![]() ![]() ![]() Once the output signal is found, the input pin could be found by connecting that output signal to any pin that is NOT ground, NOT a part of another port, and NOT showing a voltage reading - and checking whether an echo can be established. Serial modems often had the 25 pin connector and came with a 9-to-25 pin cable.įinding an undocumented (but present from a software/OS view) secondary serial port on some connector on any computer system would be most easily done by sending a constant stream of data out that port (serial ports have no idea whether anything is connected to them!) and checking pins with an oscilloscope. Two-in-one serial ports on DB25 have been implemented in some unix workstations, but the use of a DB25 connector on 1980s PC style computers for single serial ports was perfectly common and is not an indicator for such a port.Īs the above post of the full pinouts shows, the original RS232 standard allows for a lot of optional signals that were not implemented in PC hardware, so the 9-pin connector slowly became the preferred implementation. ![]()
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