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Many engineers have worked on an RS-485 application and think that since their application worked, they understand exactly what the standard is. The RS-485 standard is obsolete and has been superseded by TIA-485, but many if not most engineers and applications guides continue to use the RS designation even though it has officially changed. Just because a protocol is used with a particular implementation of RS-232 or RS-485, does not mean it is part of the standard. RS-232 defines thirteen "standard" interface types lettered A through M. None of these match IBM's implementation of its serial port. 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 intended to provide a method of interconnecting data terminal equipment and a data communication channel when each is furnished by different companies. PURPOSE AND SCOPE OF COVERAGE - This standard is applicable to the interconnection of data processing terminal equipment and data communication equipment. 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.

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. The RS-232 standard does not include a line length limit, but practical limits of the electrical signal levels prevent long lengths. There is a lot of folklore stating that RS-232 has a 50 or 100 foot limit. Serial converters are used a lot in industrial applications as a way to communicate with factory machinery or to interface with test equipment, however in one particular area the serial converter has also become very useful for consumers who need a serial port in their personal computer. A serial adapter, also referred to as a serial converter is a small electronic device which can convert one type of data signal into a different type of data signal. You can design the system to accomplish this if you pay careful attention to logic function of the driver being used, as well as the labeling of the terminal on the device.

RS-232 defines the voltages of these circuits, and the polarity of the logic generating the voltage. The UART connected to the RS-232 driver/receiver controls the protocol of the bits being transferred. Many references and examples are made to "asynchronous start-stop communication with a UART" or "UART" communications. This appendix will attempt to explain what RS-232, RS-422, and RS-485 are and are not; then discuss one of the more common implementations of RS-422 and RS-485, asynchronous start-stop ASCII communication with a UART. However, if the receiver considers the undriven wires to be a binary 0, when the driver is turned on and set to transmit a start bit, which is also a binary 0, the receiver will not see a transition, and therefore will not see the start bit. 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". The other is that the IC manufacturer's datasheets never state that the IC pin A is the same thing as RS-485's wire "A".

These annexes clearly state that they are not a part of the standard, and are included for informational (informative) purposes only. 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. Many think the RS-232 standard completely defines the IBM PC serial port, but this is incorrect. 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. Stub length, termination, and biasing resistors can have a significant impact on the performance of the network. Adding the biasing resistor to every device on the network can cause termination problems. ANSI/TIA/EIA-485, commonly called RS-485, is a standard defining the physical layer of a two wire multipoint communications network. The ground between RS-485 devices is often called a "Third Wire".

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