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Maxim Integrated Products——收发器芯片简化EIA-485实施

2007年05月10日 17:26:00 中国传动网

2007年5月10日 EIA-485 (RS-485)火热程度也许有所降低,但是这项标准采用多支路性能,仍是长距离(如4,000英尺)传输数字数据最简单的方法,认识到这一点,Maxim Integrated Products推出一套芯片,该芯片简化RS-485实施。MAX13487E和MAX13488E半双工RS-485收发器带有AutoDirection功能,简化了EIA-485实施。 该特性在传输数据时自动使能驱动器,省去了驱动器使能控制信号。AutoDirection功能线路包括一个有限状态机和一个附加接收比较仪,该比较仪测定设备是否尝试驱动总线或其他网点。 Maxim收发器中的内部状态机有两路输入:A-B,取决于微分比较仪和驱动器输入端(DI)。A是正相接收器输入/驱动器输出,B是反相接收器输入/驱动器输出。DI是内部状态。当DI处于低电平,正相输出迫使降低,反相输出迫使升高,反之亦然。有限状态机还有两路输出:DRIVER_ENABLE和RECEIVER_ENABLE。 当DI较低时,有限状态机也将驱动总线降低,当DI很高,则短时驱动总线,然后使驱动器失能。这使得外部上拉/下拉电阻能够在高电压状态下(A-B > 200 mV)支撑总线。DI每次由低到高转换的过程中,驱动器保持使能状态,直至(A-B)超过驱动器失能极限。驱动器失能,使上拉/下拉电阻将A 路和B路保持到正常状态。 MAX13487E采用回转率降低的驱动器,该驱动器减小了电磁干扰,降低了由失误中止电缆而引起的反射,防错传输高达500 kbits/s。MAX13488E驱动器回转率不受限制,最大可能为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]
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