485电路介绍

Figure 1—This general-purpose RS-485network can have up to 32 nodes. Biasing

resistors ensure that there are no false start bitswhen no drivers are enabled.

to any asynchronous serial port.Many network circuits alsorequire a port bit to control eachtransceiver’s driver-enable input.Ports designed for RS-232 com-munications can use the RTSoutput. If that’s not available, anyspare output bit will do.

Most serial-communicationstools, including Visual Basic’sMSComm, support RS-485 com-munications with RTS controlled inundefined.

software. The COMM-DRV serial-At the driver, the voltage differenceport drivers from WCSC have auto-must be at least 1.5 V, so the interfacematic RTS control built-in.

tolerates a fair amount of non-common-The main reason why RS-485 linksmode noise and attenuation.

can extend so far is their use of bal-Vendors for RS-485 transceiversanced, or differential, signals. Twoinclude Linear Technology, Maxim,wires (usually a twisted pair) carry theNational Semiconductor, and Texassignal voltage and its inverse. TheInstruments. These companies arereceiver detects the difference be-also excellent sources for applicationtween the two. Because most noisenotes containing circuit examples andthat couples into the wires is com-explanations of the theory behind them.mon to both wires, it cancels out.RS-485 is designed to be wired in aIn contrast, interfaces like RS-232daisy-chain or bus topology. Any stubsuse unbalanced, or single-ended, signals.that connect a node to the line shouldThe receiver detects the voltage differ-be as short as possible. Most links useence between a signal voltage and atwisted pairs because of their abilitycommon ground.

to cancel magnetically and electro-The ground wire tends to be noisymagnetically coupled noise.

because it carries the return currentsfor all of the signals in the interface,GENERAL-PURPOSE LINK

along with whatever other noise hasFigure 1 shows a general-purposeentered the wire from other sources.RS-485 network. Each node has a TexasAnd noise on the ground wire can causeInstruments SN75176B transceiverthe receiver to misread transmittedthat interfaces between RS-485 andlogic levels.

TTL logic levels.

The datasheets for interface chipsThe chip has a two-wire RS-485label the noninverted RS-485 line asinterface, a TTL driver input and re-line A and the inverted line as line B.ceiver output, and TTL enable inputsAn RS-485 receiver must see a voltagefor the driver and receiver. Similardifference of just 200 mV between Achips include Linear Technology’sand B. If A is at least 200 mV greaterLTC485, Maxim’s MAX485, and Na-than B, the receiver’s output is a logictional Semiconductor’s DS3695.high. If B is at least 200 mV greater thanThe circuit has two 120- termi-A, the output is a logic low. For differ-nating resistors connected in parallel,ences less than 200 mV, the output is

at or just beyond the final node at each

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CIRCUIT CELLAR® end of the link. One end of the linkalso has two 560- biasing resistors.The terminations reduce voltagereflections that can cause the receiverto misread logic levels. The receiver seesreflected voltages as output switches,and the line settles from its initialcurrent to its final current. The termi-nation eliminates reflections by makingthe initial and final currents equal.The initial current is a function ofthe line’s characteristic impedance,which is the input impedance of aninfinite open line. The value varieswith the wires’ diameters, the spacingbetween them, and the insulation type.For digital signals (which consistmainly of frequencies greater than100 kHz), the characteristic impedanceis mostly resistive; the inductive andcapacitive components are small. Atypical value for 24-AWG twisted pairis 120 .

The final current is a function ofthe line termination, the receivers’input impedance, and the line’s seriesimpedance. In a typical RS-485 linewithout a termination, the initial cur-rent is greater than the final currentbecause the characteristic impedanceis less than the receivers’ combinedinput impedance.

On a line without a termination,the first reflection occurs when theinitial current reaches the receiver.The receiver’s input can absorb only afraction of the current. The rest re-flects back to the driver. As the cur-rent reverses direction, its magneticfield collapses and induces a voltageon the line. As a result, the receiverinitially sees a greater voltage thanwhat was transmitted.

When the reflected voltage reachesthe driver, which has a lower imped-ance than the line, the driver absorbssome of the reflection and bouncesthe rest back to the receiver. This

reflection is of opposite polarity to thefirst reflection and causes the receiverto see a reduced voltage. The reflectionsbounce back and forth like this for afew rounds before they die out and theline settles to its final current.

If the line terminates with a resistorequal to the line’s characteristic im-pedance, there are no reflections. When

the initial current reaches the termi-

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