Some web sites have power pins of GTIA reversed

[atariry]

While working on my Atari on a breadboard project, I nearly blew up the GTIA chip.

 

I had wired up power, clocks and controls, but not address or data, of Sally (6502), ANTIC and GTIA. When I powered up, I was having problems programming the FPGA board, and I saw that the 5V power rail was low. I then saw that the power supply was limiting the current, and stupidly I raised the limit from 500mA to 1A. I then smelt something burning, so quickly turned off the power supply. Touching the chips, the GTIA chip was too hot to hold my finger on. Damn!!! I opened the Atari official 400/800 field service manual and to my horror saw that I had the +5V and GND connections reversed. I had googled for pinouts and had used the rather nice pictures from user.xmission.com. On this site the picture of GTIA has VCC (i.e. +5V on pin 3), as opposed to other sources which show pin 3 as ground. I will attempt to contact the webmaster to ask if the pictures can be corrected. Just my luck to use these and not double check with other sources. Lesson learned!

 

I reversed the power connections to the GTIA and powered up. All was well and current draw was no more than 300mA. Nevertheless all pins of GTIA looked dead on a scope. I re-connected the 3.57MHz source to FOSC, and finally saw a waveform on the CSYNC pin, which was horizontal sync pulses, but without and vertical sync (which I suspect/hope comes from the ANTIC chip which is not yet programmed).

 

--Atariry

[foft]

Thank you, I think you just saved me blowing up another one. Id copied the pin out from there too...

 

Your project looks great!

 

Yeah I think with no antic there is no vsync.

Edited October 8, 2018 by foft

[1050]

Problem may stem from the Atari 800XL FSM schematic which

has pin 3 with a label of VCC but is properly diagramed

as connected to ground. Pin 27 suffers the same fate

as incorrectly labeled as VSS but is properly

connected to +5 by diagram.

 

If someone went by just this official diagram that

has the mixed up labels on it things could go bad in

a hurry.

 

And a lot of people do just that without checking too.

Atari made a mistake here and everybody else just

follows suite without confirming that the labels are

wrong polarity for those functions.

[_The Doctor__]

I am of the opinion all such FSM should be corrected and replaced..... this is not a case of keeping paper as it was but preserving the machines themselves!