Question about TTL serial inverted versus non-inverted

Don M

Does the non-inverted serial sit high when idle?

Phil Pilgrim (PhiPi)

Yes, it idles high ("mark" state).

Don M

Ok then what I'm looking at is an inverted signal. It sits low when idle.

So how simple of a circuit can be made to invert the signal? I assume I can just use a transistor? I'm trying to sniff some serial data at 9600 baud. I have available 3.3V and ground. The serial data I'm sniffing is 5V level but I want to invert it and feed it into 3.3V sniffer. The sniffer is a Logomatic V2.

Can someone offer a simple schematic drawing to accomplish this?

Don M

Would this work?

And if so what would be some values for the resistors?

kwinn

Somewhere in the neighborhood of 10K for R1 and 1K for R2 should work just fine.

Duane C. Johnson

Hi Don;

Don M wrote: »

Would this work?

And if so what would be some values for the resistors?

Yes it would.
10KΩ for both would work nicely.
You should add a diode across the base and emitter. Anode to the emitter and cathode, banded end, to the base.
I know you you are running on only 5V, (which would not require the diode),
but
you might want connect to a full spec RS-232 input which can have + - 25V levels.
This would then breakdown the emitter base junction. Some transistors don't take kindly to this.
The diode protects this junction.

Duane J