With the appropriate serial cable, connect to the PTP 820G's Terminal port, and fire up a terminal window program with the following settings: 115200 bps, 8 data bits, no parity, 1 stop bit, and no flow control.
Login to the unit.
Type: platform management ip show ip-address
This returns the management IP address.
Then you can connect to the MGT port with an Ethernet cable, and go crazy!