RS-485 is a serial data interface standard drawn up and published by the Electronic Industries Association. Issued in 1983 as an extension of RS-422, it added multipoint and bidirectional communication - allowing several transmitters on the same bus - together with greater transmitter drive capability, contention protection and a wider common-mode range. It was later designated the TIA/EIA-485-A standard.
You must use the internationally accepted shielded twisted-pair cable. We recommend type RVSP2*0.5 (2-core shielded twisted pair, each core made up of 16 strands of 0.2 mm wire). Shielded twisted pair helps reduce and eliminate both the distributed capacitance between the two RS485 conductors and the common-mode interference picked up around the cable. Most installers are in the habit of using Cat 5 or Cat 5e network cable for RS485. This is a mistake, because: (1) Network cable is only 0.2 mm², which is too thin - it shortens the achievable transmission distance and reduces the number of devices that can be connected. (2) Network cable uses solid copper conductors, which break more easily than stranded wire.
An RS485 transceiver works correctly only while the common-mode voltage stays within the specified -7 V to +12 V range. Outside that range communication suffers, and in severe cases the interface is destroyed. Common-mode interference raises that common-mode voltage. One effective way to remove it is to use the shield of the RS485 cable as a ground conductor, tying together the equipment grounds of the machines, computers and other devices on the network and connecting them reliably to earth at a single point.
Keep communication cable as far as possible from high-voltage wiring. Do not run it parallel to power cables, and never bundle the two together.
A star topology produces signal reflections that disrupt RS485 communication. Keep the spur from the bus to each terminal device as short as possible - generally no more than 5 m. An unterminated spur will reflect the signal and cause strong interference, so it should be removed.
Within a single network system, use the same type of cable throughout and keep the number of joints to a minimum. Make sure every joint is well soldered and tightly wrapped so it cannot work loose or oxidise. The aim is a single, continuous signal path for the bus.
An RS485 cable consists of two twisted conductors and carries its signal as the voltage difference between them, which is why it is called differential voltage transmission.
Differential-mode interference travels between the two signal conductors and is symmetrical. It is removed by adding a bias resistor to the circuit and using twisted-pair cable.
Common-mode interference travels between the signal conductors and ground and is asymmetrical. Ways to remove it include:
(1) Use shielded twisted pair with an effective ground
(2) In areas with strong electric fields, also consider galvanised conduit for shielding
(3) Route cable away from high-voltage lines, and never bundle mains power and signal cable together
(4) Do not share a power supply with electric locks
(5) Use a linear regulated supply or a high-quality switching supply (ripple below 50 mV)
Termination resistors are usually unnecessary. They are only needed where the RS485 run exceeds 100 m, in which case a termination resistor should be fitted at each end of the bus.
One of the RS485 specifications is a length of 1.2 km with 32 nodes. Beyond that limit you must use an RS485 repeater or RS485 hub to extend the distance or the node count.
An RS485 repeater or hub lets you divide a large RS485 network into several segments, acting as a "bridge" between them. Each segment still follows the RS485 specification above - 1.2 km and 32 nodes.