NES Schematics

I'm redrawing schematics for the NES and cartridges

Similar projects worth following
As I've started working on my various NES projects, I often have to look for schematics. Many of these schematics are old scanned documents which are not always accurate. I've found several mistakes with the schematic on the NES Dev Wiki, so I'm going to try redrawing it in Altium. I'm going to start with the NROM and UNROM cartridges first.

Follow the link on GitHub to see the latest revisions. Also on GitHub, I have an Altium library which contains the parts I used in making these schematics.

One of the schematics that I often find when working on my NES is this one.

A few years ago, when I was messing with the CIC chip, I found a few mistakes with the drawing of the reset circuit. I didn't think much of it at the time, so I threw it away after I was done.

Fast forward to this year: I was trying to diagnose why my NESRGB board wasn't working. I dug up the same old schematic and found even more mistakes this time. Usually the designator or the pin numbers were just wrong. There were also no designators for transistors or caps. 

Then when I was trying to create a development cartridge, I was confused about a couple of pins. Different sources claimed the A13 pin did different things. The schematic helped a little (even though the pins 37 and 65 were wrong) and was eventually able to figure it out.

I'm currently working on a development cartridge, so I'm going to start with the NROM and UNROM cartridges. I plan to move to working on the motherboard eventually.

I'm going to start by referring to different schematics first; comparing them, looking at datasheets, and seeing if it makes sense. If anything doesn't make sense, I'll trace it out with my multimeter on a NES. Then, once I get the basics, I will verify the schematic vs PCB using my multimeter again. I don't currently plan to use any photo editing software or any pictures of the PCB. Although I may, if the need arises.

  • Started work on NES motherboard schematic...

    Matthew09/17/2017 at 02:50 0 comments

    I'm just working on correcting the schematic right now. I've got an NES motherboard, my multimeter, and a red pen.

    Here's the fixed reset circuitry. A few things to note: The reset pin of the CIC chip (pin 7) is actually active high, not active low. The NES Dev Wiki called it out wrong on the page with the cartridge connector. Pin 70 is labeled as CIC /RST. It should just be CIC RST. When you push reset button, you are applying 5 volts to the reset pin of the CIC.

    Fixed Reset Circuitry Pinout

     The LED was also in the wrong place. It looks strange now because the cathode is connected to the output of an inverter, but I promise it's right. The RST pin of the CPU is active low of course. So, when the CIC pulls RST low the inverted output is high, which makes the LED turn off. (5 volts on both side of the LED)

    This schematic is a lot worse than I originally thought. Fortunately, I found a scan of a Famicom motherboard which appears to be original. So this will help a lot. I'm still going to work through each net and check it off with a pencil as I go. So it's still going to take me a while.

    wrong pin number and wrong net on U4RA2 pullup to pin 5 on U9It makes more sense once you realize RA2 are supposed to be pull-up resistors

    This last one took me a while to figure out. It looks simple enough now that I've fixed it, but it wasn't easy at the time. At first I thought they just accidentally swapped pin numbers, but as I dug deeper, I could tell something else was just off. I knew the clock output shouldn't be going there, but that left me with the question of where the heck RA2 was supposed to go.

View project log

Enjoy this project?



Similar Projects

Does this project spark your interest?

Become a member to follow this project and never miss any updates