Hi Charles.
Thanks for posting on the forum.
With regard to this specific issue, I would recommend uninstalling your current ActivManager version, and downgrading the driver to a previous version. On the install for the driver it will ask you to install "USB" or "USB + Serial", please ensure you select "USB + Serial".
This should help with the connection issue for the serial cable.
Also, if you are using a serial to USB converter, double check to see if there is any drivers required for this device.
The older driver can be downloaded on the below link:
promethean.box.com/activdriver-5-7-x64
If you still require any help, please do not hesitate to contact Promethean.
I've created a case to support this issue you are experiencing, please quote the reference number 00426669.
Adam
Promethean Technical Support