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EIA-530 and EIA-449 are standards that specify a connector, pin assignments and RS-422 electrical levels. 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. Before we discuss polarity lets take a look at logic levels and binary states. The exact voltage level that a logic device considers ON or OFF varies by logic type, but when the voltage is high (usually but not always approaching the IC's supply voltage), the output is on and a binary 1 is on the wire, and when the voltage is approaching 0 the output is off and, a binary 0 is on the wire. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals. Then it references section 2.1.4 where the maximum capacitance of the receiver side of the interface point, including the cable, shall not exceed 2500pF. Then suggests seeing appendix A for guidance (again, the appendix states that it is not a formal part of the standard).

RS-232 is very clear that is intended for a maximum bit rate of 20kbit/s. The forward recommends EIA/TIA-530, EIA/TIA-561, and EIA/TIA-574 for operation at higher bit rates. Any bit protocol such as that from a UART is specifically not included. This protocol may include things like the start bit, number of data bits, parity and stop bit(s). A CRC or checksum can be added to the end of the data being transmitted. If either the query or response is lost through a collision or other error mechanism, RS485 standard the data will end up being retransmitted. 200mV. This can cause a problem if the RS-485 network is using a UART to transmit data. Branching the cable to a receiver will cause secondary reflections which will cause more and unpredictable ringing of the signal. In reality either or both the driver or receiver are going to meet the more modern RS-574 requirements and none of the RS-232 limits will apply. Sadly neither of these limits are in the RS-485 standard. This limits most RS-422 bidirectional networks to two devices that talk to each other. Figure 1 of the RS-485 standard is a diagram (not shown for copyright concerns) which shows the "balanced interconnecting cable" as a transmission line of two wires.

Slew rate limiting drivers will reduce the rise time and decrease the ringing of an unterminated line. In the case of 8 bit SCSI this is not the case, the drivers are single wire with a 220/330 Ω terminator at each end of the buss. At the other end of the cable is (typically) the same thing -- the connectors, the RS-485 interface IC, and a UART inside a microcontroller. Since the PCs are sitting near each other, they are on the same power circuit,the difference in the earth ground between the two computers is very small. The third installation uses the same two computers as the second example, but they are separated by several thousand feet of wire, and one of the computers is sitting next to an arc furnace that draws several thousand amps when operating. 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). Biasing has a number of uses on a RS-485 network, but first lets look at what RS422 and RS-485 have to say.

120 Ω cable should provide the best performance, but the 100 Ω CAT-X cable may you have laying around may also work. Other impedance cable may be used such as 100 Ω nominal. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. Therefore the longer the cable length, the more important the termination resistor is to improve signal quality. From a hardware point of view, full-duplex RS-485 has some major advantages over RS-232 -- it can communicate over much longer distances at higher speeds. Another RS-232 to RS485 scheme is to monitor the data stream going into the RS-485 transmitter and trigger a one-shot timer when an edge occurs. This is not technically correct since RS-232 requires the use of voltages outside those of normal logic, but the input and output of an RS-232 driver look inverted on an oscilloscope, and most data sheets show the driver as an inverter. The voltages are measured at the signal wire and are referenced to signal common. Since the driver and receiver are differential circuits, the input and output voltages are specified as differential, but these voltages are also referenced to a circuit common.