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.)
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.
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.
Obviously impractical for a KVM switch, but I wonder if you could build one out of 20 of these.
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).
Do you know of a DisplayPort KVM switch that actually does that? I tried one a few years ago (it was one of the first ones I saw for a reasonable price), but I returned it immediately because I got the brain-dead window-rearranging behavior you described whenever I switched. I've been reluctant to try another until I know it solves that issue.
I still use one of these HDMI switches (https://www.iogear.com/product/GCS62HU/), because it's EDID support actually does make it transparent to the PCs, so I can switch instantly like I want to.
I finally ordered one a while ago after I couldn't find anything cheaper with similar functionality, but it's currently on backorder so it will take some time to arrive & see how well it works in reality.
There is an interruption if you press the KVM reset button, but that's not something you should often do.
> Most of the support on (their DisplayPort 1.2 KVM offering) have been down to bad or dodgy DisplayPort cables. Seriously, it's like sort of nuts. Like you go on Amazon, you order a DisplayPort cable, it doesn't work, and it's like, "Well, I have two DisplayPort cables that work independently, but when I plug both of them into the KVM, it doesn't work." The KVM doesn't have a repeater. So if you've got two 6-ft cables that pass through the KVM, it is like as if you have one 12-ft cable, and while one 6-ft cable might work, 1 12-ft cable will not because...(Jedi hand gestures)...repeater. So use good quality cables, you'll be fine.
Ignoring the "repeater" terminology wank, that soundbite disclosed enough about the white label device's internal architecture for me to infer with confidence that even if EDID was properly implemented (the easiest part), the video interface is skimp and doesn't actually buffer frames in realtime...which means that on switch, the monitor's receiving ASIC will detect frame interruption as it starts to receive its new stream and blank for a second or two while it tries to reacquire...which is definitely not the instantaneous performance I was looking for.
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_display... https://answers.microsoft.com/en-us/windows/forum/all/how-to... https://answers.microsoft.com/en-us/windows/forum/windows_7-... (AMD workaround) https://www.overclock.net/forum/74-graphics-cards-general/12... https://superuser.com/questions/630555/turning-displayport-m... https://www.nvidia.com/en-us/geforce/forums/discover/209725/... https://www.reddit.com/r/Windows10/comments/4895u9/turning_o... https://www.reddit.com/r/nvidia/comments/9qn0dk/has_anyone_f... 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-...
Until I figured out what the hell was going I was pulling my hair every morning when I turned on my work monitor (40-something inch 4k display) and found all program windows in the top-left corner of the monitor, stacked on top of each other and resized to some svga-resolution.
Sounds colorful. Can you cite the requirement and revision of the standard referenced? I'd like to read the actual verbiage for myself.
[1] https://news.ycombinator.com/item?id=24359220
Looking at the normative language of the early DP v1.1a § 3.3 Hot Plug/Unplug Detect Circuitry:
> The HPD signal is asserted by the DisplayPort Sink whenever the Sink is connected to either its main power supply or “trickle” power. HPD signal specification is shown in Table 3-2.
To be sure, HPD assertion on "trickle" power is the monitor's requirement.
Looking closer at Table 3-2 (empty Nom col removed, Comments col separated for clarity):
| Parameter | Min | Max | Units | Comments |
| ---------------------------------- | ---- | --- | ----- | -------- |
| HPD Voltage | 2.25 | 3.6 | Volt | (a) |
| Hot Plug Detection Threshold | 2.0 | | Volt | (b) |
| Hot Unplug Detection Threshold | 0.8 | | Volt | (b) |
| HPD source termination | 100 | | kΩ | (c) |
| HPD sink termination | 100 | | kΩ | (d) |
| IRQ HPD Pulse Width Driven by Sink | 0.5 | 1.0 | ms | (e) |
| IRQ HPD Pulse Detection Threshold | 2.0 | | ms | (f) |
> (a) HPD signal to be driven by the Sink Device
> (b) HPD signal to be detected by the Source Device
> (c) Source Device must pull down its HPD input with a ≥ 100kΩ resistor.
> (d) When a Sink Device is off, it must pull down its HPD output with ≥ 100kΩ resistor.
> (e) Sink generates a low going pulse within this range for IRQ (interrupt request) to the Source
> (f) When the pulse width is narrower than this threshold, the Source must read the link / sink status field of the DPCD first and take corrective action. When the pulse width is wider than this threshold, it is likely to be actual cable unplug / re-plug event. Upon detecting HPD high, the Source must read the link / sink status field, and if the link is unstable, read the link / sink capability field of the DPCD before initiating Link Training.
Then just below Table 3-2:
> The voltage level of the HPD pin is monitored by the Source Device. TTL levels must be used for the detection.
> The Sink Device may detect the presence of the Source Device by monitoring the DC voltage level of the AUX CH lines. Source detection is an optional feature of a Sink Device.
Based on this language and description of noted workarounds, color me unconvinced that DP spec ambiguity was the root cause of this issue.
So I think what might be going on behind the curtains is that 1) DP requiring handling of hot plug 2) DP forbidding "trickle power" being sourced from the DP port 3) monitors perhaps not being able to meet 0.5 W standby if they'd entertain the DP interface 4) Operating systems handling display hotplug disgracefully combine into one heck of a bad user experience.
