Re: 800XL question


Kryten wrote:
> <stephensheppard@xxxxxxxxxxx> wrote in message
> news:1124996935.300886.41350@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> > The instructions call for a 100 Ohm resistor for the pick-off resistor
> > but the one I re-used is 75 Ohm. How much difference will this make?
>
> Try it and see.
>
> Easy way to do this is to hold a 300R resistor in parallel with the 100R.
>
> > I do have one problem. At high contrast transitions there is an
> > artefact that resembles ghosting or shadowing.
>
> Sounds like the signal is 'echoing' in the cable.
>
> The point of having 75R output impedance is that it ensures maximum signal
> transfer from source to cable, and from cable to input. At the input end it
> minimises reflected energy.
>

The resistor I used IS 75 Ohm. It's the modification instructions that
call for a 100 Ohm resistor.

>
> > Could this problem have anything to do with my
> > using a 75 ohm pick-off resistor instead of 100 ohms?
>
> Sounds probable.
>
> A bigger power rail decoupler may help. See:
> http://www.howell1964.freeserve.co.uk/Atari/800XL/Atari_800XL_Video.htm
>

Ooh! That sounds similar to my issue.

> > Is it possible to use EEPROMs in place of EPROMs? If so which
> > type/size?
>
> > what size/type EPROM is used for the 800XL?
>
> 16K for the OS and 8K for the BASIC.
>
> Perhaps you could frig them both into a 32K EEPROM.
>

I meant I wanted clarification/verification on what type of
EPROMS/EEPROMS will/might actually work.

2716's? 2818B's?

> Of course you will have to do some work yourself, checking pinouts and
> wiring in a write signal if you plan to have it reprogrammable in-site.
>

Designing circuits are probably beyond my ability and understanding.

I might find the resources on hand to be able to write EEPROMs using
other hardware, possibly. Of course, I would like to learn whether
EEPROMs will work or not, as-is, in the place of the Atari OS ROMs.

I'm seriously thinking about the Dallas SmartSocket but I sure would
like to talk to someone who has already tried that mod.

Thanks,

Stephen Sheppard.

.

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