The 422LP9R is a port - powered two - channel RS - 232 to RS - 422 converter. It converts TD and RD RS - 232 lines to balanced RS - 422 signals. The unit is powered from the RS - 232 handshake lines DTR an d RTS. One of these handshake lines must be asserted (high) in order to power the unit (See Table 1). The RS - 422 driver is enabled when RTS is asserted. Therefore, RTS must be asserted in order to transmit data. The RS - 422 receiver is always enabled. If DT R is always asserted and the RTS is used to control the driver, the 422LP9R can be used as a RS - 232 to RS - 485 4 - wire converter. To insure reception, it is recommended that DTR is asserted.In order to maximize the amount of power available to the RS - 422 d river, the RS - 232 handshake lines are not looped back (tied together). As a result the following handshake lines will appear as disasserted (low): CTS, DCD, and DSR. Care should be taken to insure that any software being used doesn't require any of these handshake lines be asserted. If existing software requires any of the handshake lines to be asserted, you can loop back the required handshake lines in your cable.