I'm planning to do some upgrades to one of my consoles, so I was looking at the CRU for available bits. It looks to me like there are 3 available, but I ran across an upgrade(s) document from Thierry Nouspikel that claims there are only two. The two that he points out are a subset of my conclusion, I am happy to say. But I have a great deal of respect for his opinion, so I'd like to get some of your opinions on mine, some time before I start hacking into this console.
To start, I agree that output bits 16 & 17 are available. In addition to those, however, it looks to me like bit 26, which appears on pin 29, is fair game, too. Bit 12 is also connected to pin 29, but that is for input or INT. I would be configuring it for output. According to the datasheet and the schematics for both the /4 and /4A, bit 26 is connected to pin 29, which is pulled HIGH by 5VDC through a 10K resistor. It has no other connections, unless BOTH of the schematics I have are in error. Is this bit available for a project, or is it commonly, or even occasionally, used for something else? Facts or opinions?
In the same doc from Thierry, if I remember correctly, he alludes to some unused NOT gates. Does anyone have, or know of, a list of logic gates in the original console, of any type at all, that weren't used?