In the picture above, the general network topology of RS485 is shown. In the picture above, noise is generated by magnetic fields from the environment. The picture shows the magnetic field lines and the noise current in the RS485 data lines that is the result of that magnetic field. When the cable is twisted, we see that in some parts of the signal lines the direction of the noise current is the oposite from the current in other parts of the cable. Termination: Proper network termination is vital to avoid signal reflections, which can lead to communication errors.Biasing: Biasing is beneficial for maintaining signal quality over long distances. For higher speeds and longer lines, the termination resistances are necessary on both ends of the line to eliminate reflections. To avoid reflections on longer cables it is necessary to use appropriate termination resitors. A: In RS485 wiring, it is essential to use twisted-pair cables with proper shielding to reduce interference from external sources.

Use 100 Ω resistors on both ends. If high noise immunity is needed, often a combination of twisting and shielding is used as for example in STP, shielded twisted pair and FTP, foiled twisted pair networking cables. Because of this, the resulting noise current is many factors lower than with an ordinary straight cable. In the straight cable, all noise current is flowing in the same direction, practically generating a looping current just like in an ordinary transformer. We also see that the maximum allowed voltage levels for all interfaces are in the same range, but that the signal level is lower for the faster interfaces. Shielding-which is a common method to prevent noise in RS232 lines-tries to keep hostile magnetic fields away from the signal lines. Rs485 cable is essentially a signal cable with a twisted pair structure, additionally shielded with 2 layers of noise protection used in industrial communication. In this setup both the TX and RX share a single pair of wires, therefore it is limited to half-duplex data transmission as the data cannot be transmitted simultaneously to and fro each device. Another advantage of RS485 is its ability to provide half-duplex and full-duplex functionality.

The advantage of the RS485 protocol is that this it can transfer data over longer distances, up to 4000 feet, and have up to 32 nodes per single network, and it can be used for multi-point networks where multiple serial devices are setup in a bus network. Each device has a communication port with two terminals, which are indicated for the sake of convenience as A and B. In these two terminals the communication cable is connected so that all the devices that take part in the communication are connected in parallel. Access control systems: Rs485 cables are mainly used to connect and transmit data between access control devices such as security gates, RFID cards, or time attendance machines and the control center. Within such a system, RS-485 serves a pivotal role as a bridge, establishing communication of flow information between the central control system and each valve box. Disruptions or interference in the differential signal can corrupt data without repetition or reception; a "fire-and-forget" system. One of the main problems with RS232 is the lack of immunity for noise on the signal lines. Noise is easily picked up and limits both the maximum distance and communication speed.

First of all we see that the speed of the differential interfaces RS422 and RS485 is far superior to the single ended versions RS232 and RS423. We also see that there is a maximum slew rate defined for both RS232 and RS423. RS232, RS422, RS423 and RS485 are serial communication methods for computers and devices. When using the default RS485 receivers with an input resistance of 12 kΩ it is possible to connect 32 devices to the network. This is great for Supervisory Control and Data Acquisition (SCADA) systems where there are many devices and it also comes at a very low cost to implement. The built-in protocol definition for parsing BACnet, a data communications protocol for Building Automation and Control Networks. In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the DMX512 protocol. That is why RS485 is currently a widely used communication interface in data acquisition and control applications where multiple nodes communicate with each other. It is the reason why RS485 is so popular with computers, PLCs, micro controllers and intelligent sensors in scientific and technical applications.

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