Expansion Module #1 supply currents

[fieroluke]

I took the time today to measure the current draws of the Atari 2600 expansion module (without cartridge). Hopefully someone finds this useful:

 

The 5V supply unsurprisingly draws most, about 270mA. I would have guessed more, but this is moderate.

 

Next is the 12V supply, coming in second at 25mA. I guess it only supplies the video amp.

 

Finally, the -5V supply: 0.0mA! Surprise, surprise, it’s apparently not used! 
 

This info may come in handy if anyone wants to turn the exp module into a standalone 2600 clone…

[Falonn]

Re: "if anyone wants to turn the exp module into a standalone 2600 clone", this is a nearly-finished place to start.

 

Once I found out you could get S-video out of a 2600--but not from the expansion module pins--it took the wind out of my sails and I abandoned the project (and just grabbed a modded 2600 from eBay instead ? ).

 

That said, that board design was far enough along that things were showing up on the screen, just... not quite correctly. ?

[fieroluke]

2 hours ago, Falonn said:

Re: "if anyone wants to turn the exp module into a standalone 2600 clone", this is a nearly-finished place to start.

 

Once I found out you could get S-video out of a 2600--but not from the expansion module pins--it took the wind out of my sails and I abandoned the project (and just grabbed a modded 2600 from eBay instead ? ).

 

That said, that board design was far enough along that things were showing up on the screen, just... not quite correctly. ?

Yes, but the interesting point was that -5V is not needed, and adding an 7805 you could run the whole thing from a single 12V supply. The 0.3A are interesting when it comes to heat sinking the 7805...

 

[Ikrananka]

7 hours ago, Falonn said:

Once I found out you could get S-video out of a 2600--but not from the expansion module pins--it took the wind out of my sails and I abandoned the project (and just grabbed a modded 2600 from eBay instead ? ).

For years I had wanted to try to replicate phattyboombatty's standalone mod but never found the time.  Then I picked up a 2600Jr. locally for a few dollars and installed a Tim Worthington RGB mod in it and have never looked back.  Stunning image quality.  I also planned on installing one of Tim's RGB mods in one of my spare expansion modules but again never found the time.  A future project perhaps.

[Falonn]

21 hours ago, fieroluke said:

Yes, but the interesting point was that -5V is not needed...

Are you completely sure?  @ChildOfCv's schematics show the -5V rail connected (through a resistor and 2.5V Zener diode) to pin 8 of each controller port (which is the "ground" pin for left/right/up/down).  During your testing did you have controllers connected?  Does the -5V rail show any current when you move the stick around?

 

(I'm hazy on Zeners, but the circuit looks like it's almost been designed to have the -5V rail be optional.  But the controller interface on the Colecovision does something similarly quizzical, so I wouldn't write it off as "not needed".)

 

16 hours ago, Ikrananka said:

Stunning image quality.

I'd be curious to see a side-by-side comparison.  At that resolution and color count, I would already describe the S-video mod as "flawless". hehe.

[fieroluke]

1 hour ago, Falonn said:

Are you completely sure?  @ChildOfCv's schematics show the -5V rail connected (through a resistor and 2.5V Zener diode) to pin 8 of each controller port (which is the "ground" pin for left/right/up/down).  During your testing did you have controllers connected?  Does the -5V rail show any current when you move the stick around?

 

(I'm hazy on Zeners, but the circuit looks like it's almost been designed to have the -5V rail be optional.  But the controller interface on the Colecovision does something similarly quizzical, so I wouldn't write it off as "not needed".)

Good point! 
i wasn’t aware of the schematics being available, I couldn’t find them when I looked for them.

indeed, I didn’t hook up a joystick (I’m not even sure I have Atari joysticks anymore!).

 

And yes, Coleco used the -5V as well to (I guess) have a „stronger“ ground using their controllers, which have diodes in them, offsetting their voltage drops? Maybe using the CV controllers on the expansion module was supported and offsetting the diodes required the -2.5V offset?

[ChildOfCv]

Yeah I think the zeners are there to make sure the CV joysticks work in the Expansion Module.  With the zener diodes in place, it pulls down to -2.5V, which ought to overcome any diode issues in the controller.  Otherwise there's a 18Ohm resistor to ground from pin 8.

[Ikrananka]

5 hours ago, Falonn said:

I'd be curious to see a side-by-side comparison.  At that resolution and color count, I would already describe the S-video mod as "flawless". hehe.

I put my RGB modded 2600 through an OSSC and then into a 20" LCD monitor via HDMI.  All I can say is that the image looks flawless, with ultra-sharp edges wherever a colour change occurs.  No fuzziness or colour blending whatsoever.  I'm not sure the same could be said for s-video.  All of this is just personal preference anyway.

[Falonn]

On 12/11/2021 at 4:47 AM, ChildOfCv said:

With the zener diodes in place, it pulls down to -2.5V, which ought to overcome any diode issues in the controller.  Otherwise there's a 18Ohm resistor to ground from pin 8.

Hmm, in the two years since this reply I seem to have forgotten I'd already asked here, so I just re-asked it over in the 2600 forum a couple hours ago before I noticed this one again.

 

But the answers here (that the -5V supply pulls 0mA and that pin 8 on the controllers would be at -2.5V) both seem at odds with the schematic:

 

(Here it is, live in the simulator.)

 

The intersection between R1, ZD1, and ZD2 is definitely at -2.5V, but the pin 8 controller GNDs are just a hair below 0V at -63mV.  And instead of 0mA, there should be a steady 43mA pouring mostly through ZD1 and R1.

 

So, if it is for Coleco controller compatibility, it's a tiny <0.1V tweak that comes at the expense of ~200mW constantly being burned.  For Atari joysticks, it seems like the -5V rail could safely be omitted.  But I still feel like I'm missing something.  Maybe I should get my Expansion Module 1 back on the bench and test the power draw, too.  I wonder if fieroluke's was behaving out of the ordinary?