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Forum Index : Microcontroller and PC projects : Forcing the VT100 terminal to VGA...

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Grogster

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Joined: 31/12/2012
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Posted: 03:42pm 27 Aug 2014
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Hi folks.

Geoff's VT100 terminal - in the July 2014 issue of SC, this unit is described, and on page 62, the text mentions that updn boot-up, the firmware looks at pin 12 to see if there is a VGA monitor connected, and if not, then it defaults to composite video.

I need it the other way around - VGA by default, even if no monitor actually connected at boot-up time.

To force this to happen, I have wired a 1k8 resistor across pin12 and ground, and this does seem to be working, but is this an acceptable way to do it, or should I be doing this a different way?

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TassyJim

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Joined: 07/08/2011
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Posted: 03:46pm 27 Aug 2014
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  Grogster said   Hi folks.

To force this to happen, I have wired a 1k8 resistor across pin12 and ground, and this does seem to be working, but is this an acceptable way to do it, or should I be doing this a different way?



That is more or less what a monitor does so it should be safe to do.

Jim
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Grogster

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Posted: 03:48pm 27 Aug 2014
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Cool, thanks.

I was just a little un-easy about that approach, in case I would be upsetting the PIC32 or the monitor.
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TassyJim

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Posted: 04:25pm 27 Aug 2014
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I misread your question and was wrong anyway. The maximite monitors pin 9 of the VGA connector.
Relying on pin 9 of the VGA connector did cause problems which might explain the change.

A better answer:
The input impedance of the 3 colour lines on a VGA monitor is 75 ohms each and there is a 220 ohm resistor on the VGA output of the CPU.
Having a 1.8k in parallel with the existing (220+75) should not put too much extra load on the CPU pin.

Jim

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Grogster

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Posted: 06:25pm 27 Aug 2014
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Acknowledged - thanks.
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