A good RS485 Standard Is...

In its simplest form, a pair of converters from RS232 to RS422 (and back again) can be used to form an "RS232 extension cord." Data rates of up to 100K bits / second and distances up to 4000 Ft. The Electronics Industry Association (EIA) has produced standards for RS485, RS422, RS232, and RS423 that deal with data communications. RS423 is another single ended specification with enhanced operation over RS232; however, RS485 standard it has not been widely used in the industry. Differential signals and twisting allows RS485 to communicate over much longer communication distances than achievable with RS232. Digital communications networks implementing the standard can be used effectively over long distances and in electrically noisy environments. By using repeaters very large RS-485 networks can be formed. The diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line during transmission of one byte (0xD3, least significant bit first) of data using an asynchronous start-stop method.

Once a character is sent the hardware reverts back into a receive mode in about 1-2 microseconds (at least with R.E. To solve the "data collision" problem often present in multi-drop networks hardware units (converters, repeaters, micro-processor controls) can be constructed to remain in a receive mode until they are ready to transmit data. The RS232 signals are represented by voltage levels with respect to a system common (power / logic ground). Therefore RS485 can overcome the practical communication speed limit of RS232. Interesting is, that RS232 is the only interface capable of full duplex communication. Shielding-which is a common method to prevent noise in RS232 lines-tries to keep hostile magnetic fields away from the signal lines. This works well and prevents the existence of ground loops, a common source of communication problems. RS485 is used as the electrical layer for many well known interface standards, including Profibus and Modbus. This robustness is the main reason why RS-485 is well suited for long-distance networking in noisy environment. It is the reason why RS485 is so popular with computers, PLCs, micro controllers and intelligent sensors in scientific and technical applications. With such an implementation of a RS485 network it is necessary that there is error detection implemented in the higher level protocol to detect the data corruption and resend the information at a later time.

All other nodes receive these data. That is why RS485 is currently a widely used communication interface in data acquisition and control applications where multiple nodes communicate with each other. Currently available high-resistance RS485 inputs allow this number to be expanded to 256. RS485 repeaters are also available which make it possible to increase the number of nodes to several thousands, spanning multiple kilometers. It is NOT necessary to introduce long delays in a network to avoid "data collisions." Because delays are NOT required, networks can be constructed, that will utilize the data communications bandwidth with up to 100% through put. RS485 drivers automatically return to their high impedance tri-state within a few microseconds after the data has been sent. The RS485 network must be designed as one line with multiple drops, not as a star. RS422. RS422 is also specified for multi-drop (party-line) applications where only one driver is connected to, and transmits on, a "bus" of up to 10 receivers.

RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. Although total cable length maybe shorter in a star configuration, adequate termination is not possible anymore and signal quality may degrade significantly. Because a mark (logic 1) condition is traditionally represented (e.g. in RS-232) with a negative voltage and space (logic 0) represented with a positive one, A may be considered the non-inverting signal and B as inverting. Circuits may be terminated on screw terminals, D-subminiature connectors, or other types of connectors. Two Tibbo Project System (TPS), Gen. 2-compatible Tibbo BASIC applications for the setup and testing of Bus Probes are available: BP-Tester-UI and BP-Tester-Web. The probes communicate using the standard Modbus RTU protocol. This allows RS-485 to implement linear bus topologies using only two wires. Care must be taken when using A/B naming. Sig- lines are twisted. RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol. The request and acknowledge data needed in most protocols does not consume bandwidth on the primary data channel of RS232. Also, RS485 drivers are able to withstand "data collisions" (bus contention) problems and bus fault conditions.