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NOP Design


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I am making some NOP's because I want some NOPs now that I am building back up my workshop to repair video games again.  Here are some pics of one being assembled.

My aim was to keep them simple but useful. I tried to come up with a standard connector for the SIG side that I can do on other NOP's. I have used GND, 2xhigh order address bit, CAT Sig Clock signal, Other Sig Clock signal (if present), Reset and 5V. Connections explained on the back.

I added LED's to what is used for start/stop, clock and reset.  Great for simple troubleshooting.  A red LED is on while RESET is active and then the others (blue) show cpu activity on RD and A15. I thought these would be a nice additions to show a stuck reset, possibly watchdog barking and the others for cpu activity.
There are pins for gnd, clock, start/stop with a signature analyzer while using the NOP module, another pin for 5V to easily connect a logic probe and reset. I left a pin gap so you can even make up a polarized connector if you want to go all out.  Kept small to make easy access to work around the cpu.

I am not sure if we can post videos, here is a link to a Clip of Z80 NOP with LEDs running.  There will be more with the same connector and LED's for Z80, 6502, 6800, 8080, 8085, 6809 and 6809E.  Once I get all these sorted I will look at the 68000, Z8002 and 8088.

Comments and suggestions welcomed.

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Here are the first set of NOPs I will be making available (6502, 6800, 6809E and Z80). You can get just the PCB's, a set of all parts needed or a set assembled and tested. Microprocessors not included. PM me if interested. Still need to test them with a signature analyzer before releasing but all meter, scope and basic NOP testing has gone well.


I have assembled the 6809, 8080 and 8085's, waiting for micros to arrive so I can test them. Then I will look at making up 8088, Z8002 and 68000 if not too hard to do. That is all I have planned for now unless someone suggests another useful one for arcade repair.

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  • 2 months later...

Progress update. I purchased a faulty HP 5004A at the start of September to fix and then use to test the NOP modules are functioning properly for signature analysis.  It went into hibernation somewhere despite paying express international shipping (no thanks to USPS). It finally arrived a few days ago and I have fixed it. That means I can get onto testing the NOP modules are working properly.

Did a very quick test on the 6502 and Cat and HP address bus signatures are all correct (depending on the clock/2 or clock pin jumper used respectively). Also did a quick test on the Z80 module and get correct address signatures. Will do more thorough testing and also of other modules by the end of next week and if they are ok I should then be able to make them available. The LED on the reset pin gives a great reminder if you have not disabled watchdog reset on the pcb being worked on.

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