2 wire RS485 Half Duplex
Simple point to point setup for bidirectional communication in half duplex mode. Half Duplex here means the two wires can either transmit or receive. They cannot transmit and receive at same time. The board automatically detects transmit at either side and enables transmit mode. In normal condition it is in receive mode.
RS485 is a half duplex configuration. RS485 using only 2 wire to communicate in both direction. With only two wire, it means that when one side is transmitting, the other side of the communication line will be receiving. Both side cannot be transmitting at the same time.
The RS485 converter allows to interface any device using an serial link to an RS485 link. The RS485 link was conceived for long haul data acquisition and control applications. Specifications allowed to network up to 32 stations on the same lines, at speeds up to 10 Mbits/s to distances of 4,000 feet (1200 mt). The link is balanced so that any electrical noise getting into one of the lines also gets into the other line thus allowing the receiver to cancel both noise signals.
RS485 links are much used in industrial process control where reliability is important. Also,the ability to communicate over a long distance at a high speed is important when it comes to industrial plants where the stations might be spread over a large area. It is very common to have a PC in charge of controlling a given process. PC's in general have an RS232 serial port (COM port) and the RS232 level signal coming out the computer serial port is converted into an RS485 signal. This way, two computers or devices using a standard serial port can communicate with each other at a maximum of 115200 Bauds over a distance of 4000 feet, something impossible to achieve using an RS232 link.
IMPORTANT: For optimal operation, a data network requires to be terminated on both ends of the link. This means that intermediate stations need NOT be terminated with resistors.
Unlike RS232 signal, RS485 implements differential voltage as the physical signal to communication. The cable for RS485 works as a pair to transmit and another pair to receive. This is important, because it means that the cable need to work as a pair in order to transmit the signal properly. Proper type of cable is required for optimal range.
For RS485 network many people use CAT5 cable used in Ethernet Networking due to easily availability and low cost. Each CAT5 cable contains 3 pairs of twisted wires from that only one pair will be used if setting up a RS485 else 2 pairs for RS422.
The picture on left is a twisted pair wire. It is recommended to use twisted pair cable to transmit RS485 signal. The reason being twisted wire can help minimize the gap between the 2 wire, therefore minimizing interference and signal distortion. This can be explain through the electromagnetic theory. The gap or loop between the two wire actually acts as the antenna intercepting nearby signal, as well as coupling it's own signal to wire next to it. These results in distortion in the signal, and ultimately reduces the data rate you can transmit over the wire pair. The twisted cable also maintain a distance, make it difficult for another wire to slip in between the wire pair. For further immunity against interference, the twisted pair is wrapped with aluminum foil acting as a shield. By twisting the cable, the gap can be reduce, therefore minimizing the interference.
Both types of wire can work, Shielded and Non shielded, But Shielded will give lesser interference.
