2007年5月10日 EIA-485 （RS-485）火熱程度也許有所降低，但是這項(xiàng)標(biāo)準(zhǔn)采用多支路性能，仍是長距離（如4，000英尺）傳輸數(shù)字?jǐn)?shù)據(jù)最簡單的方法，認(rèn)識(shí)到這一點(diǎn)，Maxim Integrated Products推出一套芯片，該芯片簡化RS-485實(shí)施。MAX13487E和MAX13488E半雙工RS-485收發(fā)器帶有AutoDirection功能，簡化了EIA-485實(shí)施。 該特性在傳輸數(shù)據(jù)時(shí)自動(dòng)使能驅(qū)動(dòng)器，省去了驅(qū)動(dòng)器使能控制信號(hào)。AutoDirection功能線路包括一個(gè)有限狀態(tài)機(jī)和一個(gè)附加接收比較儀，該比較儀測定設(shè)備是否嘗試驅(qū)動(dòng)總線或其他網(wǎng)點(diǎn)。 Maxim收發(fā)器中的內(nèi)部狀態(tài)機(jī)有兩路輸入：A-B，取決于微分比較儀和驅(qū)動(dòng)器輸入端（DI）。A是正相接收器輸入/驅(qū)動(dòng)器輸出，B是反相接收器輸入/驅(qū)動(dòng)器輸出。DI是內(nèi)部狀態(tài)。當(dāng)DI處于低電平，正相輸出迫使降低，反相輸出迫使升高，反之亦然。有限狀態(tài)機(jī)還有兩路輸出：DRIVER_ENABLE和RECEIVER_ENABLE。 當(dāng)DI較低時(shí)，有限狀態(tài)機(jī)也將驅(qū)動(dòng)總線降低，當(dāng)DI很高，則短時(shí)驅(qū)動(dòng)總線，然后使驅(qū)動(dòng)器失能。這使得外部上拉/下拉電阻能夠在高電壓狀態(tài)下（A-B > 200 mV）支撐總線。DI每次由低到高轉(zhuǎn)換的過程中，驅(qū)動(dòng)器保持使能狀態(tài)，直至（A-B）超過驅(qū)動(dòng)器失能極限。驅(qū)動(dòng)器失能，使上拉/下拉電阻將A 路和B路保持到正常狀態(tài)。 MAX13487E采用回轉(zhuǎn)率降低的驅(qū)動(dòng)器，該驅(qū)動(dòng)器減小了電磁干擾，降低了由失誤中止電纜而引起的反射，防錯(cuò)傳輸高達(dá)500 kbits/s。MAX13488E驅(qū)動(dòng)器回轉(zhuǎn)率不受限制，最大可能為16 Mbits/s。 original text： [COLOR=#708090]Maxim Integrated Products——Transceiver Chips Simplify EIA-485 Implementation May 10, 2007 EIA-485 （RS-485） may seem a little retro. But the standard is still the simplest way to drive digital data over long distances （i.e., 4000 feet） with multidrop capability. Recognizing this, Maxim Integrated Products‘ has come up with a pair of chips that simplify RS-485 implementations. The MAX13487E and MAX13488E half-duplex RS-485 transceivers achieve this simplification with a feature Maxim calls AutoDirection. This feature automatically enables the driver when transmitting data, eliminating the driver-enable control signal. The AutoDirection circuitry consists of a state machine and an additional receive comparator that determines whether the device is trying to drive the bus or another node on the network is driving the bus The internal state machine in Maxim‘s transceivers has two inputs: A-B, determined by a differential comparator, and driver input （DI）. A is the non-inverting receiver input/driver output, and B is the inverting receiver input/driver output. DI is an internal state. When it is low, the non-inverting output is forced low, the inverting output is forced high, and vice versa. The state machine also has two outputs: DRIVER_ENABLE and its complement, RECEIVER_ENABLE. When DI is low, the state machine drives the bus low. When DI is high, it drives the bus for a short time and then disables the driver. This allows external pullup/pulldown resistors to hold the bus in the high state （A-B > 200 mV）. During each low-to-high transition of DI, the driver remains enabled until （A-B） exceeds the driver-disable threshold （VDT）. The driver is disabled at this point, letting the pullup/pulldown resistors hold the A and B lines in the correct state. The MAX13487E uses reduced slew-rate drivers that minimize electromagnetic interference and reduce reflections caused by improperly terminated cables, allowing error-free transmission up to 500 kbits/s. The MAX13488E driver slew rate isn‘t limited, making throughput of up to 16 Mbits/s possible.[/COLOR]