Each PA-4E1G interface is available in either balanced (120-ohm) or unbalanced (75-ohm) mode; a unique port adapter supports each type. Neither the balanced and unbalanced modes nor the balanced and unbalanced cables are interchangeable; you cannot configure a balanced port to support an unbalanced line, nor can you attach an interface cable intended for a balanced line to an unbalanced port. Balanced interfaces typically use three conductors and three signal states: high, low, and ground. The high and low signals mirror each other. Unbalanced interfaces use only two signals: signal and ground. You can discover the mode of each interface by examining the agency approval label on each port adapter, or by using the show controller cbus command.

Refer the "PA-4E1G Serial Port Adapter Installation and Configuration" document present in the url below which will help you in troublshooting and installation:

http://www.cisco.com/en/US/docs/interfaces_modules/port_adapters/install_upgrade/serial/pa-4eig1_serial_install_config/3065over.html#wp1024647