Quick Question On DC VGA Box Schematic

[Guest_Blaine]

I'm a little confused. The numbers going to the pins on the IC:74HCT244 ...

Those aren't the pin numbers, are they? According to the sheet from TI that's not the pin order.

Am I to, for example connect pin 8 from the DC connector through a resistor to pin 4 on the IC, or to pin 2 (which it would be if we assumed that pin was second down on the left).

I'm sure I'm just being stupid but, I am never the less confused. Thanks.

[kendrick]

To be clear, you're talking about this diagram, right?

http://www.gamesx.com/grafx/dricas.gif

Sometimes for clarity's sake, the IC pin order on a schematic diagram is purposefully out of order so that lines don't have to cross over each other unnecessarily. The chips represented in the diagram are intended to be logical and not literal. The numbers on the diagram indicate which pin you're supposed to connect to, and it's assumed that you're clever enough to figure out an orderly way to arrange the wires. So in your example above, pin 8 of the DC Video out port does indeed connect to pin 4 of the chip.

Hope that helps you out. If you like, I can point you to examples of other diagrams that are similar but simpler, so you can get comfortable with the concept.

-KKC, who is awash in back pay he was owed. Drinks are on me!

[Blaine]

Ah. Thank you. That's more or less what I figured.

I have two chips: CD74HCT244E and SN74HCT244DBR.

Now the CD series is a larger socket chip and the SN is a small (actually smaller than I think I can swing) surface mount chip.

Last night I used Illustrator to try and make a crappy pcb (for use with the laser printer/iron/ferric chloride etching technique).

While I'm pretty sure it's laid out right...knowing my luck, I screwed something up. But I'll try the pcb method if only because it's a learning experience.

The attached image is the PCB at 500% or so. If anyone sees any glaring problems, please let me know.

The gaps with squares are for drilling to surface mount components (caps, resistors, switches) the circles are for where I'm just going to solder wires (such as fromt the DC connector and to the VGA connector) and the red and green dots are to be bridged with wire (since I couldn't figure out a way to do it without adding wires) and the large spot in the middle is the chip.

I figure...worst case scenario, it doesn't work and I'll use the socketed chip.

Thanks.

[Guest]

Anyone?

[Logistics]

Every manufacturer of the 74HCT244 seems to put the voltage supply on Pin 20.  In your schematic it looks like the voltage supply is on Pin 15.  At least that much seems out of place.  Actually, after comparing this to the other schematics, everything looks mixed up.  Or perhaps V in your schematic represents the vertical sync line, which still would not attach to Pin 15, but rather GND.  I can't tell what's going on in your schematic.  

Was there a particular reason you choise a Texas Instruments IC over Fairchild or Phillips, etc.?  I am just wondering if one is better than the other.


Hey, I have a question about the DCVGA Box, too!  I bought a real Sega box, and on my old 20" SGI monitor, it's kind of dark.  These monitors were originally Sync-on-Green, but I was told when I bought it that it had already been modified to accept common VGA.  Could changing the capacitors in the DCVGA box help brighten things up?  One thing I noticed is that if you use an extension cable on the 3-foot cable the monitor came with, it gets upset and starts ghosting.  Actually, this also happened when using the Creative DXR3 Card with the Video Pass-Through cable.  Whereas when using the DCVGA Box, it's just darker than normal.  Does it need some RGB amplification or something?  The Brightness and Contrast on the monitor are always up all the way in order to get a more normal picture... in fact I have to slide up the Gamma in the nVidia controls to make it look right.

[Blaine]

Ah. Yeah the V is the vert.  The 20th pin is taking the voltage. I've re-worked the board to fit a standard Rat Shack mini-enclosure (1.25x2) as well as improving the layout. After building the board, have the mini enclosure with the 15 pin DSUB. Odds are I'll short the switch so it's always in VGA mode and create a VGA cable instead of just a box. Perhaps not. I haven't decided. I'll be etching 10 boards or so - so, I'll have options.

As for why I went with the TI...honestly? No reason at all. I just knew I could get free samples from them, as an added bonus they ship it out really really quick.

Knowing that they have a very good sample program was pretty much the impetus for using TI.

I'm etching pcbs right now. I'll need to pick up some conductive pens to clean up any rough traces this weekend. But I'll probably try to seat the ic sometime tomorrow after drilling.