Hello Everyone,
Just in case anybody else may have this question in the future, we typically recommend using a serial to USB adapter for the older non-USB Summa models. Keyspan is our preferred brand and you can purchase one of the adapters here -
Keyspan High Speed USB to Serial Adapter(USA-19HS) | Tripp Lite.
I hope this information is helpful for anybody in a similar predicament.
Best regards,
Phil Johnson
Summa Inc.