MatchCableImpedance

TableOfContents

Impedance matching
http://groups.google.com/group/comp.arch.embedded/browse_frm/thread/dcb29743ac37b55a/8b54f2e2e7a55f57?lnk=st&q=RS485&rnum=11#8b54f2e2e7a55f57

Dennis, I'm in the process of typing up my notes on RS-485. I'm using it on a project and needed to get to know it better. I see you have been given a number of references, but I have a few more to add. Go to TI, National Semi., and Maxim, and B & B Electronics. I don't have the URL's, but it won't take you long to find them. In addition, Jan Axelson had a good article in Circuit Cellar ("Designing RS-485 Circuits") that is available on the internet. Now to answer some of your questions. Think of differential as being the digital signal and the inverse of it on the two lines. At least that is what you start with, depending on biasing. As you may suspect, the signal will attenuate over distance. As long as there is more than a 200 mV difference between A (the true or '+' line) and the B (or negative or '-' line), the receiver will be able to determine the state. If the difference is less than 200 mV, then the state of the bus is unknown. How does one insure that the bus state is known? You bias it by tying the A line to 5 volts through a resistor. Typical values range from 560 to 750 ohms. The B line is tied to ground with the same value resistor. You may have noticed that I specified a range of bias values. The reason has to do with termination. First, is termination needed? The answer is that depends on the signaling speed (bps) and the length of the transmission line. If the line length is much shorter than the electrical length of the bit window, then termination is not needed. When you get the documents mentioned you will see calculations to determine this. The idea is that the wave front, or Trise, needs time to dissipate before you are into the center of the bit. It is assumed that 4 or 5 round trips around the transmission path will attenuate Trise well enough. Remember the purpose of termination is to match the cable Zo to the load. 24 AWG wire has a typical Zo of about 120 ohms, so if termination is needed, there would be a 120 ohm, 1/2 watt resistor at each end. The problem with termination is that it makes the system less flexible if you plan to be adding and removing nodes at will. So you may want to do a trade-off where the speeds are slow enough that termination is not needed. The other problem with termination is that it is a load on the driver, and increases the current required. The number of drivers is not limited. Of course you can't have more than one driver on at a time because that will play havoc with the data integrity. The number of receivers is what is limited. A receiver can have 1, 1/2, 1/4, or 1/8 unit load (check with the receiver specifications). A unit load is about 12K ohms. So,if all of your receivers are one unit load, the spec. says that you can have up to 32 of them. More if you use receivers of a fractional unit load. Comparing chips is something I have to let up to you because they all have pros and cons. I'm out of time, but I hope this helps somewhat. Let me know if you have more questions.