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Determining which fault has occurred would take additional hardware. The entire subject of handshaking is obsolete with modern USB to RS-485 adapters since the PC's driver's and/or hardware handle driver enabling, and a modern PC can handle data rates much faster than can be transmitted over an RS-485 network. The driver must be electrically disconnected or "made passive" when it is not transmitting. Transmitting data bidirectionally over the same wire(s), rather than unidirectional transmission, requires a turn-around delay. 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. 10V difference. Beware of ground loops when using the shield as the third wire. 200mV. This can cause a problem if the RS-485 network is using a UART to transmit data.

Bias resistors would be required at each end of the network with these values. A single set of bias resistors could be used at one end of the network, if the resistance was halved to 720 Ω. Biasing has a number of uses on a RS-485 network, but first lets look at what RS422 and RS-485 have to say. Since a receiver may have a loading of less than one, the actual number of receivers that can be connected depend on the unit load rating of the receivers, as well as the wire, bit rate, stub lengths, biasing and termination of the network. Termination resistors will reduce the amplitude of this reflection back from the ends which will improve the signal quality. This resistor will also make any length of cable, even a short length, look like the characteristic impedance of the cable (120 Ω in the model) to the driver. For normal logic when the input of a driver is a 1 or high, the non-inverting output will go high.

A binary 1 may (or may not) be inverted by the driver before it is output. Forcing the wires to the idle condition when no driver is active is the most common reason to need biasing on the network. The UART should function correctly if the receiver considers the undriven voltage on the wires to be the idle condition. For example: An RS-422 network has an always active driver and receiver. When 5V drivers are used, the driver typically pulls one wire to common, and the other wire to 5V (and vice-versa for the opposite data). RS-422 drivers are always connected and act as their own termination at the driver's end of the wire. Therefore the total bias resistance is effectively in parallel with the termination resistor as a load to the cable. The longer the cable length, the more effect the termination resistor has on the signal level. The termination resistor should be equal to the characteristic impedance of the cable. A 120 Ω resistor on a 100 Ω cable will dramatically reduce the ringing compared to no termination. Termination is a less controversial subject than a misunderstood subject.

If you run a data rate of less than 30kBit and don't need long cables, the termination resistors are probably not required. If you are pushing any limits, the termination resistor value has to be determined empirically for the individual network. The following example is to bias the network to 200mV. You may want to bias the network to a higher differential voltage to improve the noise margin. A decent oscilloscope that is isolated from earth ground (battery operated or powered by an unplugged UPS (if you're good enough with a scope you can also subtract channel 1 from channel 2 to see the differential waveform, or use a differential probe, etc.)) can be a very useful troubleshooting tool. Ultrawide SCSI does use differential drive but whether it is RS485 compatible, I don't know. This third example would be a good place to use an isolating RS-485 transceiver. This helps to limit the current through the "third" wire. When a binary 1 is transmitted, the signal is spacing, and the voltage on the wire must be more negative than -3V. The connections range from screw terminals that bare wire is inserted into, to connectors such as a DE-9. RS-232 does not define a protocol, but the protocol that is almost always transmitted on these connectors is asynchronous start-stop ASCII (data from a UART).

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