BUILT YOUR OWN PC-TO-PC -P.Bhanu Prakash M.Sc.(Ele II-sem)

Serial communication between two PCs is made using radio wave technology.
Here a novel circuit IC MAX232, which needs only 5v power supply for level conversion is used.
The communication over short distances of 2 to 3 meters is established using infrared diodes.The range can be increased to 100 meters using laser diode module in place of infrared LEDs.
Assemble two prototypes i.e. laser modules on PCBs or bread and connect them to COM-1(or COM-2) port of each PC. Point the laser the laser beam of one module to fall on the photodiode of the module connected to the other PC, and vice versa.

Load PROCOMM or TELIX serial communication software and set the port parameters to 9600 n 8 1 (here 9600 refers boud rate, n stands for parity-none, 8 represents bits per character and 1 indicates number of stop bits) to establish the communication. File transfer is also possible. The software program for this purpose was written is 'C' language.

CIRCUIT:


Transmitter: Data signals transmitted through pin 3 of 9-pin 'D' connector of RS-232 COM port are sent to pin 8 of MAX 232and it converts these RS-232 compatible levels of +9v or -9v to 0 or 5v TTL levels. The out put pin 9 of MAX-232 IC drives the pnp transistor SK100 and powers the IRLEDs output pin 9 also drives an LED indicator (LED2) during the positive output. At logic '0' output LED2 goes 'off' but drives the pnp transistor through a bias resistor of 1k ohm (R5). The electrical pulses send by COM port are now converted to corresponding modulated pulses of IR light.

Receiver: The IR signals are detected by photodiode (D1). The detected TTL level signals are coupled to pin 10 of MAX-232 IC. These are converted to +9v or -9v levels internally.
A visible LED-1 at pin-7 of MAX-232 IC indicates that signals are deing received. Optical signals received by photodiodes are converted to electrical pulses and both PCs think that there is a null-modem cable connected between them.

Testing: Connect two transceiver modules using 3 core cables, to COM-1 ports of the two PCs. Place them 15 to 20cms apart so that the IR LEDs of each module face the photodiode detector of the other.
Power ‘on’ both the circuits to operate at stabilized 5v DC. Check if MAX-232 IC is working properly by testing pin2 for 9v positive supply and pin6 for -9v supply. Otherwise replace it.
With both PCs and supply to the transceiver modules ‘on’ throw some light with torch on photodiode. Then LED-1 should flicker. This proves that IR signals are being detected by photodiodes and converted to RS-232 compatible levels by MAX-232 and output at pin7 of MAX-232 IC.
To test the transmitter side disconnect the module from COM-1 (or COM-2). Port of PC, and with the device power ‘on’, use a short jumper wire from +5v and touch it at pin8 of MAX-232 IC to simulate a positive pulse. LED-2 should turn ‘off’ and IRLEDs and LED3 should turn ‘on’ if the wiring is correct. IRLEDs would also be glowing, although one can not see them glowing. Remove the link wire from +5v to pin8 of MAX-232 IC and connect back the ‘D’ connector to PC`s COM-1.
Run a simple communication software with the conditions given at starting and send a few characters from keyboard through COM-1 port. We should be able to see LED3 flickering for a few seconds, indicating data transmission.