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ISL32273E-77E 数据表(PDF) 2 Page - Intersil Corporation |
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ISL32273E-77E 数据表(HTML) 2 Page - Intersil Corporation |
2 / 3 page Application Note 1593 2 AN1593.0 February 1, 2011 Using Full-Failsafe Receivers for LOS Detection This full-failsafe feature can be exploited to detect the LOS bus condition. The advantage of using full failsafe receivers for LOS detection is that the VID=0V condition is now a valid input condition. If the network is set up so that a LOS causes the VID=0V condition, then the full-failsafe function can be utilized to detect the LOS. Adding an Additional Full-Failsafe Rx Allows LOS Detection Figure 2 illustrates circuitry that can implement an LOS detection function. The transceiver block can be any RS-422 or RS-485 IC with a full-failsafe Rx (see Table 1 for a list of Intersil full-failsafe Rx devices), and at least one termination resistor (RT) is required to collapse the bus voltage when the bus is not actively driven. Adding a second full-failsafe Rx - with its inputs connected opposite to those of the primary Rx - enables the LOS circuit to distinguish between the VID=0V bus state and the normal Data = 1 bus state. When the bus is actively driven, RXD and RXD will be in opposite states (see Figure 3). The only time that RXD and RXD are in the same state is when the bus voltage is 0V, because both Rx interpret this as a logic high input. ANDing the two Rx outputs yields an active high LOS signal (see Figure 4). Propagation delay differences (skews) between the transceiver Rx and the secondary Rx may cause glitches on the LOS signal. Adding a low pass filter (RF and CF in Figure 2) filters out these glitches. A good choice for the secondary Rx is the ISL3280E. It is a single Rx in a micro package (5 Ld SOT-23), has the required full-failsafe function, operates up to 20Mbps, and features IEC61000-4-2 ESD levels. Additionally, the 3V-5V supply range, and the -40°C to 125°C operating temperature range allow for the ISL3280Es use in most RS-485 and RS-422 applications. Los Detect May Not Work For All Rx Even if present at each full-failsafe Rx node, the LOS detect circuit may not properly indicate all the possible LOS conditions. As previously mentioned, a termination resistor is required to force the VID=0V condition that indicates a LOS. Unfortunately, the RS-422 Standard allows only one termination resistor on the bus, and the RS-485 Standard allows only two. Thus, only one or two of the LOS detectors can have the termination resistors. This is fine for the case where a multi-drop (only one driver on network with multiple receivers) network loses power to the driver, as the one termination resistor forces the VID=0V on the whole bus, and thus on each Rx input. But if an Rx without the termination resistor becomes disconnected from the bus, its VID may not be 0V, so the LOS circuit may not indicate the LOS condition. A possible solution is to include a larger value (e.g., 1kΩ) resistor across the inputs of the all Rx that don’t have termination resistors. Whether or not this is acceptable depends on the transceiver family used, the criticality of the termination resistance value (slow, slew rate limited transceivers aren’t as termination sensitive), and the number of nodes on the network. Multi-point systems offer another challenge. As mentioned previously, there are multiple drivers on the bus, so there are periods of inactivity when one driver disables before another enables. If the idle period is long enough, the bus voltage might collapse to 0V, causing the LOS to momentarily activate. If this is a problem, then the user might have to implement a timing circuit to ensure that the LOS signal has been active long enough to indicate a real LOS condition. D R RT RO D I RXD A/Y B/Z A B LOS RO R RXD FIGURE 2. LOS DETECTION CIRCUIT ISL3280E RF CF + - VBUS FIGURE 3. Rx OUTPUT VOLTAGES RELATIVE TO THE BUS VOLTAGE TIME (1µs/DIV) 0 1 2 3 4 5 -4 -2 0 2 4 VBUS RXD RXD FIGURE 4. AND GATE OUTPUT AND FILTERED LOS INDICATOR SIGNAL TIME (1µs/DIV) 0 1 2 3 4 5 0 1 2 3 4 5 LOS AND RXD RXD |
类似零件编号 - ISL32273E-77E |
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类似说明 - ISL32273E-77E |
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