Abstract: A circuit is described that allows four remote RS-232 transceivers to share a single UART. A dual four-to-one multiplexer permits transceiver IC1 to form a network with the four remote transceivers. RS-232 communications with one µC and more than one remote system can be problematic, because most µCs contain only one UART, which provides an interface between synchronous and asynchronous ports. The multiplexer in the figure below, IC2 , allows multiple channels (four, in this case) to share a single UART. The dual four-to-one multiplexer permits transceiver IC1 to form a network with the four remote transceivers IC3 to IC6. The table that follows defines the channel-selection codes. Selecting Channel 1, for instance, enables IC1 to communicate with IC3 without being loaded by IC4 to IC6. Pulldown resistors inside the remote transceivers force the outputs of unselected receivers to a known state. One UART and one multiplexer enable one RS-232 transceiver to communicate with four others. Channel Selection Codes Selected channel A1 A0 EN All channels disconnected X X 0 Channel 1 (IC3) 0 0 1 Channel 2 (IC4) 0 1 1 Channel 3 (IC5) 1 0 1 Channel 4 (IC6) 1 1 1 The circuit's supply-voltage range (3V to 5.5V) makes it compatible with 3V and 5V logic. IC2 receives its power directly from the V+ and V- terminals of IC1, whose ±5.5V outputs come from an internal charge pump. The multiplexer handles rail-to-rail signals, so obtaining its power from IC1 ensures that RS-232 signals pass directly through, regardless of amplitude. Each transceiver's charge pump requires four small capacitors (not shown), whose values depend on the VDD range but do not exceed 0.47µF. Note that pulling too much current from the charge-pump terminals of IC1, V+ and V-, can cause these rails to droop and can pull the IC's RS-232 transmission levels out of specification. A similar version of this article appeared in the September 28, 2000 issue of EDN. 我们期待您的反馈！ 喜欢？不喜欢？有待改善？或为我们提供建议？请与我们联系 — 我们将根据您的意见或建议改善我们的工作。 网页评价或提供建议 自动更新 需要自动接收最新发布的应用笔记吗？请订阅EE-Mail™ (English only)。 更多信息   APP 588: Sep 18, 2002 MAX3221 1µA电源电流、+3V至+5.5V、真RS-232收发器，带有AutoShutdown 完整的数据资料 (PDF, 464kB) MAX399 精密的、8通道/双4通道、低电压、CMOS模拟多路复用器 完整的数据资料 (PDF, 564kB)   下载，PDF格式 (31kB)  AN588, AN 588, APP588, Appnote588, Appnote 588