RS232 reciever schematic

  
This circuit was designed to control a 32 channel Christmas light show from the PC serial port. Originally designed with TTL logic, it has been simplified using CMOS circuits to reduce component count. It is a fairly simple, reliable circuit that requires only 4 common CMOS chips (for 8 outputs), an optical isolator, and a few discrete components. The schematic diagram (SERIAL.GIF) illustrates the circuit with 16 outputs which can be expanded with additional 8 bit shift registers.
RS232 reciever schematic - schematic

This circuit requires physical connections be made to the computer's serial port (COM1 or 2). To the best of my knowledge, it is difficult to cause damage to yourself or your computer by improper connections to this port, but there is no guarantee that damage will not result. Use caution when making any external electrical connections. Basic RS232 serial transmission Serial data is transmitted from the PC as a series of positive and negative voltages on a single wire which occur at predetermined times established by the baud rate. Both the transmitter and receiver must be operating at the same baud rate so that the receiver knows when to expect the next bit of information. For the PC serial port, baud rate and bit rate are the same thing, but this is not necessarily true with modems that can detect more than two states of the line. In the quiescent state, with no load on the line, the voltage on the transmit line (pin 2 of the 25 pin connector) will be about -12 relative to the signal ground (pin 7), which corresponds to a logical "1". The output impedance of the serial port is about 1K ohm which yields about 6 milliamps at 6 volts. A typical data transmission frame consists of a start bit, 8 data bits, and one to three stop bits. The start bit which is always positive, signals the beginning of the transmission and is used by the receiver to synchronize the clock so...



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