Things actually start to align now, because this issue started to happen en-masse around 2013-2014. And that's precisely when the more stringent standby power requirements came into force. For example, the Dell U2713H doesn't do this, while the later U2715H does. Both of these are DP 1.2 monitors, so any new mandates in the non-publicly available DP 1.3/1.4 specs shouldn't matter.
The answer seems to me unambiguous when considering the normative language in § 4.2.1.1:
> The standard external cable connector assembly must not have a wire on pin 20, DP_PWR.
...in conjunction with the normative definition of trickle power per § 1.4 (my emphasis):
> Power for Sink Device that is sufficient to let the Source Device read EDID via the AUX CH, but insufficient to enable Main Link and other sink functions. For sink to drive the HPD signal high, at least the trickle power must be present. The amount of power needed for the trickle power is sink implementation specific.
(To be sure, Figure 4-23 per DP v1.1a is single-slot PCI "Panel Cut Out Reference Dimensions", so unsure what was meant to be conveyed there).
> Note that EC demands that electronics put in stand-by / "off-mode" may not consume more than 0.5 W of wall power.
...which is ~150 mA at 3.3 V nominal. Even generously derating 30% to account for ballpark efficiency losses, we're still talking > 100 mA budget to support an explicitly constrained requirement. I mean asserting HPD in the worst case is apparently a 36 uA affair.
Anecdotally, I haven't encountered the issue despite a bias towards DP (on advise from a friend in compliance test over a random beer discussion on radiated emissions performance); all my setups discriminate to either dual U2415 or dual U2412M as a matter of personal preference.
Just thought I'd remark in passing because VESA specs never really struck me as being technically egregious in its ambiguity.
> ...which is ~150 mA at 3.3 V nominal.
Small power supplies become very inefficient due to relatively fixed losses relative to output power. The 0.5 W spec is wall power, so if you fully use the 0.5 W, you might get 0.1-0.2 W worth of output.
> Anecdotally, I haven't encountered the issue despite a bias towards DP (on advise from a friend in compliance test over a random beer discussion on radiated emissions performance); all my setups discriminate to either dual U2415 or dual U2412M as a matter of personal preference.
I'm using a Dell U2415 connected by DisplayPort to Intel graphics right now, and if I turn it off (using the soft buttons) my desktop and windows are garbled because it "disappears" from the system despite not being disconnected.
Compare this to the DVI days where I could literally switch the outlet strip powering my screens off and nothing would get garbled, and the system would never hang or crash. I'm pretty sure it's impossible for DisplayPort to support that, since the display has to be powered to avoid being un-detected by the system. So I'm not asking for that to work. Obviously a good standard would support this usage, but DP never will, so okay, fair enough. I'm just asking for the insanity of "disconnecting" monitors sent to stand-by / "soft button off" and subsequent garbling of my workspace to stop. And while they're at it, maybe fix crashes and hangs on monitor re/disconnects. Y'know, it's not really "plug and play" if I plug it in and have to hit the reset button to get my machine back.
Real world analogy: A standard for desks that dump everything on the floor as soon as you turn your back to them would never fly. But somehow the equivalent is acceptable for computers.
PS: A Japenese entrepeneur sells DPHPDMA devices, which are little dongles plugged directly into a Displayport output and contain a microcontroller powered by DP_PWR, which intercepts the EDID communication etc. and essentially performs EDID emulation in hardware (since GPU manufacturers disable software EDID emulation for consumer cards). They're like 50 bucks plus international shipping each.
> There is no mandatory time constraint on the Source Device’s response to a plug / re-plug event, but a Source Device vendor may want to impose a voluntary constraint similar to that for HPD IRQ events (for example, 100 ms) to ensure a good user experience via prompt discovery and configuration of newly attached devices.
There's a lot more going on behind the scenes than mere "scanning for a valid data frame and flinging bits onto the glass", and much of it has nothing to do with approaching technical limits, e.g. the dynamic of multiple product vendors finding incentive to reach consensus in standards development despite naturally seeking to cut each other's throats in the open market.
> PS: A Japenese entrepeneur sells DPHPDMA devices, which are little dongles plugged directly into a Displayport output and contain a microcontroller powered by DP_PWR, which intercepts the EDID communication etc. and essentially performs EDID emulation in hardware (since GPU manufacturers disable software EDID emulation for consumer cards). They're like 50 bucks plus international shipping each.
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.
The price was around $50 IIRC.
Most people aren't plugging/unplugging monitors often
They still handle it horribly though. My favorite is how macOS used to assign the logical displays randomly, so I'd have to fix the arrangement in Displays preferences several times every day.
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.
That's not possible any more. Microsoft, VESA, nVidia say no. This mustn't be possible. (Actually it used to be possible to add a registry key in Windows 7 that disabled all of this bullshit, but Microsoft wisely decided in Windows 10 to remove that. It must not be done.)
Turning compliant DisplayPort devices off while using a compliant operating system must always garble all windows and shuffle desktop icons around. The spec says so. So it must be. The will of the ancients.
(nVidia will allow you to turn this off, if you are using a Quadro card - then you can use EDID emulation even on display-ports, which causes the OS to think the display is always connected - and more importantly - turned on. One of these areas where Linux is ahead -- although Linux is back to garbling windows like the rest in case you do actually decide, as an individual with agency, not a compliant operating system, to change the arrangement of displays.)
I'm also not 100% sure if it'll freak HDCP out.
Too often is something like 20-30 times.
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.
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.
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?