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GTIA faulty or not

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#1 zylar OFFLINE  


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Posted Wed Oct 11, 2017 2:58 AM

Hello all,


I recently got an Atari 800xl rev D, now the computer booted fine after a few  reseats of the RAM and etc, but with cold reboot it boots in the diag menu and a warm reboot it gets to the "ready" stage. so i went in the schematics and saw the 3 button outputs does not go to the porkey but to the GTIA pin 12 13 and 14 and sound comes from pin 15 witch are labeled S0 to S3. When i measure on those pins i get just a few millivolt, 124 mV as maximum.


My questions are:


*Does these pins get to be pulled up by the GTIA itself or do i miss some other pullup?

*can i order a new GTIA or is there hope for this one?

#2 Rybags ONLINE  



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Posted Wed Oct 11, 2017 5:46 AM

If you get the Basic READY prompt then you should be able to enter and run this simple program:


10 ? PEEK(53279) : GOTO 10


That'll give a readout of what's coming into the GTIA CONSOL register which reflects the keys.  Bit 0=Start, 1=Select, 2=Option.  Normally read 1, will go to 0 when a key is held.

Valid values returned should be anything 0 to 7 as each key is independant.  7 should be normal state of no key pressed.

If OPTION key is stuck or GTIA somehow thinks it's held then it'll likely read 3.


Next thing to try if the key behaviour is suspect would be to remove the keyboard and see if the powerup behaviour changes.


The actual keys... I think they're internal pullup in GTIA.  The schematics have resistors but they're just inline.

Not entirely sure how the chip functions vs something like PIA - the 5200 actually uses all of the bits as outputs - extra functions being to enable analog pots and to choose which joystick's keypad is being scanned.

Edited by Rybags, Wed Oct 11, 2017 5:54 AM.

#3 ProWizard ONLINE  


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Posted Wed Oct 11, 2017 6:58 AM

Any chance to test the Atari with a diskdrive or something like that?


With other words: does it boot into selftest too then, or does it boot a disk properly?

#4 zylar OFFLINE  


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Posted Thu Oct 12, 2017 3:29 AM

well i tryed the peek command, but it was inconsistent, got all the same numbers first time 0 and 2nd time 13, the whole screen full. i did already try it without keyboard, i measured the resistance to ground with keyboard and without, it was ol without the gtia installed and went to 3 ohm when i pushed a button(after the resistor). I even tried installing an pull-up externally on the line but it went to 300mV instead of 5v. So ill order an new GTIA and see if that solves my problem.

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