Most of the KVMs I see use a simple transistor based switching circuit which can have decent bandwidth if the transistors are good enough but there's still an impedance mismatch and they chop off small part of the dark end, causing unnecessary black crush. D-link DKVM8 that I use primarly has no problem passing 2560x1600 but it definitely adds some ringing to the output and it gives some black crush that cannot be compensated for in any easy way.

Another approach I have seen is CMOS switches like Voodoo1/2 cards do, and they behave a little better than the transistor based things, assuming the chips used have decent bandwidth. I have a VGA switching board in use from a dead Adderview KVM and it has 200MHz rated switches and the image output is ok on everything but the highest resolutions. No black crush, no ringing and ghosting but prices seem to be very much not worth it...

For me, ultimate solution has been an Extron VGA matrix. I got some MVX84 for really cheap and they pass everything without any sign of ringing or other artifacts. I use mine at 1920x1440@75Hz and it is flawless as far as image goes. It can do sound too but I have not utilized that functionality yet, but judging from the parts inside, it will be adequate. Anothe bonus is that since it has properly terminated inputs and outputs, it is much less picky about cable quality. Even piece of shit cables look good, while on all the others there can be very huge ghosting and ringing issues since they don't even try to impedance match...

I have very good experience with ATEN KVM-Switches.
Very good quality build, robust and reliable. No problems since 2 years usage.

ATEN has different models also ones which switch audio as well.
You should be able to find one for your needs.

crusher wrote on 2024-09-27, 07:09:

I have very good experience with ATEN KVM-Switches. Very good quality build, robust and reliable. No problems since 2 years usag […]
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I have very good experience with ATEN KVM-Switches.
Very good quality build, robust and reliable. No problems since 2 years usage.

ATEN has different models also ones which switch audio as well.
You should be able to find one for your needs.

I can confirm this, but it seems that the two Aten kvms that I've used output the signal always at 60Hz as reported by my CRT monitor. Is there any way to output 85-100Hz? I successfully choose it at the display settings but it doesn't seem to do anything and it's obvious even with naked eyes just by looking at the displayed picture.

A lot of SOHO KVMs are coming with VGA cables barely thicker than an average USB cable, the good cables approach pinkie tip diameter.

Tiido wrote on 2024-09-27, 03:49:

Most of the KVMs I see use a simple transistor based switching circuit which can have decent bandwidth if the transistors are good enough but there's still an impedance mismatch and they chop off small part of the dark end, causing unnecessary black crush. D-link DKVM8 that I use primarly has no problem passing 2560x1600 but it definitely adds some ringing to the output and it gives some black crush that cannot be compensated for in any easy way.

Another approach I have seen is CMOS switches like Voodoo1/2 cards do, and they behave a little better than the transistor based things, assuming the chips used have decent bandwidth. I have a VGA switching board in use from a dead Adderview KVM and it has 200MHz rated switches and the image output is ok on everything but the highest resolutions. No black crush, no ringing and ghosting but prices seem to be very much not worth it...

For me, ultimate solution has been an Extron VGA matrix. I got some MVX84 for really cheap and they pass everything without any sign of ringing or other artifacts. I use mine at 1920x1440@75Hz and it is flawless as far as image goes. It can do sound too but I have not utilized that functionality yet, but judging from the parts inside, it will be adequate. Anothe bonus is that since it has properly terminated inputs and outputs, it is much less picky about cable quality. Even piece of shit cables look good, while on all the others there can be very huge ghosting and ringing issues since they don't even try to impedance match...

Yes this has also been my experience. I currently use several Extron RGB switching devices, but having to route the keyboard and mouse separately is a nuisance. Surely someone has manufactured a device that would combine the robust video buffering of the extron with simple ps/2 switching

It is in my todo list to make a proper thing but that list is endless, with several other higher priority things in front of it... but I plan to write new firmware for my KVM since it has some keyboard handling bugs, and in the process give it proper video switching capability too.

I picked up an IP based HP branded Avocent KVM on the cheap about a year ago

Positives

• supports two consoles at the same time using USB or PS/2 mice
• if the VGA input quality is good, the output quality is good
• Seems to eliminate "jail bars" on an LCD monitor. (Weird lines with Trident 8900CL ISA VGA card)
• Supports up to 16 client computers & long cable runs
• USB and PS/2 client modules available
• PS/2 client modules are cheaper than analog cables (<$10)
• Hot keys for switching between client computers
• Supports USB storage devices with some USB modules
• Clients can be named and name appears on a screen overlay

But there are some down sides:

• Doesn't do sound or serial mice
• Hotkeys can interfere with games
• Onscreen display is undesirable when gaming
• Lower quality VGA output gets fuzzy
• Compatible USB client modules are harder to find, more expensive (> $25 )
• Had a loud fan that needed work
• Requires a power cable

So great it's when doing work like building & testing & rom burning.
Not good for gaming

I wonder if for 98SE through Win7,8,10 machines, you could use "USB Speakers" which are basically a USB soundcard in speakers, when you have shared USB device capability. ... and might sorta work in 98 dos mode with AC97 SBem.

Though have a couple of different two port KVM "dongles" rather than boxes, that I thought I might not bother using, so wondering if those might get "off label" use as jailbar killers, which I always thought was an output level problem rather than a card core chipset internal problem. I guess it's not gonna work with every KVM but be worth trying any idle ones, or yeah, specifically selecting good ones if actually needing that machine on a KVM.

