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For a hobby project, I'm planning to build a 4-in-2-out VGA (yes, VGA) KVM switch/matrix circuit. The top result on Google at the time of researching is 'Building a KVM – Part 1'. I'm trying to figure out why the engineer who wrote the article used two switch ICs (TS5V330 and SN74LVC1G3157), and it's not clear from reading the article.

VGA switch schematic

The only text from the article close to explaining why there are 2 chips is maybe the following, but I'm not sure it gives me any clarity:

there’s just a switch to the IN and S lines for switching displays.

Might it be to save on cost (which the author alludes to later in the article when talking about USB switch ICs), since a full VGA switch IC could be more expensive? Or is it that VGA switch ICs are less available with HDMI/DP being the current de facto?

If not, do there exist any specific VGA switch ICs? (Not asking for product recommendation, just information on whether or not this type of IC existed at any point in time). Is there a simpler solution than using two ICs? I appreciate that IC availability might be an issue since VGA is probably dying out (but VGA is necessary for my hobby project).

I'd like to simplify the design if possible, since, if my back-of-napkin drawing of the 4-in-2-out matrix/switch is correct, I'd need a total of 8 chips (4x TS5V330 and 4x SN74LVC1G3157) which might produce a more complex circuit than is necessary. I can create a full schematic if requested in the comments (but I plan to do this later after playing with the chips).

Side notes, possibly unrelated: In my case, I'm planning on using an ESP8266/Arduino or similar microcontroller to control the switch via a web interface (as opposed to a physical switch). I know, a bit weird for a retro tech project. Not sure if this is relevant to the question though, but let me know if it is. Also, I'll be switching both USB and PS/2 connectors, which look very similar in terms of having 4 lines, VCC/G/D+/D-, so maybe I'll be able to use the same USB switcher ICs for both connector types.

Nick Bolton
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    Mainly because (ignoring EDID) there are **five** signals that need to be switched -- three color channels and two sync signals. Since the sync signals are logic-level, you can use a cheap logic mux instead of an expensive analog mux for one or both of them. – Dave Tweed Jan 26 '22 at 12:03
  • @DaveTweed That's a great answer. – Nick Bolton Jan 26 '22 at 12:13
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    "Hello, I'm the CEO of Symless, the company behind Synergy, software that lets you share one mouse and keyboard between multiple computers." This hobby project falls so close to your company's market, you are really passionate about KVM technology :D – TypeIA Jan 26 '22 at 13:41
  • Haha, you got me! I'm very passionate about using multiple computers at the same time... or maybe you could say obsessed. It seems to find it's way into everything. – Nick Bolton Jan 26 '22 at 17:02

1 Answers1

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Two chips are needed because the selected chips can't switch all the necessary signals with one chip. And because it's just someone's blog about a hobby project, it is unlikely that there has been much thought given what parts to use and why, or how suitable they are for the task.

The TS5V330 has only four switches, and it has the bandwidth to switch high speed video signals. Fourth switch is used for VSYNC, apparently, which makes no sense as HSYNC would be a logical choise, if you even would use a switch for it instead of logic buffer.

The 3157 is used for switching the HSYNC, as it is not good enough for switching video due to it's resistance, but on the other hand, it has larger bandwidth than the video switch.

Yes, actually chips that are specifically designed for switching all signals between two VGA interfaces do exist, from multiple manufacturers

Justme
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  • Thanks, after some digging I stumbled across a VGA switch IC available for purchase in 2021 (not a recommendation, but the TS5V522C seems to what I need). Edit: Ah, however, from Mouser there is a factory lead time of 52 weeks. – Nick Bolton Jan 26 '22 at 17:10
  • And, Digikey has the TS5V522C, but the minimum order is a weird number of 881 (a few more than I need for my hobby project). Perhaps the TS5V330 and SN74LVC1G3157 are my best option in terms of availability (you can purchase them in small numbers from quite a few places). – Nick Bolton Jan 26 '22 at 17:16