Serial RS232/485 communication through a DB9 connector is the most common interface for Digi radios. Many PCs have a limited number of serial ports from which to connect peripherals, and often times, laptops are not even equipped with serial COM ports. In many of these cases, USB-to-serial adaptors are used in lieu of a direct serial port connection.
There are many vendors who sell USB-to-serial adaptors. For various reasons however, Digi has found some adaptors work fairly well, while others seem unreliable or are completely incompatible. This can be attributed to the kind of oscillator used, how the control lines are configured, baud rates, etc. This can also make troubleshooting the radios much more difficult because it cannot be easily determined if the problem lies in the radio itself, or if the communication error is an effect of an incompatibility between the radio and the USB-to-serial adaptor being used.
It is recommended when troubleshooting, to eliminate the USB-to-serial device and establish a direct connection between the radio and host device. If an adapter is necessary, Digi strongly recommends testing the adapter and radio combination to verify compatibility before any field tests are performed. Please contact Digi support for recommendations for USB-to-serial adapters for greater compatibility with Digi radios.
In lieu of using a possibly unreliable adapter for another serial connection to your host device, consider adding a serial card to your host to eliminate the necessity altogether.