CNROM Dev Cart

CNROM Dev Cart
by on (#6246)
I made a CNROM devcart. There are some games that the graphics appear to have some bits missing and that shimmer. Other games work fine. The hardware seems to be fine. I notice that there seem to be places for two diodes that are not installed on my CNROM. Could there be different types of CNROM board? The diode pads would seem to connect the 161 D4 & D5 latches, maybe to control mirroring? Any ideas?

by on (#6248)
I don't know what the deal is with the spots for uninstalled diodes. I saw those on one version of the MHROM board too. Seems to be useless.

There are different versions of the boards (you'll probably see CNROM-02, CNROM-03 etc.) but they should function the same.
CNROM Dev Cart
by on (#6317)
I took the CNROM devcart apart and checked each trace and every connection was fine. Yet the graphics were still striped and sparkly. I just couldn't get it to function. Finally I cut off the LS161 section with the capacitor and resistor and grafted it on to a N-ROM cartridge. Then I took the PGROM & CHROM and put them into to the hybrid cartridge and it works perfectly. I have no idea why the CNROM cart. wouldn't work properly but now I have a cartridge that will work for mapper 0 & 3.
Re: CNROM Dev Cart
by on (#6319)
Lloyd Gordon wrote:
I made a CNROM devcart. There are some games that the graphics appear to have some bits missing and that shimmer. Other games work fine. The hardware seems to be fine. I notice that there seem to be places for two diodes that are not installed on my CNROM. Could there be different types of CNROM board? The diode pads would seem to connect the 161 D4 & D5 latches, maybe to control mirroring? Any ideas?


This is because of the way the ROM is hooked up to the 74161. Your EPROM might need VPP to be pulled up to V+. On the CNROM cart VPP goes to the 74161. So, depending on what the game does with unused bits that it writes, the game could work fine (if it wrote 1) or it could shimmer (if they wrote 0)... or it could change depending on if the bit changes depending on the bank it's in.

Some EPROMs are fine with VPP "floating in the breeze" and unconnected, or pulled to ground. Lots of older EPROMs though need VPP pulled to 5V to function properly however. So for people designing boards, it's always a good idea to run VPP to 5V. Failure to do so could cause trouble.
CNROM Dev Cart
by on (#6332)
I just checked the 27C256 sheets and you are absolutey correct. I think I may be grounding the Vpp or leaving it floating, I will have to check. However whatever I am doing I think I did it consistently on all my devcarts. When we finish our latest CNROM game Moakun, I will see what I did with Vpp and see if it is connected to Vcc. If I ever see this happening again, I will try your tip. Thanks for the pointer. As per usual: READ THE DATA SHEETS should be my byword.

by on (#7072)
I was trying to play Timezone on my TLROM devcart and it wouldn't work. I switched the Vpp's from ground to V+ and it worked perfectly. Thanks Kevtris for your advice. It's weird how the effect seems to happen on an erratic basis with different games on the same devcart and EPROM. That's why I suspected a software problem.

by on (#7078)
Hmm, that would explain why some of my carts may have been crashing in the past. I used to tie VPP to GND instead of VCC.

I'll have to give this a shot,

Thanks!

by on (#7091)
For myself, leaving VPP floating works just fine. I usually tie P to VCC because it's easy to do, it's the pin just next to it so you just have to make a giant short circuit with some solver to tie P to VCC.

by on (#7093)
It seems much simpler to just do it right from the start. Skimp now, waste time debugging later. Even if an adjacent pin doesn't have the proper signal, how hard is it to solder a thin piece of wire to the nearest Vcc/GND? I often use either solid copper sripped of insulation (since it melts anyway), or a single strand plucked from stranded wire.

by on (#7100)
Say as you wish, but I myself think that placing wires is one of the worst occupation on earth, only exeded by solder two wire-ends toghether and by solvering a standing resistor, and it's incredibly scaring. I always want my wire to roll around the pin, and then the thing quit the pin up for some random reaon, and I have to restart the whole thing again. Once it holds down the pin witout moving, solver it is really hard, scince you have to still have the wire not moving, hold your iron, hold your solder and be carefull to make a decent solvering job all this at the same times. This just makes me mad.

by on (#7110)
I find the wire-wrapping device and wire is perfect for the devcarts and the insulation doesn't melt.

by on (#7164)
Bregalad wrote:
Say as you wish, but I myself think that placing wires is one of the worst occupation on earth, only exeded by solder two wire-ends toghether and by solvering a standing resistor, and it's incredibly scaring. I always want my wire to roll around the pin, and then the thing quit the pin up for some random reaon, and I have to restart the whole thing again. Once it holds down the pin witout moving, solver it is really hard, scince you have to still have the wire not moving, hold your iron, hold your solder and be carefull to make a decent solvering job all this at the same times. This just makes me mad.


Like anything else, soldering is a skill that you acquire. To make life easier, I do this:

1) put some solder on the chip pin you wish to solder to (not ALOT but enough to give it a good coat. this is called 'tinning')

2) put some solder on the end of the wire you wish to solder also. Not a lot... if you make a tiny ball that's OK... and I mean tiny- say 1/2 the wire's diameter. if it's any bigger than this, you will have too much solder.

3) with one hand, put the wire's tinned end next to the pin and heat both with the iron. THere should be enough solder on both to make a good connection.

If done right, the two will be decently soldered.

note: If you dally too long, the flux on the solder will be gone and it won't work very good. This method relys on there being a little flux left. If you run out of flux, the joint made will be grey and dull. If done right, it'll be bright and shiny. You can also add a little flux is need be, but I never have to personally.