The Electronic Industries Associati

The Electronic Industries Association EIA−232Dspecification details the requirements for the interfacebetween data processing equipment and datacommunications equipment. This standard specifies notonly the number and type of interface leads, but also thevoltage levels to be used. The MC1488 quad driver and itscompanion circuit, the MC1489 quad receiver, provide acomplete interface system between DTL or TTL logic levelsand the EIA−232D defined levels. The EIA−232Drequirements as applied to drivers are discussed herein.The required driver voltages are defined as between5.0 and 15 V in magnitude and are positive for a Logic “0”and negative for a Logic “1.” These voltages are so definedwhen the drivers are terminated with a 3000 to 7000 resistor. The MC1488 meets this voltage requirement byconverting a DTL/TTL logic level into EIA−232D levelswith one stage of inversion.The EIA−232D specification further requires thatduring transitions, the driver output slew rate must notexceed 30 V per microsecond. The inherent slew rate of theMC1488 is much too fast for this requirement. The currentlimited output of the device can be used to control this slewrate by connecting a capacitor to each driver output. Therequired capacitor can be easily determined by using therelationship C = IOS x T/V from which Figure 14 isderived. Accordingly, a 330 pF capacitor on each outputwill guarantee a worst case slew rate of 30 V permicrosecond.