May 1, 2008
about 5 years ago
jleslie48: Ok help!!!! EE 101 help needed. how in blazes does this thing convert the TTL tx signal to 12V??? If VCC is 5V, I can’t see how it creates the 12V from the 5V VCC from the schematic.
It doesn’t really convert VCC to 12v. It simply pulls the output (pin 2 on the DB9) up to the level of VCC for “mark”. To get the lower negative voltage for “space” it uses a clever little trick of stealing and storing the negative voltage from the connected host coming in on pin 3. This is done with the diode D1 and the capacitor C1 (note how the capacitor is installed with the + side connected to ground). This negative level is then just allowed to feed back out pin 2 via R1 to the connected device when not pulled up by Q1 for a “mark”. If the connected devices does not supply a negative “space” signal then it probably doesn’t require a negative “space” just a voltage level around 0v or below for “space” and above 3v (even as low as 1.5v will work, from my experience) for “mark” as per EIA-232 specification rather than the old RS232 spec of +/- 12v.
BTW. I could be totally wrong on this too.
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