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Grogster

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Joined: 31/12/2012
Location: New Zealand
Posts: 9588

Posted: 05:31pm 12 Nov 2014

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Hi everyone.

I am about to build a custom KV(keyboard & video - no mouse) switch, and was planning to use 4066's to do it. Does anyone see any obvious problems with that?

My research indicates that the VGA frequencies are inside the 150MHz bandwidth of the 4066 switches, so I was planning to configure three 4066 chips as two SPDT switches.

Five SPDT switches - 3 for red, green & blue video, 1 for H-sync, and 1 for V-sync.

That is the plan anyway.

BOTH video signals are from two separate CMM units. I note that the on-resistance of a 4066 switch can be in the order of 150 ohms, so this would be in series with the existing 120 ohm resistors on the CMM output and the Vsync and Hsync signals - is this likely to be an issue do you think?

Although I plan to use 4066's, I am wondering if there perhaps exists a chip dedicated for VGA switching like this. If anyone knows of such a chip, can you please post it's part number and/or link.

Smoke makes things work. When the smoke gets out, it stops!

WhiteWizzard
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Joined: 05/04/2013
Location: United Kingdom
Posts: 2932

Posted: 01:17am 13 Nov 2014

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Hi Grogs,

I tried this on an AmstradCPC464 home computer to a vga monitor in the 80's and it did work ok (although I seem to remember I had to workaround some issue which I just can't remember now).

Try it on a breadboard with a spare vga lead which you can cut in two. Should only take an hour or so to test if you just switch the three RGB signals (and 3/4 of a 4066) first. If this is ok (impedance wise) then do the other lines with the additional 4066.

Keep us posted with progress . . .

WW

crez

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Joined: 24/10/2012
Location: Australia
Posts: 152

Posted: 02:33am 13 Nov 2014

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http://au.rs-online.com/web/p/multiplexer-switch-ics/7321200 /
(shame about the small surface mount)

You could also look at the cmos 4053 series which are arranged in a triple DPDT package, with similar or better on-resistance to the 4066

David

TassyJim

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Joined: 07/08/2011
Location: Australia
Posts: 6269

Posted: 09:41am 13 Nov 2014

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You are likely to need a buffer amp after the switches to restore video level.
I have used 4066's for switching composite video a long time ago.

There are plenty of commercial switches available which would be cheaper than rolling your own.

Jim

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Grogster

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Joined: 31/12/2012
Location: New Zealand
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Posted: 12:20pm 13 Nov 2014

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@ crez: thanks, will check it out.

@ Jim: Yeah, the relatively high on-resistance is one of my main concerns with the 4066 idea.

There are plenty of cheap VGA switches on eBay etc, as you say, but I wanted a specific one of my own that I can feed the CMM VGA and KB into, and have my static VGA display Micromite running on the same board, so that it is all neat and tidy(and small).

I have 50-odd DIL DPDT relays. I might just use three of those - job done, and esentially zero on-resistance. It's not going to be switching very often if at all(with any luck!), so relay's would be perfectly suitable in this respect.

Smoke makes things work. When the smoke gets out, it stops!

bigmik

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Joined: 20/06/2011
Location: Australia
Posts: 2949

Posted: 12:53pm 13 Nov 2014

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Grogster,

Personally, I think a cheap KVM (around $12) would probably be a simpler approach, Just plug it in and swap when required.. Most even have a `hot key' sequence to swap between outputs. They are also available in very small packages.. I had one (A-Ten I think) that was not much larger than the cables.

Regards,

Mick

Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<<

Grogster

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Location: New Zealand
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Posted: 01:52pm 13 Nov 2014

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Yeah, they are pretty cheap - you have a point there......

I will consider my options, but that might be the better idea - you can't really argue with that kind of price.

Smoke makes things work. When the smoke gets out, it stops!

MicroBlocks

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Joined: 12/05/2012
Location: Thailand
Posts: 2209

Posted: 07:04pm 13 Nov 2014

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You can treat the VGA signals from a cmm as being digital.

You could probably make something with a few 74hct chips.
Some AND gates with open collector outputs.
You would need five two input AND gates per VGA source.

The outputs of those can be tied together when they are open collector outputs or you can use a suitable or gate.

After that you have a digital signal similar to the signal that comes from the mcu pins that can be made suitable for VGA by adding the 3 resistors and diodes.

Microblocks. Build with logic.

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