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MC1488 Arkusz danych(PDF) 11 Page - STMicroelectronics |
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MC1488 Arkusz danych(HTML) 11 Page - STMicroelectronics |
11 / 17 page MC1488 Typical applications 11/17 6 Typical applications Figure 14. RS232C Data transmission 6.1 Application information The Electronic Industries Association (EIA) has released the RS232C specification detailing the requirements for the interface between data processing equipment. This standard specifies not only the number and type of interface leads, but also the voltage levels to be used. The MC1488 quad driver and its companion circuit, the MC1489 quad receiver, provide a complete interface system between DTL or TTL logic levels and the RS232C defined levels. The RS232C requirements as applied to drivers are discussed herein. The required driver voltages are defined as between 5 and 15V in magnitude and are positive for a logic "0" and negative for a logic "1". These voltages are so defined when the drivers are terminated with a 3000 to 7000 Ω resistor. The MC1488 meets this voltage requirement by converting a DTL/TTL logic level into RS232C levels with one stage of inversion. The RS232C specification further requires that during transitions, the driver output slew rate must not exceed 30V per µs. The inherent slew rate of the MC1488 is much too fast for this requirement. The current limited output of the device can be used to control this slew rate by connecting a capacitor to each driver output. The required capacitor can be easily determined by using the relationship C = IOS x ∆T/∆V from which Figure 15. is derived. Accordingly, a 330 pF capacitor on each output will guarantee a worst case slew rate of 30V per µs. The interface driver is also required to withstand an accidental short to any other conductor in an interconnecting cable. The worst possible signal on any conductor would be another driver using a plus or minus 15V, 500 mA source. The MC1488 is designed to indefinitely withstand such a short to all four outputs in a package as long as the power-supply voltages are greater than 9.0V (i.e., VS ≥ 9.0V; VEE ≤ 9.0V). In some power-supply designs, a loss of system power causes a low impedance on the power-supply outputs. When this occurs, a low impedance to ground would exist at the power inputs to the MC1488 effectively shorting the 300W output resistor to ground. If all four outputs were then shorted to plus or minus 15V, the power dissipation in these resistors would be excessive. Therefore, if the system is designed to permit low impedances to ground at the power-supplies of the drivers, a diode should be placed in each power-supply lead to prevent over-heating in this fault condition. |
Podobny numer części - MC1488_06 |
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Podobny opis - MC1488_06 |
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