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MC26LS30D Datasheet(PDF) 11 Page - ON Semiconductor |
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MC26LS30D Datasheet(HTML) 11 Page - ON Semiconductor |
11 / 16 page MC26LS30 http://onsemi.com 11 SYSTEM EXAMPLES (Pin numbers refer to SO–16 package only.) Differential System An example of a typical EIA–422–A system is shown in Figure 17. Although EIA–422–A does not specifically address multiple driver situations, the MC26LS30 can be used in this manner since the outputs can be put into a high impedance mode. It is, however, the system designer’s responsibility to ensure the Enable pins are properly controlled so as to prevent two drivers on the same cable from being “on” at the same time. The limit on the number of receivers and drivers which may be connected on one system is determined by the input current of each receiver, the maximum leakage current of each “off” driver, and the DC current through each terminating resistor. The sum of these currents must not exceed the capability of the “on” driver ( ≈60 mA). If the cable is of any significant length, with receivers at various points along its length, the common mode voltage may vary along its length, and this parameter must be considered when calculating the maximum driver current. The cable requirements are defined not only by the AC characteristics and the data rate, but also by the DC resistance. The maximum resistance must be such that the minimum voltage across any receiver inputs is never less than 200 mV. The ground terminals of each driver and receiver in Figure 17 must be connected together by a dedicated wire (or the shield) in the cable to provide a common reference. Chassis grounds or power line grounds should not be relied on for this common connection as they may generate significant common mode differences. Additionally, they usually do not provide a sufficiently low impedance at the frequencies of interest. Single–Ended System An example of a typical EIA–423–A system is shown in Figure 18. Multiple drivers on a single data line are not possible since the drivers cannot be put into a high impedance mode. Although each driver is shown connected to a single receiver, multiple receivers can be driven from a single driver as long as the total load current of the receivers and the terminating resistor does not exceed the capability of the driver ( ≈60 mA). If the cable is of any significant length, with receivers at various points along its length, the common mode voltage may vary along its length, and this parameter must be considered when calculating the maximum driver current. The cable requirements are defined not only by the AC characteristics and the data rate, but also by the DC resistance. The maximum resistance must be such that the minimum voltage across any receiver inputs is never less than 200 mV. The ground terminals of each driver and receiver in Figure 18 must be connected together by a dedicated wire (or the shield) in the cable so as to provide a common reference. Chassis grounds or power line grounds should not be relied on for this common connection as they may generate significant common mode differences. Additionally, they usually do not provide a sufficiently low impedance at the frequencies of interest. Additional Modes of Operation If compliance with EIA–422–A or EIA–423–A Standard is not required in a particular application, the MC26LS30 can be operated in two other modes. 1) The device may be operated in the differential mode (Pin 4 = 0) with VEE connected to any voltage between ground and –5.25 V. Outputs in the low state will be referenced to VEE, resulting in a differential output voltage greater than that shown in Figure 6. The Enable pins will operate the same as previously described. 2) The device may be operated in the single–ended mode (Pin 4 = 1) with VEE connected to any voltage between ground and –5.25 V. Outputs in the high state will be at a voltage as shown in Figure 10, while outputs in a low state will be referenced to VEE. Termination Resistors Transmission line theory states that, in order to preserve the shape and integrity of a waveform traveling along a cable, the cable must be terminated in an impedance equal to its characteristic impedance. In a system such as that depicted in Figure 17, in which data can travel in both directions, both physical ends of the cable must be terminated. Stubs leading to each receiver and driver should be as short as possible. In a system such as that depicted in Figure 18, in which data normally travels in one direction only, a terminator is theoretically required only at the receiving end of the cable. However, if the cable is in a location where noise spikes of several volts can be induced onto it, then a terminator (preferably a series resistor) should be placed at the driver end to prevent damage to the driver. Leaving off the terminations will generally result in reflections which can have amplitudes of several volts above VCC or several volts below ground or VEE. These overshoots/undershoots can disrupt the driver and/or receiver, create false data, and in some cases, damage components on the bus. |
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Descripción similar - MC26LS30D |
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