9 pin D-SUB female connector
An RS-232 to RS-485 converter cable translates the single-ended, point-to-point TIA/EIA-232-F signals from a PC serial port into the balanced differential signalling of TIA/EIA-485-A. RS-485 supports a multipoint half-duplex bus of up to 32 standard unit loads (nodes), with data rates to 10 Mbit/s and cable runs to approximately 1,200 m at lower baud rates. A transceiver IC such as the TI SN75176B performs the conversion inside the dongle, with the RS-232 RTS signal — or an automatic turnaround circuit — controlling the RS-485 driver-enable line to switch the bus between transmit and receive.
Notes
EIA-485 cable is usually made with twisted pair (like EIA-422) and may span up to 1,200 metres. The recommended arrangement of the wires is as a connected series of point-to-point nodes — a daisy-chain or multidrop bus topology. Ideally, the two ends of the cable will have a termination resistor connected across the two wires, and two powered bias resistors to hold the lines apart when no driver is active. The value of each termination resistor should equal the cable’s characteristic impedance (typically 120 Ω for twisted pairs), in accordance with TIA/EIA-485-A.