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If you are pushing any limits, the termination resistor value has to be determined empirically for the individual network. 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. While it may seem quite rational to assume that an RS-485 driver IC pin that is labeled A matches the RS-485 standard's wire "A", this is actually an assumption and is in fact not the case. It may sound complicated for those who don’t know what is serial communication, so here’s an explanation. A binary 1 may (or may not) be inverted by the driver before it is output. However, if the receiver considers the undriven wires to be a binary 0, when the driver is turned on and set to transmit a start bit, which is also a binary 0, the receiver will not see a transition, and therefore will not see the start bit. In this situation, bandwidth can be used for almost 100%. There are other implementations of RS485 networks where every node can start a data session on its own.

Especially when there are difficulties with the installation of new terminal devices, any troubleshooting should start by checking the bus polarity. Difference measurements (measuring Bus A vs. The receiver simply evaluates the difference between the two lines, so that common mode noise on the transmission line will not result in a falsifying of the actual signal. Only end devices will have a single pair of wires terminated into the communication connector, except for necessary End of Line (EOL) terminators or bias circuits. Line A voltage, implying A, the green wire, is indeed connected to the driver inverting signal, as seen in a whitepaper. The current could cause damage to the wire, or the RS-485 port. An ESD shock to the laptop could cause the two RS-485 wires to increase in potential against the earth ground to several thousand volts. Although designed for long distances, between which potential shifts are generally unavoidable, the standard for RS485 interfaces does not directly prescribe galvanic isolation.

Due to this and it being able to transmit data over long distances, the RS485 is used commonly as a protocol for POS, industrial and telecom. 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. 500 m. The devices are connected to this cable through a stub cable of max. ISO 8482 standardizes in addition the cabling topology with a maximum length of 500 meters. All devices, or units, on the link generate an electrical load to produce or detect this voltage, increasing with the addition of more devices and units. The signal is a voltage change across a load as low as 1.5 volts differential from a driver, with a receiver detecting a differential voltage as minute as 200 millivolts. These rates won’t degrade signal quality over the maximum network cable length of 1200 meters (4000 ft).

Cable length: typically max. Cable length: Defined by the protocol used, typ. RS485 is also commonly used for building automation as the simple wiring configuration and lengthy cable length are ideal for joining remote devices. As in the case of RS422 interfaces, the serial data are transmitted without a ground reference as a voltage differential between two corresponding lines. Disruptions or interference in the differential signal can corrupt data without repetition or reception; a "fire-and-forget" system. Since the signal is attenuated as it travels, the "reflected" signal decreases in amplitude until its level stabilizes. RS-485 typically uses a linear network with bidirectional signal wires: There are typically many devices along a RS-485 shared cable. It’s reasonable to question why one wouldn’t simply transmit signals at the quickest possible speed to hasten network updates.The answer lies in the degradation of communication signals due to transmission losses on the cable when the product of the communication trunk length and the baud rate exceeds the recommended limit. It is generally recommended to use shielded 24awg twisted pair cables for daisy-chaining devices together, RS485 standard avoiding loop or star-shaped wiring configurations.

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