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The RS-232 standard does not include a line length limit, but practical limits of the electrical signal levels prevent long lengths. The RS-232 standard resides in layer one of the OSI model, or the physical layer. RS-485 is the physical layer for many higher-level protocols, including Profibus and other fieldbus systems, SCSI-2, SCSI-3, and BitBus. With Modbus, BACnet and Profibus, A/B labeling refers A as the negative green wire and B as the positive red wire, in the definition of the D-sub connector and M12 circular connector, as can be seen in Profibus guides. Yet the signaling waveforms in Figure 1 of the RS-485 standard clearly show that when a binary 0 (ON) is on the wires, the voltage on wire "A" is positive with respect to "B" and conversely, when a binary 1 (OFF) is on the wires, the voltage on wire "A" is negative with respect to "B". This standard specifies the electrical characteristics of a single transmitter and up to ten receivers on a single pair of wires.

Typical UART communications with an RS-422 device, uses both a transmitter and receiver on a network of two balanced pairs of wires for a total of four wires. The physical layer may include a connector, wires, and electrical levels, but does not include the bit and framing protocol of the UART. The original "RS-232 compatible" serial port used a UART to drive the RS-232 electrical signal levels and many came to believe that the UART protocol was a part of the RS-232 standard. Many references and examples are made to "asynchronous start-stop communication with a UART" or "UART" communications. 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). 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.

Other reasons that some RS-232 ports were half-duplex: some very old UARTs may be half-duplex limiting the system, and some very old computers drove the RS-232 drivers directly from the processor without a UART. These old and slow microprocessors did not always have the horsepower to monitor the timing of both the incoming and outgoing bits, limiting the system to half-duplex. This computer had limited functionality with a keyboard and monitor as the primary peripherals. Half-Duplex RS-232 is usually limited by an additional communications device between the two farthest ends that are communicating. The modem may be limited to half-duplex communications, and therefore the link between the two computers is half-duplex. The signals appear differentially on these two wires. 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. Two circuits are for transmitting and receiving the serial data, circuit "BA" (TX data on pin 2) and "BB" (RX data on pin 3). A number of circuits are for handshaking and other functions. The RS-232 standard includes secondary serial data circuits (secondary TX data, and secondary RX data), three clocks (timing elements) for synchronous serial data transfer, Signal Quality Detector, and more.

3V, and when a binary 1 is being transmitted, the voltage must be more negative than -3V. When a binary 1 is transmitted, the signal is spacing, and the voltage on the wire must be more negative than -3V. Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption. This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. In addition to the 21 circuits, the shield may be connected to pin 1, pins 9 and 10 are "Reserved for Testing", and pin 11 is "unassigned". While it may seem quite rational to assume that an RS-485 driver IC pin that is labeled A matches the RS-485 standard's wire "A", this is actually an assumption and is in fact not the case. The driver must produce a differential voltage between 2 and 10V into a loaded/terminated cable. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). Another thing is that different cables have different characteristic impedances at different frequencies. This length of cable would be expected to have significant DC losses if termination resistors were used.

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