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The cable's length, impedance, terminations, stub lengths, and data rate will all have an impact on signal quality. Any number of characters can be sent, and the transmitter will automatically re-trigger with each new character (or in many cases a "bit-oriented" timing scheme is used in conjunction with network biasing for fully automatic operation, including any Baud rate and/or any communications specification, eg. When choosing a transmission line for RS-485, it is necessary to examine the required distance of the cable and the data rate of the system. One of the factors limiting total cable length is the capacitive load. Typically one device (node) is addressed by the host computer and a response is received from that device. The line can be driven by one device at a time, and other drivers are usually put in high-impedance mode when they are not in use. In short, the RS232 port was designed to communicate with local devices, and will support one driver and one receiver. 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. It shows a diagram of the driver and receiver with two wires connecting them, and a third point "C" that is called a common.

From a software point of view, full-duplex RS-485 looks very similar to RS-232. If both are implemented then the connection is full-duplex. When communicating at high data rates, or over long distances in real world environments, single-ended methods are often inadequate. The extremely high volume of Category 5 cable used makes it widely available and very inexpensive, often less than half the price of specialty RS422/485 cabling. Another choice is the same cable commonly used in the twisted pair Ethernet cabling. This avoids conflict with both Ethernet (Pins 1-3, 6) and analog phone (Pins 4-5). Grounding can be problematic depending on the application. RS485 communication has a limited bandwidth compared to other communication protocols such as Ethernet. It is essentially a form of serial communication. In its simplest form, RS485 standard 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. RS-485 and RS-422 can interoperate with certain restrictions. While the RS-485 specification does not specify cabling, these recommendations should be used for RS-485 systems as well. Single master systems (many other communications schemes are available) offer a straight forward and simple means of avoiding "data collisions" in a typical 2-wire, half-duplex, multi-drop system.

To show you how simple this is, I wired up a K-30 CO2 sensor to communicate with our free GasLab software via RS485 in a few minutes from stuff on my desk, zero software, in between some other projects. The RS232 interface of RS232 TO RS485 (B) connects to the PC with a serial extension cable or USB TO RS232 cable. The RS485 interface is, like the RS422 interface, intended for serial high-speed data transmission over long distances. The Laureate interface board P/N L485 with dual RJ11 jacks allows multiple Laureate meters, counters, timers and remote displays to be daisy-chained to the same RS485 line and to be addressed in command mode. A true multi-point network consists of multiple drivers and receivers connected on a single bus, where any node can transmit or receive data. A termination resistor can increase the signal's quality (reduce ringing), but it will also reduce the signal's amplitude. When the cable is cut to any length and terminated with this Characteristic Impedance, measurements of the cable will be identical to values obtained from the infinite length cable. 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.

And, even if the diff-mode impedance is given, the common-mode impedance is never mentioned (on high-performance STP cables, you can hazard a guess - see below). Through use of a symmetrical transmission procedure in combination with low-capacitance and low-attenuation, twisted-pair cables, extremely reliable connections over long distances at high transmission rates can be achieved over a distance of up to 500 m. With the introduction of "automatic" repeaters and high-impedance drivers / receivers this "limitation" can be extended to hundreds (or even thousands) of nodes on a network. When using the default RS485 receivers with an input resistance of 12 kΩ it is possible to connect 32 devices to the network. Suitable for Multi-Vendor Environments: The standardization of RS-485 ensures interoperability between devices from different vendors. Among various topological structures, the Daisy Chain topology is considered the optimal choice for RS-485 due to its minimal impact on signal integrity. Plenum cable is generally more expensive due to the sheathing material used.