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The term network is used to define the wires, terminations, bias networks, and all devices connected as a whole system. The physical layer may include a connector, wires, and electrical levels, but does not include the bit and framing protocol of the UART. The new IBM PC/AT serial port no longer used the RS-232 25-pin connector or signal levels, but IBM took care to ensure that the new serial port’s electrical levels would still work with existing serial port peripherals, requiring only a 9-pin to 25-pin adapter. 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. But, since existing RS-232 compatible peripherals still functioned with it, it was still regularly called an "RS-232 port". While this is incorrect, many to this day still believe that the RS-232 standard includes things such as baud rate and the bit stream protocol used in the IBM serial port. The termination also includes pull up and pull down resistors to establish fail-safe bias for each data wire for the case when the lines are not being driven by any device. This document is primarily directed at discussion of RS-485 but also includes information about RS-232 and RS-422.

These annexes are discussed in this document but it should be clear that they are not a part of the standard even though they are included in the standard. This "third wire" is not officially part of the standard and is discussed further in the grounds and grounding section. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. Care must be taken that an SC connection, especially over long cable runs, does not result in an attempt to connect disparate grounds together - it is wise to add some current limiting to the SC connection. Failure to stay within this range will result in, at best, signal corruption, and, at worst, damage to connected devices. 12 V, i.e. ±7 V on top of the 0-5 V signal range. The connections range from screw terminals that bare wire is inserted into, to connectors such as a DE-9. 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).

Another example of misunderstanding the RS-232 standard is the 9-pin connector. The 9-pin connector and electrical levels were eventually documented in EIA/TIA-574. When IBM changed to the new connector they also changed the electrical signal levels. It can’t be denied that the UART IBM selected for its implementation of the serial port is considered by many as the definition of RS-232, but these parts are not in the RS-232 standard. In the early 80’s IBM introduced the IBM PC. Line A voltage, implying A, the green wire, is indeed connected to the driver inverting signal, as seen in a whitepaper. The logic function of the driver and receiver are not defined, only the binary state of the differential voltages on the wires. RS-422 allows for multiple receivers on each pair, but only one driver. RS-485 standard conformant drivers provide a differential output of a minimum 1.5 V across a 54-Ω load, whereas standard conformant receivers detect a differential input down to 200 mV. It would require an isolated power supply and optically isolated drivers.

Lower voltage drivers are now available to allow operation from battery powered devices and the open circuit voltages may be lower than ±12V. Digital logic usually uses the same convention (but not the same voltage) as the lamp example. RS-232 uses point-to-point unidirectional signal wires: There are only two devices connected to a RS-232 cable. RS-232 uses point-to-point unidirectional signal wires: There are only two devices connected to a RS-232 cable. 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". It is not clear as to what the actual standard name is for each release. The RS-485 standard is obsolete and has been superseded by TIA-485, but many if not most engineers and applications guides continue to use the RS designation even though it has officially changed. The EIA has officially disbanded and the standard is now maintained by the TIA as TIA-485, but engineers and applications guides continue to use the RS-485 designation. 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.

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