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http://www.chipkin.com/rs485-cables-why-you-need-3-wires-for-2-two-wire-rs485/
RS485 needs 3 conductors and a shield. Many people say its a two wire network but it is not.
Two conductors are used to carry the RS485 Differential voltage signal.
The Shield is connected to earth/ground at one end only and provides shielding against induced noise.
So why the 3rd conductor ?
The driver sends data by modulating the differential voltage. The receiver must sense and decode the differential. There are limits to the voltages the transmitters and receivers can work with. These limits are specified by the code. They are -7Volts to +12Volts. What happens if you have two devices and a ground potential exists between the two devices of 24 volts ? You can see that one of the devices will be operating outside the specified voltage range. While you might expect that all the electrical equipment in an installation is ultimately connected to the same ground in practice this is rare especially in cold climates where building architecture and frozen ground can conspire against you. That why you need the 3rd conductor – to connect the ground (of each RS485 driver) to the same reference. Now we don’t care about ground potentials.
Ever wonder why you blew a 485 device when you connected your laptop or computer ?
Its this problem – there is a ground potential. Thats why its good practice to connect your laptop’s 485 ground conductor before you connect the differential conductors.
Can you get away with 2 conductors ?
Yes. Thats why lab or factory tests suddenly stop working when installed at a site. In your lab or on your desk you can be sure the devices are all commonly grounded. Now if you measure the difference between the ground of a RS485 driver on one device and another device you will find zero volts.
Can you get away without the shield?
The value of the shield is controversial. If you are using a twisted pair and you don’t ruin the twists by unwinding them by more than an inch or two at each end then the shield probably is of little value. But, most cables come with a shield. If its not easy to terminate the shield or you cant be bothered then at least have your design drawings tell the installer to to coil and tape the shield drain wire so that you can use it if you want to.
Where can you purchase 3 wire 485 cable ?
I don’t know. Purchase two twisted pairs with an overall shield/drain. Use one pair for the differential and join the conductors of the other pair to make your ground wire.
What size conductors ?
The bigger the better. Most installations are done with 24awg but remember the higher the baud rate the greater the signal frequency and the greater all kinds of captives and inductive effects. If you can, get bigger conductors.
What kind of cable ?
Choose one with designed to present a almost constant nominal impedance because it will make the addition of terminating resistors easy – just read the cable jacket and get a resistor of the same impedance. Most cables listed for 485 use are designed to present an almost constant nominal impedance. Almost constant means a cable who impedance is reasonably independent of length.
What about biasing ?
If a RS485 driver is idle then what ? The line is disconnected from the driver in its idle state which means that the + and – are floating. What happens if for an instant there appears a differential voltage of more than 0.2 Volts? Well a differential greater than 0.2 is considered a signal and hence constitutes data noise. Solve this problem by pulling the lines to no voltages when they are idle. To do this connect them to ground or some other voltage using pull up/pull down resistors. Nice vendors include this. Nicer vendors provide a choice of biasing resistors selected by switches/jumpers. The reason you might not always use the same value is that the cumulative effect of many device’s biasing resistors may make it impossible to signal at all. How do you calculate the value of a biasing resistor ? How would you know where to connect it ? Good luck getting answerers to these questions. An easier question to answer is this: How do you know if you need to bias the idle state ? If you have a scope you can see what the idle state floating voltages are ? Don’t forget you can only measure this when the device is idle.
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原文地址：http://www.cnblogs.com/aardvark/p/5865657.html
