Posibus Peripheral Emulator

[m. thompson]

For quite a while I have been working on a self education project using a SmartFusion FPGA running uC Linux. Fortunately I have a lot of friends who know about projects like this and were willling to help. I used a small buffer circuit on a Wire-Wrap board to connect the Posibus on a PDP-8/L to the 3.3V FPGA. I put the paper tape reader controller flag flip-flip, and address and IOT decoding in the FPGA, and wrote a Linux device driver to provide a Linux device to a paper tape reader application. The Linux application reads a paper tape image from flash and writes it to the Linux paper tape reader device.

It actually works! I was able to load Focal in about two seconds from the emulated paper tape reader. That is about 10x faster than the real paper tape reader. The speed is probably limited by the PDP-8.

After a little more software development I will switch to a larger Emcraft FPGA board that has more I/O. With a little more work it should be able to emulate any programmed I/O device.

With more FPGA and sofware development it should be able to emulate data-break peripherals. That would add disks and tapes to the list of emulated peripherals.

You can find more details on the project here:
http://www.ricomputermuseum.org/Home/equipment/pdp-8-l/making-a-posibus-periperal-emulator