RS-422's annex has a chart of empirical data using 24 AWG copper UTP telephone cable. 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. There is no graph of cable length vs. The RS-485 standard calls for a differential signaling scheme that will operate over a much longer line length than RS-232 can. When configuring an IBM PC’s serial port, the parity, number of data bits, and number of stop bits must be set; therefore, many think this must be part of the RS-232 standard. Many think the RS-232 standard completely defines the IBM PC serial port, but this is incorrect. The serial port used a D-sub, B-shell, 25-pin connector (commonly called a DB-25) and RS-232 electrical signal levels, which made it "RS-232 compatible" or compatible with the RS-232 standard. Some manufacturers have avoided this confusion by changing the name of the output pins labels to X and Y. There are two things to be noted about this so called "pin mislabeling".

This defines all 25 pins of the connector. The 9-pin "RS-232" connector used for the IBM PC-AT serial port is not specified in RS-232. When IBM introduced the PC/AT they also changed the physical format of the serial port connector to a D-sub, E-shell, 9-pin connector (a DE-9 commonly miscalled a DB-9). This serial port was no longer a "RS-232 compatible" serial port since the connector and signal levels no longer matched the RS-232 standard. 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". The modem may be limited to half-duplex communications, and therefore the link between the two computers is half-duplex. Another implementation of two RS-485 networks is BitBus, which uses one of the RS-485 networks for bidirectional SDLC data communications (very different from asynchronous start-stop communications with a UART), and a second, optional, RS-485 network for RTS (direction control if a repeater is used). This means that in a master/slave configuration, a master can talk to multiple slaves, but only one slave can talk back to the master. When the input of an RS-485 driver goes high, one of the outputs will go high in reference to circuit common and the other will go low.

A third point on the driver shown as "C" is labeled as a common, but is not shown connecting to any wire. Since the laptop has no connection to earth ground, there will normally be little current through this third wire. The annex says this connection can be wired directly or made through a 100 Ω resistor. Clearly a physical connection to the signal is necessary. Slew rate limiting reduces the maximum bit rate, but will improve signal quality on networks with poor characteristics. If you are trying to get two ancient pieces of equipment (both of which use true RS-232 drivers and receivers) to work with a long cable, the capacitance limit in RS-232 may apply, but RS-232 does not take bit rate into account. The bias resistance needs to be taken into account when selecting the termination resistor. Low data rates are primarily limited by the DC resistance of the cable (the effects of the DC resistance of the cable are made worse if a termination resistor is used) and high data rates are limited by the AC effects of the cable on signal quality. The driver's impedance (when active) is not specified, but the driver needs to be capable of driving 32 unit loads and a termination resistance as low as 60 Ω.

For example: An RS-422 network has an always active driver and receiver. 25 wires. Not all of the EIA-530 signals use RS-422 levels. This is why RS-485 states "The use of a cable termination is normally required".. Its annex also states "It is desirable for proper operation over the interchange cable that the DC wire resistance not exceed 25 Ω per conductor." Any cable that meets the capacitance requirement can be used. 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). Another example of misunderstanding the RS-232 standard is the 9-pin connector. Again, this protocol is not part of the RS-232 standard, even though many engineers think of this protocol as "RS-232". Even though the logic function of the generator and receiver are not defined by RS-485, it makes sense to many engineers to have a binary 1 appear on the RS-485 wires when a binary 1 is being transmitted. RS-422 includes the voltage levels of the two wires when a binary 0 or 1 is on the RS-422 lines, but specifically excludes the logic function of the generator or receiver.

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