These days, the term "network" basically means plugging into a wireless access point to access the Internet and perhaps files on other computers. To consumers who are accustomed to USB plug-and-play and perhaps an Ethernet cable for a reliable high-speed connection to a router, the terms RS232, RS422, or RS485 mean little. However, leave the world of consumer electronics, and RS-485 remains an important network standard for technical and industrial applications such as device control, data acquisition systems, remote peripheral management and so on. So let's revisit RS-485, what it is and what it can do.
First, unlike the RS232 standard, RS485 does not specify a data protocol or pin-outs. Instead, it describes the electrical characteristics of drivers and receivers in balanced digital multipoint systems.
Like RS-422 connections, RS-485 systems use "balanced" signals with negative and positive voltages (ranging from -6 to +6 Volts) on twisted pair wires. This makes them much more immune to interference than "unbalanced" RS232 signals and also allows for speeds is up to10 mbps over short distances an a maximum connection distance of 4,000 feet at lower speed.
However, while RS-422 is limited to a single driver that communicates with up to ten nodes or receivers, RS-485 is a true multipoint system with a maximum of 32 drivers and 32 receivers. This means that up to 32 drivers can communicate over the same pair of wires. What if 32 isn't enough for an application? Then a repeater allows adding another 32. If that is still not enough, additional repeaters can be added, up to the addressing maximum of the system, which is usually 256 devices.
As far as connectors go, the RS-485 standard doesn't specify any particular type. As a result, you can use standard DB9 or DB25 serial connectors, but also RJ11 or RJ45 jacks, a terminal block, or a variety of DIN-style connectors. Connection to computers with standard RS232 serial ports requires either a RS485 interface card or a RS232 to RS485 converter. They come in commercial and industrial-grade varieties. If voltage spikes, ground loops or transient surges are an issue, look for an opto-isolated converter to protect the computer.
Note that RS485 networks may be configured either as 2-wire or 4-wire systems. On a 2-wire system drivers and receivers are connected via just one twisted pair (plus a dedicated signal ground connection). Nodes can communicate with one another, but the communication is only half-duplex. On a 4-wire system (plus ground), a driver transmitter is connected to all the node receivers on one pair, and the node transmitters are connected to the driver receiver on a second pair. This allows for full-duplex communication, but also means nodes cannot directly communicate with one another. Each system has its uses and advantages.
Since RS485 or, as they are properly called, EIA-485 signals are not limited to any particular standard, they can be used in a very wide variety of control and automation systems. Add the minimal wiring requirements and great resistance to electromagnetic interference and you have a standard that will likely remain important for years to come.
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Chris Robertson is a published author of Majon International. All Rights Reserved. Copyright 2011 (Fri Jan 01 2010) Majon International. Majon International is one of the worlds MOST popular internet marketing and internet advertising companies on the web. Visit their main business resource internet marketing web site at: http://www.majon.com
