Hello [EMAIL PROTECTED], since you wrote:
>
> DTR is one of those control signals that should float asserted, so
> removing the plug should let output go through - and get lost.
>
> Not true. All rs-232 signals float to 0 volts, which is undefined. They
> have to be actively pulled to at least plus or minus 5 volts or you don't
> have a valid signal.
Hmm, but under RS232 the physical levels are +25..+3 Volt and -25..+2
Volt, that's why RS423 is RS232 "compatible", i.e. RS423 has:
5 Volt = 1
0 Volt = 0 (like all good slow computing equipment)
RS232 has:
(25..3V) 12 Volt = 0
(-25..2.5V) -12 Volt = 1 (for signals, reverse 0 and 1 for controls)
so that writing the complement of the byte to an RS423 port gives you
the byte on an RS232 line. Thus floating to 0 Volt (as you commonly
do when electricity is off) gives you an RS232 1 (deassert) on the line,
___ ___ ___ ___ ___
which, being defined as RTS, CTS, DSR, DCD, or DTR (i.e. active low, which
means high), says OK, go ahead, send data, I am here.
But please, lets not get into a flame war on inverted logic, it really is
quite an esoteric subject.
Thomas
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