Four Times the RAM, Same Great Price

A project log for Mackerel 68k Computer

A computer engineering journey through the Motorola 68k CPU family

Colin MaykishColin Maykish 02/27/2022 at 23:340 Comments

After spending all afternoon soldering, Mackerel now has a full 2 megabytes of super fast SRAM. I decided to go with the ISSI IS61C5128AL RAM chips for this project. They're pretty similar in function to the more common AS6C4008 series from Alliance, but they are rated for a blistering 10ns access time, more than 5x faster than the Alliance RAM. They're also slightly cheaper. The downside is that they're only available in surface mount packages. By the time the adapter boards are factored in, the cost difference is negligible, but the speed is still really nice. No wait states here!

The SOJ-36 packages are not really that bad to solder by hand. With lots of liquid flux, I was able to get (in my humble opinion) really nice joints connecting them to the DIP breakout boards. These breakout boards won't really be necessary when I start designing PCBs, but they do make the ICs easy to reuse during prototyping.

I've also thought about designing a SOP-36 to DIP-32 adapter which would convert the footprint of these ISSI chips to match the more standard DIP-32 SRAM pinout, allowing either style of SRAM to be used in the same socket.

Here's the full RAM board with four chips installed for a total of 2048KB:

The back side is a little less attractive:

I'm not sure how well this wiring arrangement will hold up at higher clock speeds, but I was able to verify reads and writes to the entire 2MB range using a simple test program.

One thing I don't really like with this design is that all of the chip-select logic is off-board. Every SRAM chip has its chip-select line connected to a separate pin on the backplane. I've got plenty of pins to spare at the moment, but if this is made into a PCB, I would like to move the address decoding for each individual module on board, freeing up a bunch of backplane connections.