Date: 07/04/2018
By: Deltayrpqrp
Subject: Что за ошибка частотника?
parallel end effective at high bit rates high power series low power suitable only for 2-node lines AC low power suitable only for low bit rates, short cables parallel with open-circuit biasing ensures valid logic level when open high power, requires 2 additional resistors per bus parallel with open and short-circuit biasing ensures valid logic level when open or shorted requires 4 additional resistors per node TIA-485-A says that RS-485 drivers must be able to drive 32 unit loads plus a parallel termination of 60Q. The total load, including the driver, receivers, and terminations, must be 54Q or greater. On a full-duplex line, each termination resistor has its own pair of wires, so each driver sees a resistance of 120Q. In a cable with two termination resistors across the same pair of wires, the parallel combination of two 120Q resistors is 60Q.The input impedance of 32 receivers at 1 unit load each decreases the total resistance of the load slightly, while the output resistance of the driver and the series resistance of the lines increase the total resistance of the load. Effects of Terminations You don’t have to understand why transmission lines behave as they do to design a system that works. But for the curious, the following is an introduction to transmission-line theory without attempting mathematical proofs. A transmission line has two wires, one to carry the current from the driver to receiver and another to provide a return path back to the driver. An RS-485 system is a little more complex because it has two signal wires that share a termination plus a ground return, but the basic principles are the same. In one sense, there’s nothing different about how long and short lines behave. The same laws of physics apply whether the driver is slow or fast and whether the signals travel a short or long distance. Both long and short lines can have voltage and current reflections due to an impedance mismatch. 115 Chapter 7 In all cases, the reflections happen quickly prom-electric.ru
