MC1488

5

MOTOROLA ANALOG IC DEVICE DATA

APPLICATIONS INFORMATION

The Electronic Industries Association EIA–232D specification

details the requirements for the interface between data processing

equipment and data communications 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 EIA–232D defined

levels. The EIA–232D requirements as applied to drivers are

discussed herein.

The required driver voltages are defined as between 5.0 and 15 V

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 EIA–232D

levels with one stage of inversion.

The EIA–232D specification further requires that during transitions,

the driver output slew rate must not exceed 30 V per microsecond.

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 12 is derived.

Accordingly, a 330 pF capacitor on each output will guarantee a

worst case slew rate of 30 V per microsecond.

1000

10

100

1.0

10

333 pF

30 V/

µs

Figure 12. Slew Rate versus Capacitance

for ISC = 10 mA

C, CAPACITANCE (pF)

10,000

100

1,000

1.0

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 15 V, 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 9.0 V (i.e., VCC

q 9.0 V; VEE p – 9.0 V). 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 300

Ω output

resistors 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 overheating in

this fault condition. These two diodes, as shown in Figure 13, could be

used to decouple all the driver packages in a system. (These same

diodes will allow the MC1488 to withstand momentary shorts to the

± 25 V limits specified in the earlier Standard EIA–232B.) The

addition of the diodes also permits the MC1488 to withstand faults

with power supplies of less than the 9.0 V stated above.

Figure 13. Power Supply Protection

to Meet Power Off Fault Conditions

VCC

14

MC1488

VEE

7

MC1488

14

MC1488

14

7

1

17

1

The maximum short circuit current allowable under fault conditions

is more than guaranteed by the previously mentioned 10 mA output

current limiting.

Other Applications

The MC1488 is an extremely versatile line driver with a myriad of

possible applications. Several features of the drivers enhance this

versatility:

1. Output Current Limiting – this enables the circuit designer to

define the output voltage levels independent of power supplies and

can be accomplished by diode clamping of the output pins. Figure 14

shows the MC1488 used as a DTL to MOS translator where the high

level voltage output is clamped one diode above ground. The

resistor divider shown is used to reduce the output voltage below the

300 mV above ground MOS input level limit.

2. Power Supply Range – as can be seen from the schematic

drawing of the drivers, the positive and negative driving elements of

the device are essentially independent and do not require matching

power supplies. In fact, the positive supply can vary from a minimum

7.0 V (required for driving the negative pulldown section) to the

maximum specified 15 V. The negative supply can vary from

approximately – 2.5 V to the minimum specified – 15 V. The MC1488

will drive the output to within 2.0 V of the positive or negative supplies

as long as the current output limits are not exceeded. The combination

of the current limiting and supply voltage features allow a wide

combination of possible outputs within the same quad package. Thus

if only a portion of the four drivers are used for driving EIA–232D

lines, the remainder could be used for DTL to MOS or even DTL to

DTL translation. Figure 15 shows one such combination.