OK Andrew.
Assuming you have correct cable setup (DTE to DCE). The interface with the DCE connection will require the clockrate setting. eg.
interface Serial0
ip address 10.1.1.1 255.255.255.252
clockrate 128000
The other end will be DTE
interface Serial0
ip address 10.1.1.2 255.255.255.252
This should allow you to test back to back