Single Attached Station (SAS)
Just has one card, e.g. HPs with a single connection S-PORT with 2 pins, Pin and Pout. Date enters via Pin and leaves via Pout
Dual Attached Stations (DAC)
Has two cards known as the A port and the B port. There are two rings, one providing backup for the other. Data enters the A port via Pin and leaves the B port via Pout. The B port then connectes to the A port of another station or to a concentrator.