7 ms·

Would somebody like to help me understand why KVM switches are so sophisticated? A DisplayPort cable has 20 pins, so why can't it just be a 20 pole switch? (Or

Would somebody like to help me understand why KVM switches are so sophisticated? A DisplayPort cable has 20 pins, so why can't it just be a 20 pole switch? (Or 30 pole for two USBs and a headphone jack, etc.)

bravo22 6y ago
Signal integrity. High speed signals degrade with even slightest mismatch in trace length or stubs. For high-speed signals you use a chip which is essentially an electrical repeater to redirect the output to port A or B.
adrianpike 6y ago
It can be, eBay has HDMI physical switches for like $8. Your computers will see it as a monitor unplug, though, and do fun things with adjusting windows/icons/resolutions, and then have to re-handshake on every switch. Not a blocker, just a pain. I'm also not 100% sure if it'll freak HDCP out.
- 8ytecoder 6y ago
  I bought a HDMI switch and a USB switch. Total cost was under $30. I now have to press two buttons instead of one but that's about it. However, I don't know how to scale that for two monitors. I prefer a single monitor setup so it works for me.
- fock 6y ago
  iirc most of this switches (and cheap chinese capture cards) work around this problem by "breaking" HDCP. Problem solved.
myself248 6y ago
At the multi-gigabit speeds of a modern video signal, the components of a normal mechanical switch are antennas.
- modeless 6y ago
  So are the pins in cable connectors, but cables are (relatively) cheap and switching signals by replugging connectors works just fine.
  - formerly_proven 6y ago
    In my experience displayport and DVI cables of all brands and prices are incredibly fragile and definitely stop working well soon if you plug/unplug them once too often. Too often is something like 20-30 times.
  - bronson 6y ago
    No, cables and connectors these days are shielded and extremely tightly controlled. The amount of work that goes into designing them is staggering. (and that's before considering active cables)
Ductapemaster 6y ago
A lot of it has to do with the electrical properties of such a switch. Even older versions of DisplayPort have massive data rates, meaning each electrical signal switches many millions of times a second. Also, each signal may have to switch states close to the same time as the other signals, meaning there needs to be some sort of matching between the different signal paths. Add in noise sources from the power supplies, your cell phone, and other sources, and it becomes a challenging physics problem. Not to mention, you don't want those signals leaking out either. At these data rates, wires are both transmitting and receiving antennas. Switches are inherently very mechanical devices, so designing one that can operate in these conditions reliably would be both complex and expensive. Much easier to have some silicon do it for you.
- stefan_ 6y ago
  Isn't this what SPDT and other RF switches are designed to do? Obviously impractical for a KVM switch, but I wonder if you could build one out of 20 of these.
  - bronson 6y ago
    RF switches are fairly expensive and aren't likely to meet the electrical spec anyway. If they're SPDT, you'll probably need at least 10 of them. What's the motivation for using them?
- ric2b 6y ago
  But I can just pull the plug on any USB 3.0/HDMI/DP cable without any warning to the devices and nothing breaks, why can't a rudimentary switch do the same?
  - Ductapemaster 6y ago
    These standards are robust to step-function changes like unplugging and plugging in, through smart electrical, mechanical, and software design. There's a ton of detail here, so if you're interested feel free to DM me. However, the challenges I am referring to above mostly have to do with the steady state operation of the bus, after it has negotiated it's final data rate and is transmitting at full-blast. That's when you run into the transmission problems (eg. radiation of your signals), or your setup just won't allow you to reach the full data rate due to interference.
stonogo 6y ago
In addition to the signal issues outlined in sibling comments, a good KVM switch presents the HID devices full-time, so instead of the computers seeing USB disconnect/reconnects, they don't see any difference at all. This requires a complete USB implementation including HID protocols and something to translate real-keyboard and real-mouse signals into the virtual USB devices. There are similar considerations regarding DDC passthrough on monitors for resolution negotiation and so forth. A bad KVM presents to the computer as though the user is disconnecting all the input devices and the monitors every time you switch. These are indeed much simpler and cheaper devices but they cause all kinds of quality-of-life issues when you use them regularly.
- modeless 6y ago
  Why shouldn't a computer be able to handle reconnection of peripherals without quality of life issues? It's pretty silly if this pure software issue is the reason KVM hardware has to be so complex and expensive.
  - stonogo 6y ago
    Some peripherals require configuration after connection. Waiting for this can suck. I'm including monitors here -- a bad implementation will present, as an example, monitors out of order. Another problem you run into is inadvertently cycling through connections too quickly, which can show as devices being disconnected during initialization, causing some systems to flag the device as malfunctioning and refuse to initialize it next connection. Of course it's easy to say that these are silly problems that should have pure software fixes, but Microsoft and Apple don't exactly accept patches. Let's not pretend that software is optimal.
  - aetch 6y ago
    You can see for yourself by disconnecting your monitor in a multi monitor setup or opening your laptop lid if you use the laptop on a dock. Computer windowing systems seem to take a moment to reposition all the windows and might not remember to redistribute your windows when the monitor count changes.
