Worse is the fact that annex A states that this graph is a conservative guide based on empirical data of 24AWG telephone cable (POTS). The graph in RS-422's annex A is not an absolute limit, but a guide to what should always work with cheap telephone wire. The standard defines the common-mode voltage as being referenced to ground, it defines a term "ground potential difference" as the difference in the signal ground between the driver and receiver, but it does not say that this is earth ground or just a a third wire common. Another figure in the annex (again, the annexes are not considered to be part of the standards) discusses connecting the "Green Wire Ground of Power System" or "Protective Ground or Frame Ground" to "Circuit Common or Circuit Ground" and "SC Signal Common". 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. 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.

As a rule of thumb, the speed in bit/s multiplied by the length in metres should not exceed 108. Thus a 50-meter cable should not signal faster than 2 Mbit/s. The cable's length, impedance, terminations, stub lengths, and data rate will all have an impact on signal quality. Since a receiver may have a loading of less than one, the actual number of receivers that can be connected depend on the unit load rating of the receivers, as well as the wire, bit rate, stub lengths, biasing and termination of the network. 120 Ω cable should provide the best performance, but the 100 Ω CAT-X cable may you have laying around may also work. Pull a cable between the two devices and see if they can talk to each other. 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-422's annex has a chart of empirical data using 24 AWG copper UTP telephone cable. The annex says this connection can be wired directly or made through a 100 Ω resistor.

200mV as undefined, but the IC manufacturer can put the threshold for a 0 and a 1 anywhere they want. 200mV. This can cause a problem if the RS-485 network is using a UART to transmit data. When an RS-485 driver gets its data directly from a UART (with no added inversion), you would expect the "A" and "B" wires to match the voltages in the RS-485 standard for the voltages on the wires, but they will not (unless the driver inverts its input). 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. In reality either or both the driver or receiver are going to meet the more modern RS-574 requirements and none of the RS-232 limits will apply.

Another RS-232 to RS485 scheme is to monitor the data stream going into the RS-485 transmitter and trigger a one-shot timer when an edge occurs. If reducing the bit rate is not an option, then you could try an RS-232 to RS-485 converter at both ends. Then the annex states that consideration should be given to various things including grounding arrangements. 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). Section 1.3, Data Signalling Rates, states; this standard is applicable for use up to a nominal limit of 20,000 bits per second. The third point C is discussed in the section on grounding. A third point on the driver shown as "C" is labeled as a common, but is not shown connecting to any wire. This helps to limit the current through the "third" wire. 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. The forward to RS-485 references TSB-89 which has topics including data signaling rate vs.

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