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RS232 splitter cable

There are various RS232 splitter cable schematics around, but the one that I find the most charming is displayed on the picture below.

Thanks to resistors (about 10kohm) connected in parallel to diodes, the circuit above feeds negative voltage level to masterís RX line during idle state or when Ďonesí are sent. Of course, because of these resistors some current is drained into the non-talking slave but drivers are usually capable to cope with it.

The circuit is so simple that there is nothing more to say about it. Still, I want to tell some general stuff about the RS232 serial line:

  • the RS232 uses rather high positive and negative voltages
  • during idling, the negative voltage is held on the TX line
  • the start bit has a positive level
  • regarding data bits, logical zeros are sent as positive levels, and logical ones as negative levels
  • the least significant data bit (LSB) is sent as the first bit of the data train. Sending 0x55 character repetitively using N-8-1 settings will create perfect square waveform signal output.
  • some RS232 implementations will accept TTL levels at RX lines
  • using Windows API, you can directly play with RS232 serial ports on your old computer (newer computers do not have the RS232 port). You can use RTS and DTR lines as general digital outputs and CTS and DSR lines as general digital inputs.

The USB takes over and these days the RS232 can only be found on old computers. Unfortunately I donít find USB charming at all. It is too difficult to use (from programmerís perspective). It doesnít seem that handy for playing.

Danijel Gorupec, 2012


See also the AVR programmer article.


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