By default Port 1 and Port 2 on the SD4002 are both configured as RS232. However Port 2 can also be software selected to be an RS485 or RS422 port. In this configuration Port 2 connects through the screw terminal block(shown below):
RS-422 uses a full duplex transmit on TX+/TX- pair, receive on RX+/RX- pair.
RS-485 uses half duplex over single pair. The SD4002 supports half duplex 'party-line' communications over a 2-wire RS-485 bus (D+/D-). This is enabled by choosing the RS-485 option (instead of RS-232 or RS-422) for "Signaling Protocol" from the "Serial Port: Configuration" link on the Web management console. In addition two short cable loops are required between the RX+/TX+ pins and RX-/TX- pins. This is because the SD4002 uses universal differential transceivers that support 4-wire (RS-422) and 2-wire (RS-485) operation.
In RS-485 mode, Port2 on the SD4002 listens on the 2-wire bus for receive data until it is required to send data. In RS-485 send mode it stops receiving, enables its transmitters when there is data to be sent, transmits the data and returns to receive mode. This eliminates the possibility of collisions with other devices which share the RS-485 bus and avoids receiving stale echoed data.