The TX3-USB-AD Kit converts RS-485 signals to USB. The MD-993 Module mounts above the TX3 Telephone or Card Access controller boards in the locations shown below, using the four # 6-32 screws and spacers provided. The MD-993 Module ribbon cable connects to the P4 modem connector on the Telephone Access System controller board and to the P3 modem connector on the Card Access System controller board.