Isolated Full-duplex Rs232 Interface

Posted on Feb 6, 2014

This self powered interface circuit electrically isolates the TxD and RxD lines from the PC serial port and protect the PC from direct connection to hazardous voltages. The isolator is intended to provide electrical isolation between a computer and the equipment connected to its serial port. This can be necessary when the target system works at a

Isolated Full-duplex Rs232 Interface
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completely different voltage level, or when earth loops must be avoided. Figure 1 shows how the electrical isolation is achieved. Connector K1 is linked to the serial port of the PC, power to the PC side of the circuit is derived from the signal lines DTR and RTS. Positive supply is derived from RTS and -ve one from the DTR line, therefore it is necessary for the user program to set the RTS status to logic zero & DTR to logic one in order to get the proper supply levels at the output. IC U1 is used to isolate the TxD line while IC U2 isolates the RxD line. The other side of the isolator carries TTL levels. This side is powered by the target system power supply. IC U3 is used to buffered the signals for the opto isolators and also drives the data indicating LEDs. The interface has been tested at the baud rate of 19. 2k baud.

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