piclist 1999\09\19\223337a >
Thread: rs-232 w/o level convert?
picon face BY : Mike Keitz email (remove spam text)

On Sun, 19 Sep 1999 16:54:51 +0100 Dan Rosenfeld
<drosenfeldspam_OUTspam.....SMARTMATTER.COM> writes:
> I'm also
> wondering
> whether I can get away with running this all at Vdd = 3.0V (thus the
> LF84)
> or will I need run at some higher voltage while debugging.

You can make a direct connection if the PC is going to accept the 0 to 3V
signal.  I haven't tried but I suspect that many will.  If you don't use
a level converter the PIC software needs to send out bits of opposite
polarity than if a level converter is used.  The serial out line of the
PIC should idle near Vss for a direct connection.  With a level converter
it would idle near Vdd, the level converter will convert that to a
negative voltage for the RS-232 line.

If your PC doesn't work with such a small voltage signal it would
probably be better to include a level converter circuit in the PC adapter
circuit rather than increase the PIC supply voltage.  The PIC circuit
would then be unchanged for debugging other than tapping off a 0 to 3V
logic line for serial output.

> I'm pretty clear on the PC side (terminal emulator app or basic
> program),
> but I'm not certain of the exact electrical connections on the pic
> side.
> I've seen mention of using anywhere from 1k to 1meg in series w/o
> tx/rx
> lines and along with some mention of pulldowns on these lines.

If you are transmitting only from the PIC no resistors are necessary.  It
may be a good idea to use a small resistor (100 ohms) in series with the
data line to limit current in case of a fault.  So you'd connect PIC
ground to PC ground and PIC pin (possibly through resistor) to PC data

> Also, do I need to do anything special with any of the other rs-232
> lines
> such as RTS/DTS, etc. or can I just let them float? (Assuming the pc
> software is configured correctly.)

Most PC software will can be set to ignore the handshaking lines, but
some would like them driven high.  A conventional way to do that is
connect CTS to RTS and DSR to DTR at the PC connector.  The PIC circuit
would just connect to two pins: ground and data in.

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<19990919.222244.-312989.1.mkeitz@juno.com> 7bit

See also: www.piclist.com/techref/io/serials.htm?key=rs%2D232
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