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The primary difference between RS-422 and RS-485 is that RS-485 requires two wires in a single pair which all devices transmitters and receivers are connected to, and RS-422 typically uses two or more pair in which only one transmitter and up to ten receivers are connected to a single pair. It is interesting to measure the humidity of the ground, but we have also thought about other things; and then we have TXtemp to measure the temperature (one of the uses of TXtemp is in association with SuperClock), and for practically any type of measurement we have TXdata (RS232, RS485 ModBus) - which can be connected to thousands of existing third-party professional RS232 or ModBus meters (and there is something for all needs: temperature and humidity meters, anemometers, voltage meters, energy, light meters, water meters, presence sensors, brightness sensors, soil humidity sensors, etc. ; practically all professional meters are available in ModBus or RS232 standard; and not just meters). The RS485 protocol employs a differential voltage system that allows it to operate effectively in environments with higher levels of electrical noise. Environments that exhibit high levels of electrical noise and variable ground potential impact RS232’s ability to transfer data efficiently and can lead to data loss or corruption.

A pair of wires has capacitive coupling and a finite (usually very high) resistance between the wires. The other pair is dedicated to peripheral-to-PC communication. Then the software will work unchanged when connected to a full-duplex RS-485 network, a RS-232 network, and a variety of other communication media. The appendix discusses capacitance and resistance of the cable, then gives an example calculation where the capacitance of the cable per foot (30pF/foot) multiplied by the cable's length, plus the capacitance of the receiver (100pF) gives a maximum cable length of 80 feet. Then it references section 2.1.4 where the maximum capacitance of the receiver side of the interface point, including the cable, shall not exceed 2500pF. Then suggests seeing appendix A for guidance (again, the appendix states that it is not a formal part of the standard). This "third wire" is not officially part of the standard and is discussed further in the grounds and grounding section. Unfortunately RS-485 section 4.5.3 states that the total load between "A" and "B" should be no less than 54 Ω. RS-485 specifies a number of other things, but they are all electrical values. If you are pushing the data rate above 100kbit or the line length above 1000 meters, you may want to use a better grade of wire.

If multiple devices start to talk at the same time, the data may be corrupted. Not all will provide the same performance. The performance of the cable should work for the application. The annex discusses the fact that many applications can handle greater amplitude and timing distortion, and that practical experience has shown that the cable length can be extended to several kilometers at lower data rates. Even POTS telephone wire may work for many applications. Since the majority of RS-485 applications are driven by a UART, the data rate is usually below 100kbit. In this case, POTS telephone wire should work for quite a long line. RS-422's annex has a chart of empirical data using 24 AWG copper UTP telephone cable. In reality, if you are using relatively modern equipment (say 1990 or later) and a low baud rate, line lengths of 1,000 feet (300 m) or more are possible. Low capacitance cable becomes important when pushing line length and/or data rates to the maximum. This should give you an approximate limit for that cable in feet.

There is a lot of folklore stating that RS-232 has a 50 or 100 foot limit. From a hardware point of view, full-duplex RS-485 has some major advantages over RS-232 -- it can communicate over much longer distances at higher speeds. 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. 120 Ω cable should provide the best performance, but the 100 Ω CAT-X cable may you have laying around may also work. Many cable manufacturers can recommend a 120 Ω cable intended to work with RS-422 or RS-485. If you have to know that it will work before you pull the cable, then get the required length of the cable and move the two devices next to each other, connect them and see if they can talk. RS-485 discusses that it is used for devices up to 10Mbit/S, then says they need not be limited to 10Mbps. It also states that "the upper bound is beyond the scope of this Standard".

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