15 Jan
2004
15 Jan
'04
12:52 p.m.
I've had problems in the past with voltage levels
on RS-232.
One device runs -12/+12V, the other 0/+5V, and they
refused to talk
bi-directionally. Putting in a homemade level shifter (forced the
0/+5 to -9/+9) made it work.
I think a sending device is required to swing at least below -3V and
above +3V for it to be RS-232 (though I may have the voltages wrong,
it's been a while - but I'm sure they're symmetric about 0). I ran
into this when connecting Sun keyboards to vanilla serial ports - I
found I needed a level shifter and an inverter (of course, the inverter
may be unnecessary depending on whether the level shifter you use
inverts or not). (Sun keyboards are RS-232 except for the voltage
levels in question, and possibly impedances.)
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