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ISL3170E-79E датащи(PDF) 1 Page - Intersil Corporation |
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ISL3170E-79E датащи(HTML) 1 Page - Intersil Corporation |
1 / 3 page 1 Application Note 1593 Author: Jeff Lies Add a Loss of Signal (LOS) Indicator to Your RS-485 or RS-422 Transceiver Introduction A desirable feature on any network may be a Loss of Signal (LOS) indicator. Such an indicator alerts a monitoring controller that a potentially serious network error has occurred, thereby allowing for notification and/or intervention. In an RS-485/ RS-422 network, a LOS may occur if a connector disconnects from a board, if a cable fails, or if the network driver fails or loses power. When this occurs, bus activity ceases - as shown in Figure 1 - and an LOS detector would assert its output. This application note discusses the RS-485/RS-422 receiver (Rx) operation, explains how the Rx interprets the differential bus voltages, and introduces a method for generating the desired LOS indicator. RS-485 Receiver (Rx) Basics The RS-485/RS-422 Rx is a differential circuit (comparator) that compares voltages at the noninverting and inverting inputs (A and B), and outputs a corresponding logic level. Per the Standards, if A-B (i.e., the differential input voltage) is greater than +200mV, then the Rx outputs a logic high; if A-B < -200mV, then the Rx outputs a logic low. Any differential voltage between -200mV and +200mV is undefined, and the Rx might output a logic high, a logic low, or may even oscillate. Rx Failsafe In RS-485/RS-422 terminology, “failsafe” is used to indicate Rx functionality that drives the Rx output to a known state when common bus faults occur. The logic high state typically represents the idle (no transmission in progress) state, so driving the Rx output to a logic high under fault conditions is the usual failsafe implementation. Without the failsafe function, an Rx output’s inadvertant logic low, or oscillation, might be interpreted as a message start bit. This could cause a processor to waste valuable time attempting to service phantom messages. The most common form of failsafe is failsafe open, with “open” referring to the condition where the Rx inputs are floating. This can occur when a network bus is cut, or if the bus connector separates from the networked device containing the Rx. Most RS-485 Rx available today incorporate this function, and it is typically accomplished via an internal pull-up resistor on the noninverting (A) input. The pull-up is sized to ensure that if the Rx inputs float, then the Rx input circuitry creates enough of a positive offset so that the Rx comparator interprets the input condition as a logic high. The “Full-Failsafe” RS-485 Receiver An improved differential Rx includes all the features mentioned previously, plus the addition of circuitry to ensure that the Rx is failsafe if the differential input voltage (VID) is 0V (failsafe “shorted”). This condition may occur due to a cable error (e.g., crimped), a connector error (shorted pins), or in normally operating networks where multiple drivers operate on a terminated bus (multi-point network). Multi-point operation requires that all drivers be tri-statable, and there are periods of time when all drivers are simultaneously tri-stated. When this occurs, the differential termination resistor(s) causes the bus voltage to collapse to the VID=0V condition. As mentioned before, VID=0V is an undefined RS-485 level, so an Rx with only failsafe open functionality may generate erroneous start bits. The full-failsafe Rx solves this problem by ensuring that the VID=0V condition is recognized as a logic high input, so the Rx output remains in the idle state. Full-failsafe functionality is accomplished by designing the Rx so that the minimum input high level is slightly negative (-10mV to -25mV), rather than the +200mV used for standard RS-485 and RS-422 Rx. Note that this new threshold definition remains RS-422 and RS-485 compliant. FIGURE 1. TYPICAL LOS OPERATION TIME (1µs /DIV) -5 -3 -1 1 3 5 0 VBUS LOS CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 | Copyright Intersil Americas Inc. 2011. All Rights Reserved. Intersil (and design) is a trademark owned by Intersil Corporation or one of its subsidiaries. All other trademarks mentioned are the property of their respective owners. February 1, 2011 AN1593.0 |
Аналогичный номер детали - ISL3170E-79E |
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Аналогичное описание - ISL3170E-79E |
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