chrisseaton 6y ago
I think KVMs provide the illusion of the monitor still being there when you switch away from that machine? So it doesn't think you just yanked the cable.
formerly_proven 6y ago
Using a literal switch or multiplexer would have the link disappear, so switching causes ~5-10 seconds of delay, assuming the machine doesn't crash or hang because somehow that's okay behavior for computers in 2020...?! Additionally: The DisplayPort spec mandates that operating systems have to throw a huge temper tantrum and garble your desktop and windows when you disconnect or turn off a display. (Yes, nVidia and Microsoft consistently deny implementing an option to turn this utterly braindead behavior off, saying that it's not a bug, but intended behavior because DisplayPort and VESA say so). What KVMs instead contain is similar to the rescaler ASICs found in a monitor, they receive multiple streams of display data and mix/forward/scale one of these. But since they constantly receive all of the streams, switching can be instant, or at least as quick the monitor can modeswitch (which, for some reason, takes screens at least a second).
- JoshTriplett 6y ago
  Can you provide more information about the details of DisplayPort you're referring to, and what actual spec-required behavior you're referencing?
  - formerly_proven 6y ago
    That'd be the most gracious DisplayPort hot-plug detect. I assume garbling windows isn't technically mandated by the spec, it's just what DWM does when Windows tells it to "rearrange" all windows on a desktop that has lost a bunch of displays, because Windows helpfully noticed that they were turned off. https://www.reddit.com/r/nvidia/comments/66opvp/this_displayport_hotplug_madness_finally_needs_to/ https://www.reddit.com/r/nvidia/comments/66opvp/this_display... https://answers.microsoft.com/en-us/windows/forum/all/how-to-disable-the-automatic-monitor-detection-or/37da9cd5-1e3e-4970-a9fb-1ed97a6dbe3e https://answers.microsoft.com/en-us/windows/forum/all/how-to... https://answers.microsoft.com/en-us/windows/forum/windows_7-hardware/windows-7-dual-monitors-turning-off-a-monitor/e0b8bb93-5960-4273-b959-4e3177946f8d?page=3 https://answers.microsoft.com/en-us/windows/forum/windows_7-... (AMD workaround) https://www.overclock.net/forum/74-graphics-cards-general/1235582-dealing-displayport-hdmi-autodetect.html https://www.overclock.net/forum/74-graphics-cards-general/12... https://superuser.com/questions/630555/turning-displayport-monitor-off-disables-monitor-completely https://superuser.com/questions/630555/turning-displayport-m... https://www.nvidia.com/en-us/geforce/forums/discover/209725/annoying-desktop-resets-on-monitor-power-off/ https://www.nvidia.com/en-us/geforce/forums/discover/209725/... https://www.reddit.com/r/Windows10/comments/4895u9/turning_off_a_displayport_monitor_samsung_results/ https://www.reddit.com/r/Windows10/comments/4895u9/turning_o... https://www.reddit.com/r/nvidia/comments/9qn0dk/has_anyone_figured_out_a_way_to_disable_dp/ https://www.reddit.com/r/nvidia/comments/9qn0dk/has_anyone_f... https://www.reddit.com/r/windows/comments/788l4f/put_one_displayport_monitor_to_sleep_on_a/ https://www.reddit.com/r/windows/comments/788l4f/put_one_dis... and many, many more, like this one: https://what.thedailywtf.com/topic/24548/displayport-is-the-world-s-biggest-piece-of-shit https://what.thedailywtf.com/topic/24548/displayport-is-the-...
zachrose 6y ago
Thanks everyone!
6nf 6y ago
Check out a DisplayPort cable teardown. I was surprised how complicated they are! Long gone are the days of simple 'copper only' cables. These high bandwidth cables have electronics on both sides to negotiate with the devices they connect to and to optimise data throughput from one end of the cable to the other.
fizzled 6y ago
USB is a differential signal, not a digital one (like a GPIO square wave), and once you lose the training the connection is lost: it's basically a replug, but you can't do it too quickly, so you need to build a circuit to switch it; or just have a smart hub that electrically switches it by maintaining all of the connections (phy links).
flatfilefan 6y ago
There isn’t a tb3 kvm switch available on the market, that would do what I need. So I’m just re-plugging the tb-3/usb-c docking station downlink cable between two laptops. All the peripherals are connected to the docking station so I don’t need to switch them. Probably could build a mechanical contraption to do that on a lever push. So that’s what I’m curious about too: why yet no mechanical switch if I’m literally doing it manually already?
bronson 6y ago
Back in the old days (early 90s), they actually were many-pole switches. If you opened one up, it looked like a stack of washers with a bunch of wires coming out of it, more like a variable capacitor than a normal switch. They were very expensive and very unreliable. If something was weird, click it back and forth a few times and maybe it fixes itself. Nowadays the signaling is too fast to go through a physical switch and most devices would be unhappy to be plugged/unplugged so rapidly.
mncharity 6y ago
> A DisplayPort cable has 20 pins, so why can't it just be a 20 pole switch? I use magnetic 20-pin USB C adapters to connect laptop thunderbolt and usb c to a DisplayPort headset and misc. Less worry about yanks; easy dis/re-connect; simplified cable and headset handling. No issues so far. I like them. Though I've not been pushing the DP bandwidth hard. And because they disconnect much more easily than a plug, I can't for instance move the laptop around and expect the cabling to simply follow. But to use them for frequent hot swap, compared with a real switch, I'd be worried about duty cycle lifetime. And the dis/connect isn't hidden from the environment, which might be inconvenient.

