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MC1489/MC1489A Application information
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8 Application information
The Electronic Industries Association (EIA) has released the RS-232C specification
detailing 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 RS-232C defined levels. The RS-232C requirements as
applied to receivers are discussed herein.
The required input impedance is defined as between 3000Ω and 7000Ω for input voltages
between 3.0 and 25V in magnitude; and any voltage on the receiver input in an open circuit
condition must be less than 2.0V in magnitude. The MC1489 circuits meet these
requirements with a maximum open circuit voltage of one V
BE
.
The receiver shall detect a voltage between - 3.0 and - 25V as a Logic "1" and inputs
between + 3.0 and + 2.5V as a Logic "0". On some interchange leads, an open circuit of
power "OFF" condition (300Ω or more to ground) shall be decoded as an "OFF" condition or
Logic "1". For the reason, the input hysteresis thresholds of the MC1489 circuits are all
above ground. Thus an open or grounded input will cause the same output as a negative or
Logic "1" input.
8.1 Device characteristics
The MC1489 interface receivers have internal feedback from the second stage to the input
stage providing input hysteresis for noise rejection. The MC1489 input has typical turn-on
voltage of 1.25V and turn-off of 1.0 V for a typical hysteresis of 250mV. The MC1489A has
typical turn-on of 1.95V and turn-off of 0.8V for typically 1.15V of hysteresis.
Each receiver section has an external response control node in addition to the input and
output pins, thereby allowing the designer to vary the input threshold voltage levels. A
resistor can be connected between this node and an external power supply. Figure 4.,
Figure 6. and Figure 7. illustrate the input threshold voltage shift possible through this
technique.
This response node can also be used for the filtering of the high-frequency, high-energy
noise pulses. Figure 11. and Figure 12. show typical noise-pulse rejection for external
capacitors of various sizes.
These two operations on the response node can be combined or used individually for may
combinations of interfacing applications. The MC1489 circuits are particularly useful for
interfacing between MOS circuits and MDTL/MTTL logic systems. In this application, the
input threshold voltages are adjusted. (with the appropriate supply and resistor values) to
fall in the center of the MOS voltage logic levels (see Figure 13.).
The response node may also be used as the receiver input as long as the designer realizes
that he may not drive this node with a low impedance source to a voltage greater than one
diode above ground or less than one diode below ground. This feature is demonstrated in
Figure 14. where two receivers are slaved to the same line that must still meet the RS-232C
impedance requirement.
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