some extent inspiration for USB <3.0 and Firewire "one big bus" high level model). Edit: then there is third approach: simple keyboard/input devices with serial interfaces, where host provided clock is used to both clock the interface and keyboard scan logic. In essence the whole keyboard then looks like one big shift register. Keyboards that works this way includes original Macintosh, most Wyse and DEC terminals and MIT/LMI/Symbolics Lisp machines (and probably pre-sun4 Suns, sun4 and later have rs232-derived ionterface), also this is the way how controllers for Nintendo consoles before Wii work (IIRC Nintendo calls that "EXI bus") and how PlayStation 1/2 controllers work. IMHO this is to some extent where the idea behind SPI comes from.
- yathern 9y agoFor what it's worth, that answer is now 8 years old - I'm curious if this has changed since then.
- SebiH 9y agoA more recent video claims that the PS/2 advantages are negligible even for serious gamers (around 3:40) https://www.youtube.com/watch?v=AWkvzycD5PE https://www.youtube.com/watch?v=AWkvzycD5PE
- ghusbands 9y agoThe message therein directly states that latency isn't noticeable for those keyboards and hence, according to the article, is simply uninformed.
- sturmeh 9y agoMost importantly it removes the penalty of using a 1000hz scanning input which can have an adverse affect on the cpu whilst providing a better (yet insignificant) result.
- revelation 9y agoI've gotten a FTDI down to 2 ms RTT, so if done right (using isochronous transfer) you can get USB down to 1 or max 2 ms. Looking at the latencies quoted here and in the article it's certainly not the problem. Notice that just getting a thread scheduled every 2 ms is already impossible for Windows, certainly one running a game. You'll get a bunch of outliers within the second. So even if you got your keyboard down to 5 ms, great, but you are not running an operating system that can reliably do something within that timespan!
- eertami 9y ago>So even if you got your keyboard down to 5 ms, great, but you are not running an operating system that can reliably do something within that timespan! Maybe I'm not understanding, but that doesn't seem correct. Most people (gamers) are running mice at 500/1000Hz polling rates and you can easily verify the movement made in each 1-2ms update. (And it is most definitely a noticeable difference going from a standard 125Hz rate to even 500Hz.)
- 9y ago
- theandrewbailey 9y agoSaying that PS/2 boards are "scanned" is inaccurate, as they trigger an interrupt immediately on a keypress.
- rubatuga 9y agoDoesn’t the micro controller in the keyboard still scan, and then send a ps/2 signal?
- valarauca1 9y agoMost mechanical PS2 keyboards are a diode cascade. You pressing the key triggers a wave of electricity cross the keyboard which is converted into the PS2 waveform, and pumped out at the same rate of the incoming clock waveform. OLD PS2 keyboards don't generally have internal digital micro-controllers, they're effectively analog, the logic they do contain is blindly simple. This is why some struggle with N-Key Rollover. --- OFC this is if you are using an -old- PS2 keyboard. I'm still using an 80's IBM Model M and above it roughly how it works.
- kmill 9y agoI am having a very difficult time believing this. - I cannot find about "diode cascades" having anything to do keyboards. The closest is the usual diode matrix to deal with simultaneous key presses. - The PS/2 interface involves sending scan codes, parity bits, and start and stop bits. How exactly does a "wave of electricity" get converted into a PS/2 waveform without some digital electronics? - All the old schematics for mechanical keyboards I could find were the usual matrix scan. Even a textbook from the 1970s. Anyway, reference please. I'd find it very interesting to see how such a thing would work.
- duskwuff 9y agoThis is absolutely untrue for any PS/2 keyboard, as well as for most pre-PS/2 PC keyboards (incuding AT and XT keyboards). All keyboards have always had a microcontroller in them which is responsible for scanning the key matrix and sending output over the serial link. (In PS/2, it's also responsible for reading input to change the state of keyboard LEDs.) There is nothing analog about them whatsoever. Here is a set of pictures of an IBM Model M, for instance. The microcontroller is clearly visible in the ceramic DIP40 package. http://www.clickeykeyboards.com/model-m-gallery/1985-ibm-model-m-1390120/ http://www.clickeykeyboards.com/model-m-gallery/1985-ibm-mod...
- HugoDaniel 9y agodo serious gamers avoid v-sync ? even at 120Hz they get at least ~8ms * 2(double buffer) latency right ?
- dsr_ 9y agoYes, they want freesync or g-sync monitors with high refresh rates.
- a_t48 9y agoOr a good enough machine so that you never dip below native framerate.
