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The modem may be limited to half-duplex communications, and therefore the link between the two computers is half-duplex. For example; Two computers communicating with each other over an older modem. 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. If both the driver and receiver of the devices on the network have no inversion, RS485 standard (or If both the driver and receiver of the devices are inverted) then the A and B lines of the devices should be connected together. Bias resistors would be required at each end of the network with these values. But this is only true if there is no ringing or noise on the line that switches the receiver back to a 0 at the end of the stop bit.

By converting from single-ended RS232 to differential RS422 and then, converting back from RS422 to RS232 at the other end of the line, distance and noise immunity can be greatly improved. This allows for the creation of multipoint networks where several devices can communicate over the same twisted-pair cable. The standard 120-Ω value for termination resistors corresponds to the differential-mode characteristic impedance of the twisted-pair bus wires. The best cables in terms of impedance and RF attenuation are not available in a true fire-resistant variety (and vice versa). Although this voltage may be insufficient to switch the receiver state directly, an internal bias current within the attenuation network creates a voltage difference at the receiver comparator input. Failure to terminate the transmission line properly, or terminating with a value different from the cable’s impedance, introduces mismatches that cause reflections at the network ends. While the standard only supports low data rates and short line length (50ft.) it is still widely used and, very useful in many applications. This type of port was needed to transfer data from/to a printer, modem, or in rare cases a mouse.

This impedance is determined by factors such as wire gauge, insulation type and thickness, and the number of twists per unit length, which collectively influence the impedance detected by high-speed data signals. This means that at certain times, all RS-485 transceivers on the bus may present a high impedance state, resulting in no actively driven logical state. Another approach is to use additional external resistors to establish an external bias on the idle bus. To add a little extra noise margin, use 680 Ω resistors and you have one of the more common biasing networks used on a RS-485 network. The drivers and receivers, called as transceivers link to a primary cable trunk through short network stubs in this arrangement. But, since existing RS-232 compatible peripherals still functioned with it, it was still regularly called an "RS-232 port". 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 facilitates the robust transmission of moderate data rates across long distances in multipoint communication applications.

Balanced interface: RS-485 uses a balanced transmission line, meaning it has two signal wires carrying equal and opposite signals to reduce noise and improve signal integrity. The RS-232 standard does not include a line length limit, but practical limits of the electrical signal levels prevent long lengths. The RS-485 standard calls for a differential signaling scheme that will operate over a much longer line length than RS-232 can. There is no way to say that a termination resistor will always increase the line length the network. There are two functions of the resistor-divider network present on the A and B inputs of the comparator. These other standards provide the protocol and other requirements that are needed for two devices to transfer information. A common and incorrect assumption is that the RS-232 standard defines the bit protocol of the data being transferred asynchronously through 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.