STC International Limited
http://www.mcu-memory.com
STC232E Datasheet Rev.0.1, Dec.2005
5
Page
of
6
RS-232 Receivers
ESD Protection
Pin Configurations and Typical Operating Circuits
The receivers convert RS-232 input signals to inverted TTL signals. Since the input is usually from a transmission line,
where long cable lengths and system interference can degrade the signal, the inputs have a typical hysteresis margin of
500mV. This ensures that the receiver is virtually immune to noisy transmission lines. The input thresholds are 0.8V
minimum and 2.4V maximum, again well within the 3V RS-232 requirements. The receiver inputs are also protected
against voltage up to 25V. Should an input be left unconnected, a 5kO pull down resistor to ground will commit the
output of the receiver to a high state. In actual system applications, it is quite possible for signals to be applied to the
receiver inputs before power is applied the receiver circuitry. This occurs, for example, when a PC user attempts to print,
only to realize the printer wasn’
t turned on. In this case an RS-232 signal from the PC will appear on the receiver input
at the printer. When the printer power is turned on, the receiver will operate normally. All of these enhanced devices are
fully protected.
STC232E devices have standard ESD protection structures incorporated on the pins to protect against electrostatic
discharges encountered during assembly and handling. In addition, the RS232 bus pins (driver outputs and receiver
input) of these devices have an extra level of ESD protection. Advanced ESD structures were designed to successfully
protect these pins against ESD discharge of 15kV when powered down.
C1+
1
16 Vcc
STC232E
V+
2
15 GND
C1-
3
14 T1OUT
C2+
4
13 R1IN
C2-
5
12 R1OUT
V-
6
11 T1IN
T2OUT
7
10 V2IN
R2IN
8
9 R2OUT
DIP/SO
+
C1
1
3
4
5
C2
C5
C3
+5V INPUT
16
C1+
C1-
C2+
C2-
V+
V-
2
6
+10V
-10V
C4
Vcc
+5V TO +10V
VOLTAGE DOUBLER
+10V TO -10V
VOLTAGE INVERTER
+5V
400KΩ
TTL/CMOS
INPUTS
11
10
T1IN
T2IN
+5V
400KΩ
T1OUT
T2OUT
14
7
RS-232
OUTPUTS
TTL/CMOS
OUTPUTS
12
9
R1OUT
R2OUT
R1IN
5KΩ
R2IN
13
8
RS-232
INPUTS
5KΩ
GND
15
TOP VIEW
+
+
+
+