MC1488 View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

MC1488

RS-232 quad line driver

STMicroelectronics 

MC1488

6

Typical applications

Figure 14. RS232C Data transmission

Typical applications

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 7000Ω 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.

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