FYL DB9 DB 9 RS-232 Male to Female Mini Null Modem Coupler Adapter Converter 100% Brand New, High Quality Null modems are used to connect two computers or other devices together to achieve file transfer. Null modem connectors are different from straight through connectors because the receive and transmit conductors are crossed. This is a miniature null modem with a male connector on one side and a female on the other.