bravo22 6y ago

Signal integrity. High speed signals degrade with even slightest mismatch in trace length or stubs. For high-speed signals you use a chip which is essentially an electrical repeater to redirect the output to port A or B.

adrianpike 6y ago

It can be, eBay has HDMI physical switches for like $8. Your computers will see it as a monitor unplug, though, and do fun things with adjusting windows/icons/resolutions, and then have to re-handshake on every switch. Not a blocker, just a pain. I'm also not 100% sure if it'll freak HDCP out.

8ytecoder 6y ago
I bought a HDMI switch and a USB switch. Total cost was under $30. I now have to press two buttons instead of one but that's about it. However, I don't know how to scale that for two monitors. I prefer a single monitor setup so it works for me.
fock 6y ago
iirc most of this switches (and cheap chinese capture cards) work around this problem by "breaking" HDCP. Problem solved.

myself248 6y ago

At the multi-gigabit speeds of a modern video signal, the components of a normal mechanical switch are antennas.

modeless 6y ago
So are the pins in cable connectors, but cables are (relatively) cheap and switching signals by replugging connectors works just fine.
- formerly_proven 6y ago
  In my experience displayport and DVI cables of all brands and prices are incredibly fragile and definitely stop working well soon if you plug/unplug them once too often. Too often is something like 20-30 times.
- bronson 6y ago
  No, cables and connectors these days are shielded and extremely tightly controlled. The amount of work that goes into designing them is staggering. (and that's before considering active cables)

Ductapemaster 6y ago

A lot of it has to do with the electrical properties of such a switch. Even older versions of DisplayPort have massive data rates, meaning each electrical signal switches many millions of times a second. Also, each signal may have to switch states close to the same time as the other signals, meaning there needs to be some sort of matching between the different signal paths. Add in noise sources from the power supplies, your cell phone, and other sources, and it becomes a challenging physics problem. Not to mention, you don't want those signals leaking out either. At these data rates, wires are both transmitting and receiving antennas. Switches are inherently very mechanical devices, so designing one that can operate in these conditions reliably would be both complex and expensive. Much easier to have some silicon do it for you.

