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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. 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. PURPOSE AND SCOPE OF COVERAGE - This standard is applicable to the interconnection of data processing terminal equipment and data communication equipment. 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. It is not clear as to what the actual standard name is for each release. ANSI/TIA/EIA-485, commonly called RS-485, is a standard defining the physical layer of a two wire multipoint communications network.

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. This is the 7 or 8-bit protocol commonly associated with serial ports. 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. The RS-232 standard does not include a line length limit, but practical limits of the electrical signal levels prevent long lengths. This problem can also be fixed by adding bias resistors to force the line to idle condition when a driver is not connected, but this has to be done on a network basis, not a device basis. 200mV. This can cause a problem if the RS-485 network is using a UART to transmit data. PURPOSE AND SCOPE OF COVERAGE - This standard is intended to provide a method of interconnecting data terminal equipment and a data communication channel when each is furnished by different companies. It defines a means of exchanging control signals and binary serialized data signals between data terminal equipment and a data communication channel in cases where interchange point.

This type of port was needed to transfer data from/to a printer, modem, or in rare cases a mouse. But, since existing RS-232 compatible peripherals still functioned with it, it was still regularly called an "RS-232 port". This computer had limited functionality with a keyboard and monitor as the primary peripherals. 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. 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). Many think the RS-232 standard completely defines the IBM PC serial port, but this is incorrect. Like much of the RS-485 standard, fail-safe operation is left wide open and implementation is not part of the standard. 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. The RS-485 standard calls for a differential signaling scheme that will operate over a much longer line length than RS-232 can.

There are two other devices in this figure (receiver and transciever) that are connected to the transmission line the same way. When a "bad message" shows up on the cable, it is more difficult (but not impossible) to figure out which node(s) transmitted that message when you have a shared-medium with a dozen nodes connected to the same single cable, compared to a point-to-point medium with only 2 nodes connected to any particular cable. Please note that a RS-485 network is a single pair of wires, but there is nothing in the RS-485 standard that prevents more than one RS-485 network from being used by each device. The physical layer may include a connector, wires, and electrical levels, but does not include the bit and framing protocol of the UART. The term network is used to define the wires, terminations, bias networks, and all devices connected as a whole system. The term "RS-232 compatible serial port" was often shortened to "RS-232 port".

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