Well I just picked up an AVM216 Adderview Matrix 16 Port KVM Switch from ebay for $60 shipped. A little pricey but if it works as advertised I'd say it's well worth it. The ability to setup two user stations and then connect *all* my retro builds to them is pretty appealing. The only thing left to sort is audio.

Haven't looked at anything else since I got my Black Box Servswitch Wizard (SW652A). Crystal clear picture, works with everything I throw at it, serial mouse emulation, etc, etc.

It doesn't do audio, but that can easily be solved with a simple passive mixer. This can of course be expanded to as many inputs as you want. I'm using 2k2 resistors in mine, the suggested 10k is a bit high in my opinion.

The only downside is that it's quite bulky. There's a lot of air in it though, the enclosure can easily be modded to less than half the original size.

kaputnik wrote on 2024-09-30, 08:44:

Haven't looked at anything else since I got my Black Box Servswitch Wizard (SW652A). Crystal clear picture, works with everything I throw at it, serial mouse emulation, etc, etc.

Serial mouse emulation? How does that work?

douglar wrote on 2024-09-30, 11:12:

kaputnik wrote on 2024-09-30, 08:44:

Haven't looked at anything else since I got my Black Box Servswitch Wizard (SW652A). Crystal clear picture, works with everything I throw at it, serial mouse emulation, etc, etc.

Serial mouse emulation? How does that work?

http://www2.blackboxab.se/Manualer/S/SW/SW651A-SW652A.pdf

Basically you solder together the cable on page 55 - gotta love BB for including special cable diagrams in the manual btw - and connect it between the computer's serial port and the PS/2 mouse port of whatever computer port on the KVM switch you use. The KVM switch then does translation between PS/2 and serial mouse protocols. Very useful for computers without PS/2 mouse port 😀

Do the larger KVMs have ground loop isolation or something? Unless I was gonna connect to a pile of laptops, I'd be nervous about having more than 5 machines powered per circuit (US/Can 110-120v 15A)

Though I guess I should stick a meter on them for worst case scenario, because I doubt anything pre-pentium often gets over 80W, whereas your XP overkill beast could be more like 400W at the wall, when 2 GPUs and a CPU are at full load.

There's no such thing on any of them from what I see.

BitWrangler wrote on 2024-09-30, 11:37:

Do the larger KVMs have ground loop isolation or something? Unless I was gonna connect to a pile of laptops, I'd be nervous about having more than 5 machines powered per circuit (US/Can 110-120v 15A)

Though I guess I should stick a meter on them for worst case scenario, because I doubt anything pre-pentium often gets over 80W, whereas your XP overkill beast could be more like 400W at the wall, when 2 GPUs and a CPU are at full load.

Continuity tested between a few ground pins on different ports on my switch, no ground loop isolation there at least.

kaputnik wrote on 2024-09-30, 11:22:

http://www2.blackboxab.se/Manualer/S/SW/SW651A-SW652A.pdf

Basically you solder together the cable on page 55 - gotta love BB for including special cable diagrams in the manual btw - and connect it between the computer's serial port and the PS/2 mouse port of whatever computer port on the KVM switch you use. The KVM switch then does translation between PS/2 and serial mouse protocols. Very useful for computers without PS/2 mouse port 😀

Looks like your adapter

Is similar same as this dual mode adapter:
https://old.pinouts.ru/InputCables/MousePs2Se … al_pinout.shtml

Edit: I reviewed the documents for my HP AF617A KVM switch again. No sign of similar functionality. There's serial modules for it, but they are for VT100 communication, not mice.

So I realised I had a 2 port unit with serial ports intended for mice, but I haven't determined yet if they are just passive for mice that already support serial or not, or whether it does the conversion. The input is only PS/2 but it has serial and PS/2 for both outputs.

The one I have is the Cybex SwitchView 520-154 2port. The 4 port version of that appears to be the Cybex SwitchView 520-147. I do not see more ports than 4 on any at the moment. So Cybex became Avocent at some point, noughts maybe, and all the Avocent ones I've seen dropped the serial ports, but there could have been some crossover models. The ones with serial ports look "fat" kinda, twice as tall as typical for KVM boxes. It looks though like Avocent retained much of the Cybex tech, so if the vid quality is good on Avocents, it is hopefully good on the prior Cybex units also. I do not recall what it was like on this one last time I ran it, but I was only likely using it for 800x600 or less, so not a demanding test.

maxtherabbit wrote on 2024-09-27, 19:31:

Well I just picked up an AVM216 Adderview Matrix 16 Port KVM Switch from ebay for $60 shipped. A little pricey but if it works as advertised I'd say it's well worth it. The ability to setup two user stations and then connect *all* my retro builds to them is pretty appealing. The only thing left to sort is audio.

Update - this Adderview Matrix is everything I ever wanted, audio support notwithstanding.

I've run up to 1080p through it over VGA and as long as you use coax cables the output quality is flawless. No ghosting, no blur, no loss of range.

Plus the ability to matrix with a second station is awesome, and it also has a very handy OSD. I give it a 8/10. Audio support would make it a perfect 10 for retro computing.

This sounds pretty good, perhaps I should attempt to aquire one myself ~