For the past few months, I've been working on a design for the PowerBook 100 - a 6MB RAM expansion containing 12 512k x 8 PSRAM chips. I have a few PB100s, but only one had an upgrade past the stock 2MB expansion card.
Based on what I read in the Developer's Note, if I choose to do so, I can partition the upper and lower write bytes to allow for compatibility with a 32bit bus PowerBook. Because I knew my card wouldn't physically fit in a PowerBook 140/170, I opted to ignore this to make PCB routing easier.
Upon assembly and testing, I found that when all RAM ICs were populated, the system gave the Sad Mac at bootup. Eventually I got the card working, but only with 4 RAM ICs installed - not any better than the 2MB stock Apple card! For what it's worth, I was able to boot into System 6 with 8 chips populated (6MB total RAM), but the system was very glitchy.
In my post-testing troubleshooting, I stumbled on some early PB100 schematics. Looking at the RAM section, they laid it out identically to how I did it. I am really scratching my head here as to why it's not working. Unless there's a mistake in the expansion connector pinout listed in the Developer Note...?
I've attached both the OEM RAM schematics, and my own... please let me know if you see anything funky with my work. A second set of eyes is sometimes all it takes!
Based on what I read in the Developer's Note, if I choose to do so, I can partition the upper and lower write bytes to allow for compatibility with a 32bit bus PowerBook. Because I knew my card wouldn't physically fit in a PowerBook 140/170, I opted to ignore this to make PCB routing easier.
Upon assembly and testing, I found that when all RAM ICs were populated, the system gave the Sad Mac at bootup. Eventually I got the card working, but only with 4 RAM ICs installed - not any better than the 2MB stock Apple card! For what it's worth, I was able to boot into System 6 with 8 chips populated (6MB total RAM), but the system was very glitchy.
In my post-testing troubleshooting, I stumbled on some early PB100 schematics. Looking at the RAM section, they laid it out identically to how I did it. I am really scratching my head here as to why it's not working. Unless there's a mistake in the expansion connector pinout listed in the Developer Note...?
I've attached both the OEM RAM schematics, and my own... please let me know if you see anything funky with my work. A second set of eyes is sometimes all it takes!