After it’s first release, the NES went through a number of revisions to its motherboard. The most notable of these changes were based on increasing the efficacy of the NES lockout chip. As time went on, unlicensed publishers found better ways of defeating the NES lockout system, and this lead to a sort of arms race between Nintendo and the unlicensed game studios. While the initial CPU revision (NES-CPU-04) is vulnerable to every common lockout defeat method, by the end of its production and the last revision (NES-CPU-11), the NES was very robust against most methods to circumvent the lockout system. This is why some unlicensed cartridges (most notably Codemasters cartridges) feature a switch on the cartridge. This allows for multiple defeat strategies to be used to help adjust to the motherboard revision present in the consumer’s NES.
So, why might you want to know what CPU you have? Well, quite simply, if you have an early CPU revision, you can play just about any unlicensed game cart without problem. If you have a later revision, however, you will find that some cartridges just will not play on your console. It is good to know what motherboard revision you have, and if you have a later revision you can go through the steps to permanently disable the lockout system in your console, allowing any game to be played.
The only steps for identifying the motherboard revision on your NES are outlined in my post on Accessing the NES Motherboard. Once you have the motherboard out, it is simply a matter of looking on the component side of the motherboard, and looking for ‘NES-CPU-##’ silkscreened onto the PCB. In the picture below, you can see the motherboard revision for my NES (NES-CPU-05):
Once you have your CPU identified, you can determine whether you need to take steps to allow you to play whatever unlicensed game of your choice. The following lists the motherboard revisions I am aware of currently – I am trying to gather information relating to what changed between the different revisions. If you have any information, please leave a comment and I will incorporate it into the list.
- NES-CPU-04 – The original retail release of the NES. I know of no compatibility issues with unlicensed games.
- NES-CPU-11 – The final retail release of the NES. Very robust to lockout defeat schemes.