Specifications

CONSTRUCTION
2 (receiver pin) of 9-pin (or pin 3 of 25-
pin) ‘D’ connec-
tor used for the
serial port in the
PC, so that the
data may be
read. The optical
signals received
by the photo-
diodes are in fact
converted to
electrical pulses
and both PCs ‘think’ that there is a
null modem cable connected between
them. Table II shows the correspon-
dence between the various pins of a 9-
pin (or 25-pin) ‘D’ connector of serial
port of PC. In some PCs, the serial
port is terminated into a 9-pin ‘D’ con-
nector and in some others into a 25-
pin ‘D’ connector.
Testing
Assemble two transceiver modules and
connect each of them, using 3-core cables,
to Com-1 ports of the two PCs. Place
them 15 to 20 cms apart so that the IR
LEDs of each module face the photodiode
detector of the other.
Power ‘on’ both the circuits to oper-
ate at stabilised 5V DC. You may alter-
natively use a 7805 regulator IC with a
9V DC source to ob-
tain regulated 5V
supply.
Check if the
MAX232 IC is work-
ing properly by test-
ing pin 2 for 9 to 10V
positive supply and
pin 6 for
-9V supply. MAX232
(refer Fig. 1) uses
1µF, 25V capacitors
C1-C5 as a charge
pump to internally
generate ±9V from 5V
supply. Generally, de-
fective MAX232 ICs
will not show a volt-
age generation of +9V
and -9V at pins 2 and
6, respectively. Re-
place ICs, if required.
Although 1µF, 25V ca-
pacitors are recom-
mended in the
datasheet, the circuit
works well even with
10µF, 25V capacitors, which are easily
available.
With both the PCs and supply to the
transceiver modules ‘on’, throw some light
DB 9 Pin DB25 Pin Signal Direction Description
1 8 In DCD (data carrier detect)
2 3 In RX (receiver data)
3 2 Out TX (transmit data)
4 20 Out DTR (data terminal ready)
5 7 — GND (signal ground)
6 6 In DSR (data set ready)
7 4 Out RTS (request to sent)
8 5 In CTS (clear to send)
9 22 In RI (ring indicator)
TABLE II
TABLE III
Base Address for the Communication Ports
Communication Base address port
COM1 03F8H
COM2 02F8H
COM3 03F8H
COM4 02F8H
Fig. 2: Communication between two PCs for a short range using IR diodes, or longer distance
using laser
Fig. 3: Modified circuit diagram for PC-to-PC communication using 38kHz modulated pulses
received. Pin 7 is also connected to pin
176