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This protocol may include things like the start bit, number of data bits, parity and stop bit(s). 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. RS-485 specifies a number of other things, but they are all electrical values. There are other electrical characteristics in the standard, but you should be getting the idea that RS-232 specifies the connector, signals, and electrical characteristics. Again many thought that the UART protocol used in the serial port were part of the RS-485 standard, but the RS-485 standard contains even less of the OSI model layer one than RS-232 does. A serial port could be added with a card that plugged into one of the ISA expansion slots. Converters that changed the RS-232 electrical levels to RS-485 levels were developed which allow the serial port to transfer data over much longer distances than the RS-232 levels allowed. 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.

PURPOSE AND SCOPE OF COVERAGE - This standard is applicable to the interconnection of data terminal equipment (DTE) and data circuit-terminating equipment (DCE) employing serial binary data interchange. PURPOSE AND SCOPE OF COVERAGE - This standard is applicable to the interconnection of data terminal equipment (DTE) and data circuit-terminal equipment (DCE) employing serial binary data interchange. Clearly not all of these circuits or signals are included in IBM’s implementation (very few implementations use secondary data lines or many of the other circuits). For example, one of the circuits is "AB". A fourteenth interface type, lettered Z, is reserved for applications not covered by types A through M. All of the circuits on interface type Z (except Signal Common) are optional and are to be specified by the supplier. In practice this type of isolation is rare. The signal will eventually stabilize (until the next bit transition) so the slower the bit rate the more time the signal will have to stabilize before it is sampled (usually by a UART). I.e. only one device can talk to the other devices on the RS-485 network at a time. In a RS-485 cable, data typically flows in both directions along any particular wire, sometimes from the "A" of the first device to the "A" of the second device, and at a later time from the "A" of the second device to the "A" of the first device.

This particular implementation of two RS-485 networks is often called 4-wire or full-duplex RS-485. The signals appear differentially on two wires of each pair. This standard is for a network of a single balanced pair of wires. EIA/TIA-574 is the standard for 9-pin connectors, and EIA/TIA-561 is a standard for 8-pin connectors that are commonly used with serial ports. RS-232 is very clear that is intended for a maximum bit rate of 20kbit/s. The forward recommends EIA/TIA-530, EIA/TIA-561, and EIA/TIA-574 for operation at higher bit rates. But the baud rate and bit framing must also match. A final note on transmission lines is that the termination resistor should match the characteristic impedance of the cable. POTS telephone cable's impedance is less controlled than CAT cable and can vary from 600 Ω at lower audio frequencies to less than 100 Ω at higher frequencies. 120 Ω cable should provide the best performance, but the 100 Ω CAT-X cable may you have laying around may also work. There is a lot of folklore stating that RS-232 has a 50 or 100 foot limit. This document is primarily directed at discussion of RS-485 but also includes information about RS-232 and RS-422. RS-422 specifies the electrical levels of one transmitter to one of more receivers on a single pair of wires.

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. It is commonly considered that a 12k resistance is 1 unit load, but unit load is more complex than than a single resistance. 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. Another example of misunderstanding the RS-232 standard is the 9-pin connector. RS-485 does not include any connector. When IBM changed to the new connector they also changed the electrical signal levels. Often, use of shielded CAT5/6 cable can give an adequate signal ground, although this is not recommended. RS-422 is full duplex so it can use the X-On / X-Off protocol, if needed. Stub length, termination, and biasing resistors can have a significant impact on the performance of the network. The cable's length, impedance, terminations, stub lengths, and data rate will all have an impact on signal quality.

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