A null modem is often required to connect these USB serial adapters to DCE such as modems with the USB representing the DTE when plugged into a computer. The DCE port usually has a male DB9 which implies a null-modem is required.

Some adapters for RJ45 ports have roll-over wiring which avoids the need of a null modem for convenience. These cables usually have a female DB9 on one end and an RJ45 connector on the other. There are also combined RJ45 plug to USB serial adapter cables that integrate the cross-over cabling in their wiring between USB-A and RJ45 plugs.