- sgarman 9y agoEven so you would never use vsync as it will introduce input lag in most gaming engines.
- sgtmas2006 9y agoMost competitive players will have their own configs for competitive play etc. I have tournament/LAN configs, online play configs, and my video settings mostly stay the same. All tuned for lowest latency and highest visibility.
- dlubarov 9y agoThat's tangential though. Whether or not you have skipped frames, G-Sync/FreeSync tend to reduce input lag, because the panel's refresh events will be timed such that they're closer, on average, to the times when new frames become available.
- sturmeh 9y agoOf course, they've turned to gsync or leave vsync off.
- 482794793792894 9y agoPro gamers in highly competitive, reaction-based games (such as most first-person shooters) usually disable it, yeah. And I'm not an expert in this either, but I'm pretty sure double buffering is the norm, whether you use V-Sync or not. One buffer in which your graphics card works on frames (I'll call this "graphics card buffer") and then a buffer into which finished frames are moved, so that your screen can read them out in peace (I'll call this "pre-screen buffer"). There's also "triple buffering", which introduces another one of those graphics card buffers, so that when a finished frame is being transferred from that first graphics card buffer to the pre-screen buffer, then your graphics card doesn't have to wait for that transfer to finish and can instead start working in the second graphics card buffer right away. (And then it transfers frames from those graphics card buffers in alternating fashion.) So, the delay that V-Sync introduces is not that. What V-Sync does, is that instead of transferring finished frames from the graphics card buffer(s) into the pre-screen buffer as soon as the frame is finished, it waits with the transfer until your screen has finished with reading out the previous frame. If you don't wait (have V-Sync disabled), your screen will read out some part of the previous frame and then read out the rest from the new frame. On the screen, you'll see this break between previous and new frame as screen tearing. So, assuming your screen can display 120 frames per second and your graphics card happens to finish 120 frames in a second, then V-Sync will not introduce a delay. It'll wait once with transferring the first frame, but then they'll be in sync and no further delay should occur. However, if your graphics card is able to calculate 240 frames per second (and your screen still does 120 frames per second), then with V-Sync, the graphics card buffer will be transferred into the pre-screen buffer only 120 times per second, making the graphics card slow down to 120 frames per second as well. Without V-Sync, the graphics card buffer will be transferred 240 times per second, regardless of how often your screen can read it out from the pre-screen buffer. This means a frame will get loaded into the pre-screen buffer as your screen is reading it out. As a result, you'll get screen tearing and one half of your screen will be from the new frame, with a 1/240 s = 4.167 ms delay, the other half is still from the previous frame with a 2/240 s = 1/120 s = 8.33 ms delay. So, a few numbers: 120 FPS with 120 Hz screen = 8.33 ms delay. 240 FPS with 240 Hz screen = 4.167 ms delay. 240 FPS with 120 Hz screen = ½8.33 ms + ½4.167 ms = 6.25 ms delay on average. 480 FPS with 120 Hz screen = ¼(1/480) + ¼(1/360) + ¼(1/240) + ¼(1/120) = 4.34 ms delay on average. 960 FPS with 120 Hz screen = 2.83 ms delay on average. 960 FPS with 240 Hz screen = 2.17 ms delay on average.
- sp332 9y agoThe PS/2 protocol is incredibly slow. Just sending two bytes over the wire via PS/2 takes 1.3 ms.
- deleted 9y ago[deleted]
- ComputerGuru 9y agoTrue. But that is throughout, not latency. Key presses are latency limited, not bandwidth limited.
- sp332 9y agoBut the latency of your keyboard will be increased by 1.3 ms just by putting it on a PS/2 port. That's more than USB polling at 1000Hz.
- kllrnohj 9y agoI'm unaware of any pro gamer (or streamer) that's using PS/2, and that's about as serious a gamer as you get. Most are using a late model mechanical keyboard connected via USB, often with lots of RGB just because. I think the only PS/2 holdouts you'll find are the old Korean Starcraft players that are still using a Qsenn DT-35.
- sturmeh 9y agoPS/2 supports lots of RGB, I'm using one, because using the USB costs CPU where as the PS/2 does not, and PS/2 gives NKRO and lower latency, however insignificant they are. It's entirely a preference based thing, the most relevant factor would be whether or not you need to hot swap it.
- GuB-42 9y agoPro gamers are paid to use whatever their sponsors want them to use. And if their new fancy keyboard is good enough not to negatively affect their game, I don't see why they would turn down the money.