What is the purpose of this resistor ?

If you want to ask questions about how the machine works, peculiar details, the differences between models, here it is !
How to program the oric hardware (VIA, FDC, ...) is also welcome.
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Voyageur
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What is the purpose of this resistor ?

Post by Voyageur » Fri Jul 19, 2019 9:34 am

Its reference is R26 and its value is 2k2... It pulls down to ground the line between ULA and 74LS365 / Prom 256x4....
R26.jpg
Is it an help / a need for generating the Red color ? Why is there nothing like that with the Green and the Blue colors ?
Or is it only used with the PROM ?

I don't understand why there is such a single resistor just there...
Does anybody have information about it ?
Thanks for teaching me ! :)
Oric Atmos : a new life !

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mikeb
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Re: What is the purpose of this resistor ?

Post by mikeb » Fri Jul 19, 2019 5:54 pm

Secret purpose.

Gateway to Narnia.

Do not mess with it :)

It's been sat there looking out-of-place, and yet completely innocent, all these years, and its purpose is to stop that video "output" line accidentally floating up to +5v. Spoiler Alert: This pin is also an input, but its not documented, until I fell over it while doing this ...

http://oric.signal11.org.uk/files/pub/u ... Schems.pdf

The answers are in the above!

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Voyageur
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Re: What is the purpose of this resistor ?

Post by Voyageur » Sat Jul 20, 2019 5:13 am

Thank you Mikeb !

That is bright ! I couldn't imagine the concerned chip was the ULA !
A different design for the Red ! A pin with Output and Input :

19 OUT Video R
+ IN VC Reset 2

Now I can see why there is a "lost" resistor.
What a good job made with this document ! Congrats !

Merci encore ! :D
Oric Atmos : a new life !

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