stefan_ 6y ago
Isn't this what SPDT and other RF switches are designed to do? Obviously impractical for a KVM switch, but I wonder if you could build one out of 20 of these.
- bronson 6y ago
  RF switches are fairly expensive and aren't likely to meet the electrical spec anyway. If they're SPDT, you'll probably need at least 10 of them. What's the motivation for using them?
ric2b 6y ago
But I can just pull the plug on any USB 3.0/HDMI/DP cable without any warning to the devices and nothing breaks, why can't a rudimentary switch do the same?
- Ductapemaster 6y ago
  These standards are robust to step-function changes like unplugging and plugging in, through smart electrical, mechanical, and software design. There's a ton of detail here, so if you're interested feel free to DM me. However, the challenges I am referring to above mostly have to do with the steady state operation of the bus, after it has negotiated it's final data rate and is transmitting at full-blast. That's when you run into the transmission problems (eg. radiation of your signals), or your setup just won't allow you to reach the full data rate due to interference.

stonogo 6y ago

In addition to the signal issues outlined in sibling comments, a good KVM switch presents the HID devices full-time, so instead of the computers seeing USB disconnect/reconnects, they don't see any difference at all. This requires a complete USB implementation including HID protocols and something to translate real-keyboard and real-mouse signals into the virtual USB devices. There are similar considerations regarding DDC passthrough on monitors for resolution negotiation and so forth. A bad KVM presents to the computer as though the user is disconnecting all the input devices and the monitors every time you switch. These are indeed much simpler and cheaper devices but they cause all kinds of quality-of-life issues when you use them regularly.

modeless 6y ago
Why shouldn't a computer be able to handle reconnection of peripherals without quality of life issues? It's pretty silly if this pure software issue is the reason KVM hardware has to be so complex and expensive.
- stonogo 6y ago
  Some peripherals require configuration after connection. Waiting for this can suck. I'm including monitors here -- a bad implementation will present, as an example, monitors out of order. Another problem you run into is inadvertently cycling through connections too quickly, which can show as devices being disconnected during initialization, causing some systems to flag the device as malfunctioning and refuse to initialize it next connection. Of course it's easy to say that these are silly problems that should have pure software fixes, but Microsoft and Apple don't exactly accept patches. Let's not pretend that software is optimal.
- aetch 6y ago
  You can see for yourself by disconnecting your monitor in a multi monitor setup or opening your laptop lid if you use the laptop on a dock. Computer windowing systems seem to take a moment to reposition all the windows and might not remember to redistribute your windows when the monitor count changes.

chrisseaton 6y ago

I think KVMs provide the illusion of the monitor still being there when you switch away from that machine? So it doesn't think you just yanked the cable.

formerly_proven 6y ago

Using a literal switch or multiplexer would have the link disappear, so switching causes ~5-10 seconds of delay, assuming the machine doesn't crash or hang because somehow that's okay behavior for computers in 2020...?! Additionally: The DisplayPort spec mandates that operating systems have to throw a huge temper tantrum and garble your desktop and windows when you disconnect or turn off a display. (Yes, nVidia and Microsoft consistently deny implementing an option to turn this utterly braindead behavior off, saying that it's not a bug, but intended behavior because DisplayPort and VESA say so). What KVMs instead contain is similar to the rescaler ASICs found in a monitor, they receive multiple streams of display data and mix/forward/scale one of these. But since they constantly receive all of the streams, switching can be instant, or at least as quick the monitor can modeswitch (which, for some reason, takes screens at least a second).

