Section 1.3, Data Signalling Rates, states; this standard is applicable for use up to a nominal limit of 20,000 bits per second. 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. Since the laptop has no connection to earth ground, there will normally be little current through this third wire. 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. None of these values are an absolute, they are going to change with frequency. I.e., you should be able to change the resistor when it goes up in smoke. When the input is high the inverting output goes low, and when the input is low the inverting output goes high. Conversely, the output that goes low when the input goes high is often called the "inverting" output.

The driver's output must be limited to 250mA peak output current, but may be limited to much less. The opposite is true of an inverting output. If you are trying to get two ancient pieces of equipment (both of which use true RS-232 drivers and receivers) to work with a long cable, the capacitance limit in RS-232 may apply, but RS-232 does not take bit rate into account. The driver has the capability of driving 10 receivers of 4k impedance, but the actual number that can be driven depend on the actual input impedance of the receivers, bit rate, wire, stub lengths, biasing and termination of the network. And, of course, the installation can make or break the network performance. The annex also says that certain applications may cause the resistor to fail so the installation must allow access for inspection and replacement. 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.

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. The first bit a UART transmits is the start bit (0) and the receiving UART should see this transition. When the driver is transmitting a 1 (from the UART for example), then the voltage on the wire must be less than -3V. The driver must not exceed 6V differential, or 6V common mode. This difference in earth ground may be high enough to cause damage to the RS-485 devices, but connecting a third wire between their circuit commons/earth grounds would try to bypass the power earth common (this is often called a ground loop) causing excessive current in the third wire. There is no wire shown connecting this third point between driver and receiver.

This helps to limit the current through the "third" wire. That being said, what is the practical line length limit? Try to find the capacitance of the cable being used (in pF/foot) and divide 2500 by it. A CRC or checksum can be added to the end of the data being transmitted. Unlike the terminator, the biasing divider does not have to be placed strictly at the end of the bus. 120 Ω cable should provide the best performance, but the 100 Ω CAT-X cable may you have laying around may also work. POTS telephone cable's impedance is less controlled than CAT cable and can vary from 600 Ω at lower audio frequencies to less than 100 Ω at higher frequencies. A 120 Ω resistor on a 100 Ω cable will dramatically reduce the ringing compared to no termination. The ringing occurs when the data transitions, and will eventually damp out.

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