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.
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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
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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