Technically none, only a willingness to learn things by reading external resources (research, not looking up solutions), and/or to spend a lot of time thinking and trying things out yourself.
I had no experience with it, beyond being a programmer and thus being more knowledgeable on binary than the average person, and I managed to get through the game just fine.

Having more knowledge will obviously help, but it'd also trivialize a lot of the game and may end up being too rudimentary if you're "overly familiar" with it. Some of the people who are really good at architecture design build really advanced stuff instead of following the "intended" designs, but the current game engine can't quite handle simulating things at a decent speed with too much stuff going on. The eventual future update the dev's working on will solve that, but there's no telling when that will be done. The current version's probably better for people trying to learn than it is for the pros - those might be better off with VCB until then.

Originally posted by Goblin:

Technically none, only a willingness to learn things by reading external resources (research, not looking up solutions), and/or to spend a lot of time thinking and trying things out yourself.
I had no experience with it, beyond being a programmer and thus being more knowledgeable on binary than the average person, and I managed to get through the game just fine.

Having more knowledge will obviously help, but it'd also trivialize a lot of the game and may end up being too rudimentary if you're "overly familiar" with it. Some of the people who are really good at architecture design build really advanced stuff instead of following the "intended" designs, but the current game engine can't quite handle simulating things at a decent speed with too much stuff going on. The eventual future update the dev's working on will solve that, but there's no telling when that will be done. The current version's probably better for people trying to learn than it is for the pros - those might be better off with VCB until then.

Thanks. I have no experience with logic circuits or programming, but I was intrigued.

I bought Scrap Mechanic because it had a working physics engine and logic circuitry.

What's interesting about that system and what EVERYONE basically gets WRONG is that the logic gates in that games are not INSTANTANEOUS. They have "CLOCK" which is a VERY BIG DEAL. Long story short, I had a lot of fun building automated farming systems that eventually outpaced the weight limit of the physics engine but had some fairly impressive button logic with simple programming and patterns.

I learned how to make sense of that with nothing but truth tables and watching outputs. The confusing factor of clock, then unknown, and certain quirks of memory storage in the gate logic of the game made it both entertaining and excruciating.

You can do the same thing here, but without the physics or robot attacks and none of the invisible surprises.

In theory : none. Everything needed is explained.
In reality : good grasp on binary in order to be able to pass the several binary "race" levels.

Assembly langage programming is a huge bonus. It'll also help you having nicer opcodes on the second CPU (you know the proper result intended).

Some digital electronic background may also help but it is not a requirement. On the other hand, if you want, you may have a look at the "Digital Design" by M Morris Mano (Pearson Prentice Hall ISBN 0-13-165472-1) or a latter edition (up to the 3rd Edition, 4th dropped several things like Quine Mc Clusky method in favor of HDL langages). This will be a very nice compliment as the book will take you through the same process (creating a CPU).

Basic knowledge of the boolean operations (AND, OR, XOR, NOT) and of using them in a computer langage are also a big plus.

I'd have said none but I've recently watched non-IT/non-STEM people play some pretty trivial programming games and I have my doubts.
Basically all you need is logic and an understanding of the AND/OR/XOR/NOT concepts.
(Basically what @vapula said)
I don't believe you need an electronic background though. I'm a computer scientist with only a basic understanding of electronics and none of it was required.