Unfortunately these days finding an alternative Linux distro which supports an installation process which uses the serial terminal interface is really, really hard these days. So the device sits unused in one of my desk drawers.
It was sort of a pain, at first, to get pfsense on them. Its been a while since I’ve done it, but heres the short version.
1. I found a version of linux (voyage maybe?) that had some pxe boot stuff setup automatically. I just had to set the dhcp range and a few other options. Any distro with pxe boot works but I’m lazy.
2. Plugged the soekris box into my laptops ethernet port. The linux distro was just a vm. Set up the vm so its nic could communicate with the physical laptop ethernet port.
3. Hooked a serial cable up to the soekris and set it to pxe boot. I had to find the manual to figure out what speed i needed for the serial port. I want to say default was 9600, but double check. Then it got the image from the linux vm and was enough to get me a cmd line via serial port.
4. I copied the pfsense cf (i think its called that) to a exfat USB flash drive and plugged it into the USB port on the soekris.
5. I mounted the USB flash drive on the soekris and used dd to copy the image to the internal cf card.
6. Reboot the soekris when done, change boot order on it, if applicable, and from there plug the soekris into the network or somewhere where it can get a dhcp address so you can finish setup from a web browser.
If you’re interested i might be able to find my notes on it, but its probably been 5+ years.