There are two types of devices that can communicate over a serial interface: DCE and DTE. A DCE provides a physical connection to a network and forwards traffic. A DTE connects to a network through a DCE device. Typically, a DTE device is connected to a DCE device (or vice versa) rather than another DTE device.
Check the documentation that ships with your serial device to determine if it is a DTE or DCE device. If you cannot find the information in the serial device documentation. This table summarizes serial devices that can be connected to the router serial port and whether the device is considered a DTE or DCE device. The Cisco 805 router is a DTE device by default.