On a single AF-5600 ACM system, the only potentially free serial port is going to be serial #1, which, if not already in use by a remote audio panel, would be free to use for the G5 backup ADAHRS function. You can access PFD#1 through pins 12/25 on the ACM Audio panel connector.

70080 ACM-ECB Serial Port Diagram Dip Switch.pdf