MC1488

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TYPICAL APPLICATIONS: RS232C Data Transmission

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

from which Figure 12 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

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 15 V, 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. These two diodes, as

shown in Figure 13, could be used to decoupled all the driver packages in a system. (These same diodes

will allow the MC1488 to withstand momentary shorts to the ±15V limits specified in the earlier Standard