JoshTriplett 6y ago
Can you provide more information about the details of DisplayPort you're referring to, and what actual spec-required behavior you're referencing?
- formerly_proven 6y ago
  That'd be the most gracious DisplayPort hot-plug detect. I assume garbling windows isn't technically mandated by the spec, it's just what DWM does when Windows tells it to "rearrange" all windows on a desktop that has lost a bunch of displays, because Windows helpfully noticed that they were turned off. https://www.reddit.com/r/nvidia/comments/66opvp/this_displayport_hotplug_madness_finally_needs_to/ https://www.reddit.com/r/nvidia/comments/66opvp/this_display... https://answers.microsoft.com/en-us/windows/forum/all/how-to-disable-the-automatic-monitor-detection-or/37da9cd5-1e3e-4970-a9fb-1ed97a6dbe3e https://answers.microsoft.com/en-us/windows/forum/all/how-to... https://answers.microsoft.com/en-us/windows/forum/windows_7-hardware/windows-7-dual-monitors-turning-off-a-monitor/e0b8bb93-5960-4273-b959-4e3177946f8d?page=3 https://answers.microsoft.com/en-us/windows/forum/windows_7-... (AMD workaround) https://www.overclock.net/forum/74-graphics-cards-general/1235582-dealing-displayport-hdmi-autodetect.html https://www.overclock.net/forum/74-graphics-cards-general/12... https://superuser.com/questions/630555/turning-displayport-monitor-off-disables-monitor-completely https://superuser.com/questions/630555/turning-displayport-m... https://www.nvidia.com/en-us/geforce/forums/discover/209725/annoying-desktop-resets-on-monitor-power-off/ https://www.nvidia.com/en-us/geforce/forums/discover/209725/... https://www.reddit.com/r/Windows10/comments/4895u9/turning_off_a_displayport_monitor_samsung_results/ https://www.reddit.com/r/Windows10/comments/4895u9/turning_o... https://www.reddit.com/r/nvidia/comments/9qn0dk/has_anyone_figured_out_a_way_to_disable_dp/ https://www.reddit.com/r/nvidia/comments/9qn0dk/has_anyone_f... https://www.reddit.com/r/windows/comments/788l4f/put_one_displayport_monitor_to_sleep_on_a/ https://www.reddit.com/r/windows/comments/788l4f/put_one_dis... and many, many more, like this one: https://what.thedailywtf.com/topic/24548/displayport-is-the-world-s-biggest-piece-of-shit https://what.thedailywtf.com/topic/24548/displayport-is-the-...

zachrose 6y ago

Thanks everyone!

6nf 6y ago

Check out a DisplayPort cable teardown. I was surprised how complicated they are! Long gone are the days of simple 'copper only' cables. These high bandwidth cables have electronics on both sides to negotiate with the devices they connect to and to optimise data throughput from one end of the cable to the other.

fizzled 6y ago

USB is a differential signal, not a digital one (like a GPIO square wave), and once you lose the training the connection is lost: it's basically a replug, but you can't do it too quickly, so you need to build a circuit to switch it; or just have a smart hub that electrically switches it by maintaining all of the connections (phy links).

flatfilefan 6y ago

There isn’t a tb3 kvm switch available on the market, that would do what I need. So I’m just re-plugging the tb-3/usb-c docking station downlink cable between two laptops. All the peripherals are connected to the docking station so I don’t need to switch them. Probably could build a mechanical contraption to do that on a lever push. So that’s what I’m curious about too: why yet no mechanical switch if I’m literally doing it manually already?

bronson 6y ago

Back in the old days (early 90s), they actually were many-pole switches. If you opened one up, it looked like a stack of washers with a bunch of wires coming out of it, more like a variable capacitor than a normal switch. They were very expensive and very unreliable. If something was weird, click it back and forth a few times and maybe it fixes itself. Nowadays the signaling is too fast to go through a physical switch and most devices would be unhappy to be plugged/unplugged so rapidly.

mncharity 6y ago

> A DisplayPort cable has 20 pins, so why can't it just be a 20 pole switch? I use magnetic 20-pin USB C adapters to connect laptop thunderbolt and usb c to a DisplayPort headset and misc. Less worry about yanks; easy dis/re-connect; simplified cable and headset handling. No issues so far. I like them. Though I've not been pushing the DP bandwidth hard. And because they disconnect much more easily than a plug, I can't for instance move the laptop around and expect the cabling to simply follow. But to use them for frequent hot swap, compared with a real switch, I'd be worried about duty cycle lifetime. And the dis/connect isn't hidden from the environment, which might be inconvenient.