The 485SD9TB is a port-powered two-channel RS-232 to RS-485 converter. It converts the TD and RD RS-232 lines to balanced halfduplex RS-485 signals. The unit is powered from the RS-232 data and handshake lines whether the lines are high or low. An external power supply can be connected to two terminals on the RS-485 connector if no handshake lines are available. The 485SD9TB has a DB-9 female connector on the RS-232 side and a terminal block connector on the RS-485 side.
Although the 485SD9TB uses handshake lines to power the converter, no handshaking is required to control the RS-485 driver. The RS-485 driver is automatically enabled during each spacing state on the RS-232 side. During the marking or idle state, the RS-485 driver is disabled and the data lines are held in the marking state by the 4.7K ohm pullup and pull-down resistors. The value of these resistors may need to be changed to a different value when termination is used in order to maintain the proper DC bias during the idle state. See B&B Electronics’ RS-422/RS-485 Application Note for more information on termination and DC biasing of an RS-485 network. The 485SD9TB has an internal connection to prevent data transmitted from the RS-232 port from being echoed back to the RS-232 port. The 485SD9TB is used as a two wire (half duplex) RS-485 converter
