MC1489/MC1489A
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.
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. Figures 2, 4 and 5 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.
Figures 9 and 10 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
11).
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 12 where two receivers are slaved
to the same line that must still meet the RS-232C impedance requirement.
Figure 13 :
Typical Turn-on Threshold vs
Capacitance from Response Control Pin to GND
Figure 14 :
Typical Turn-on Threshold vs
Capacitance from Response Control Pin to GND
8/13
STMICROELECTRONICS [ STMICROELECTRONICS ]
