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Awesome! Okay, so I’m back to this chip. 😆

I’m having a hard time finding where these can be purchased. I found an alternative on Mouser, but it’s 3.3V only and the inputs are not 5V tolerant. A couple of level shifters would solve that issue. We would still need a 3.3V power supply. Unless you can point me to some readily available 5V...

What I meant in the part about refresh rates was more about the address lines than actual rates. I was originally confused about that chip having an A11 pin, because A10 should be the MSB for that size of memory chip. Then I noticed the little blurb in the datasheet block diagram regarding the...

We would need to use four of these 4Mb chips just to have the correct number of data bits, and I think it would end up providing 8MB of memory, half of it being wasted unless we tried to do something tricky and find some extra address space for a RAM disk? I don’t know much about that nor the...

I’m pretty sure I bought mine on eBay. I don’t reallly remember my reasoning for settling on that part other than the low price. I appreciate you searching for parts which are easier to obtain! This 1Mx16-bit (2MB?) chip you linked looks like it should work great, since it has separate lower...

This PDF talks a little about the SCSI section of the Mac Plus, and includes details on the memory map: https://ia800705.us.archive.org/26/items/Mac_Hardware_Info_Mac_Plus/Mac_Hardware_Info_Mac_Plus.pdf And here’s the schematic for the Mac Plus, which uses an additional Apple custom IC to...

I found this build thread for reverse engineering a MacSnap SCSI card. It uses some clever, slightly complicated, hacks to make it work using only the lines available at the ROM sockets, but I don’t think that’s necessary if we are already soldering leads to various points on the logic board...

A feature that the memory expansion board Adrian found on that 512K logic board was that you could hold the mouse button down at power on (like you would do to make the system eject a disk) to have the memory board disable itself leaving the system with only 512K of memory. System reset...

Adrian Black released a follow up video to the memory upgrade board on a Mac 512K logic board he was fixing up (I believe these are on his second channel). It got me thinking that bodge wires leading into a connector as a removal means for a memory upgrade mod isn’t such a bad way to go. Sure...

No I did not. I see that they (~CAS0 annd ~CAS1) aren’t necessary to take back to the logic board and could instead be pulled up high. I’m not sure if it would be necessary because original memory should be disabled as their ~RAS line is pulled high. Maybe the DRAMs still respond somehow to ~CAS...

I considered using SIMM sockets and SIMMS as DRAM but ultimately ended up settling on the ICs I have as I could get some decently cheap on eBay. I think a future final design should use SIMMs so folks can replace them easily if needed.

Wow! They desoldered the entire CPU! I’ll check this out too. I originally searched high and low for some of those IC sockets which snap on to the top of an IC and came up with very little results. That would be way nicer than desoldering and socketing a bunch of rare ICs. I think someone would...

Thank you! I’ll try to dig into all of this soon.

Maybe dual ported isn’t the phrase for it, but the DRAM datasheet indicates a data input and data output pin: https://datasheetspdf.com/pdf-down/H/M/5/HM50256_Hitachi.pdf The Mac’s schematic shows the DRAM data inputs are connected directly to the data bus, and the outputs are connected to some...

Correction: I meant to say 800K drive

On top of that, these breadboards are a little worn out from use in other projects, and they were very low quality to begin with. And thanks! I’d appreciate the second set of eyes.

I’ve got the 128K ROM. I know this because they’re installed in a third party SCSI card, and the system has a 400K drive. Good catch!

Thank you both for the encouragement. I’m pretty motivated to make this work. It would just be very disheartening to layout and PCB, pay to get a few made, then build one out just to find out the entire design was doomed from the start. If you know of anyone who may be interested in taking a...

For fun, here are some photos of my failed bread boarding attempt. I got some pretty flashing lines, and with enough wiggling of the wires, I could barely make out the image of a sad Mac through the lines!

Here’s a link to the build thread I started: 'Early Macintosh home brew 4MB memory upgrade board development' https://68kmla.org/bb/index.php?threads/early-macintosh-home-brew-4mb-memory-upgrade-board-development.47308/