RS-422 does not specify any connector. RS-232 provides a list of circuits and says if you use a circuit; this is what it must do and it must be on this pin of the connector. Section 1.3, Data Signalling Rates, states; this standard is applicable for use up to a nominal limit of 20,000 bits per second. Often, use of shielded CAT5/6 cable can give an adequate signal ground, although this is not recommended. RS-485 typically uses a linear network with bidirectional signal wires: There are typically many devices along a RS-485 shared cable. In addition to using the best cable, there are many different drivers, receivers, and transceivers available. If your intention is to push the cable length or bit rate to extremes, you should pay careful attention to the cable, drivers, and installation. This means that low data rates can handle an improperly terminated (or even unterminated) cable better than high data rates. Again, this protocol is not part of the RS-232 standard, even though many engineers think of this protocol as "RS-232". This is considered "inverting" logic by many engineers. Before we discuss polarity lets take a look at logic levels and binary states. 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.

The RS-485 standard includes the voltage levels of the two wires when a binary 0 or 1 is on the two RS-485 wires, but specifically excludes the logic function of the generator or receiver. 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". 3V, and when a binary 1 is being transmitted, the voltage must be more negative than -3V. The annex of RS-485 states that "When employed, the shield shall be connected only to frame ground at either or both ends depending on the specific application." Shields usually have a lot more copper (and/or aluminum) than a single wire and can therefore carry a lot more current.

The annex also says that certain applications may cause the resistor to fail so the installation must allow access for inspection and replacement. Even POTS telephone wire may work for many applications. Devices may be designed to operate at lower data rates for "economically specific applications". If you have to know that it will work before you pull the cable, then get the required length of the cable and move the two devices next to each other, connect them and see if they can talk. 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. The ringing occurs when the data transitions, and will eventually damp out. All of the application guides and data sheets that say RS-485 has a limit of 1200 meters or 10Mbit are flat out wrong.

RS-485 requires the driver and receiver to function if the common mode voltage is shifted against circuit common (see the section on voltages for more information). The standard defines the common-mode voltage as being referenced to ground, it defines a term "ground potential difference" as the difference in the signal ground between the driver and receiver, but it does not say that this is earth ground or just a a third wire common. When an RS-485 driver gets its data directly from a UART (with no added inversion), you would expect the "A" and "B" wires to match the voltages in the RS-485 standard for the voltages on the wires, but they will not (unless the driver inverts its input). A CRC or checksum can be added to the end of the data being transmitted. The graph at right, which is often shown in these app-notes, shows a limit of 1200 meters/DC at one end and 14 meters/10 Mbit at the other. It should be noted that this model is only valid for one frequency. Many cable manufacturers can recommend a 120 Ω cable intended to work with RS-422 or RS-485. Now being used commonly in the pro audio industry to control digital audio and signal processors such as the DBX driverack and other manufacturers equivalent products.

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