What is RS-485?
RS-485 simply stands for Recommended Standard #485, and it’s been around since 1983. It is used for Serial communication. Devices that use Serial ports usually have a 9 pin D connector or a terminal block connector. Today the standard is maintained by the Telecommunications Industry and its correct name is TIA-485-A (although no one calls it that). It enables high speed networks over a low cost two wire twisted pair cable with data transmission speeds of 10 Mbit/s up to 35m and 100 Kbit/s at 1200m.
6 reasons to choose RS-485 for data transmission
RS-485 is a long established technology but it’s not out-dated. It can be used by PLCs to connect to a sensor network, factory automation and process control equipment, lighting and stage equipment in theatres, to control video surveillance systems or broadcast your favourite TV show. Here we highlight some of the reasons it is just as relevant today.
RS-485 cabling is not expensive to install and has few overheads. When putting in your cable think about the future, what will your requirement be? How awkward is it to install the cable? Do you need to build in redundancy? Consider putting in a 3 or 4 pair cable rather than just a 2.
RS-485 allows a high speed network – 35 Mbit/s up to 10m and 100 Kbit/s at 1,200m cable. As a rule of thumb: the speed in bit/s multiplied by cable length in meters should not exceed 10E8, therefore a 50 metre cable should not signal faster than 2 Mbit/s.
Maximum Network Length for RS-485 is 1,200 metres (4,000ft) – that’s roughly 11 lengths of the pitch at Wembley stadium. The cabling is simple just 2 wires and sheath and it can cover this long distance with no need for repeaters, routers or switches. Ethernet cable can be 100m long but then it will need a repeater, router, hub or switch so you can run the next 100m. RS-485 can go for 1200m on a single cable.
Factories are full of EMI (Electro Magnetic Interference) sources with long cable runs acting like antennas that pick up electrical noise easily. With other cable standards, RS-232 for example, noise can interfere with the signal. The longer the cable the more susceptible it is to interference. RS-485 cables are noise resilient because of their differential twisted pair structure where the two inner wires are literally twisted together over and over again along the length of the cable. The cable experiences the same noise as any other cable but by sending data down both wires of the twisted pair, one + and one -, there is double the signal and the common noise can be cancelled out. Data travels down one wire as +data and gets inverted, -data, down the other. They both experience the same noise (n) so the signals (+data + n) and (-data + n) are both recieved. If one is subtracted from the other we get (+data + n)-(-data + n) which can be solved to give +2data. As a bonus the opposite polarity signals in the differential pair cancel out each other’s radiated emissions and so contribute much less EMI to other devices than do un matched lines.
- Topology – Conceptually Simple Network
- Almost NO Protocol Overhead
RS-485 can accommodate all but the biggest installations. Its simplicity can save time and money in the installation process. RS-485 allows up to 32 bus loads per segment in a straight line or bus topology. This allows you to connect 32 devices using 1 load each on a single cable run. Typical devices are 1, ½ or ¼ unit load. If you use ½ load devices you can connect up to 64 devices, ¼ load allows up to 128 devices per segment.
An alternative to RS-485 would be to use Ethernet with a TCP/IP stack. This would require each device on the network to use a powerful processor with lots of memory, potentially the use of an memory & computationally heavy Operating System. The result would be a power hungry solution, that is more expensive to set up and difficult to debug creating additional overheads. There are almost no overheads with RS-485 and it is practically easy to implement. It’s a simple UART data stream with no complex TCP/IP stack or high performance microprocessor needed.
Good Fit to Device Needs & Capabilities
The capability of the technology is matched to the needs of the connected devices. Most sensors and actuators are very basic, the information
they produce is small, often at a timed interval. You don’t need to overdo the connection. For example reading a temperature accurate to 0.25°C between 0-10k°C every second takes less than 10 bytes of data payload. At 19,200 Baud: 1 bit time ~50µ seconds. The Serial data stream is easily generated by a microprocessor with dedicated UART. Moore’s Law has it that the electronic technology from yesteryear will be cheaper today, making microprocessors with dedicated UARTs very cheap.
Data Pay Loads, 32 devices at 19,200 bits/second gives 1,920/32 = 60 bytes per device per second. Even with overhead for a protocol this is a good fit for sensors and actuators in industrial automation and process control.