Working on my #powerbook 165c RAM expansion card, and the Developer Note for the PB160/180 (https://powerbook.micahgartman.com/dev/dn_160_180.pdf) is a godsend as it contains the expansion port pin-out, as well as some hints for how to build an expansion card.
That said, it's.. not perfect. A small overview of confusing things:
1. The physical dimension guidelines (pic 1) seem to have no correlation to the actual physical size of produced cards (pic 2), and using the guidelines, I don't see how you could possibly build a 10MB card.
2. The data bus is 32 bits wide, which you are expected to service using 4 512kx8bit PSRAM chips. However, Apple wanted to support flexible writes (to any byte in a dword), so they have 4 write-enable lines, named /LLW, /ULW, /LUW, and /UUW. Obviously, /LLW refers to the lowermost byte (bits 0-7), and /UUW the uppermost byte (bits 24-31). In the pinout, Apple calls /LUW "Lower middle write byte" and /ULW "Upper middle write byte", which to me sounds like bits 8-15 and 16-23, respectively. However, when I verified this with the card I have, I discovered they are the other way around, which linguistically makes no sense to me, to the extent that I'm still doubting myself here. In general, why would you even use this confusing naming scheme? Just call them /W0, /W1, /W2, /W3, and specify the bits they are associated with!
3. The expansion connector has 6 chip select lines, which each select a 2MB RAM bank (made up of those 4 chips). That implies the max expansion RAM is 12MB, but Apple specifies the max RAM as 10MB and makes no mention of the extra CS line 🤔