View Full Version : Should I use a resistor? If so what value?

03-22-2009, 01:45 AM
i am connecting a parallel port to my prop and i thought i might need a resistor. Do I? If i do should i use a 10k or something else?


03-22-2009, 01:48 AM
Your best best is to read the doc on how to interface the prop to 5V signals here, to avoid surprises:


Beau Schwabe
03-22-2009, 01:56 AM

Ale is correct, your Parallel port will typically be 5V so you need a level shifter of some kind. I would typically stay away from a single resistor solution unless the resistor value is fairly large only because with this solution, the ESD diodes that are built into the IO's are now part of your active circuit.

Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

Mike Green
03-22-2009, 02:00 AM
Everybody needs resistors. The large ones can be used for pony-tails or decorating dread-locks. They make nice decorative chains with their cute color bands. You can cut the wire ends and use those for jumpers on your home-made printed circuit boards. ...

If you're asking about electrically matching a standard parallel port to a Propeller's I/O pins, the issue is that a parallel port is based on +5V logic levels and a Propeller is based on +3.3V logic levels. If you connect a +5V logic high signal to a Propeller's I/O pin, the Propeller will be damaged. The +5V signal will go through a protective diode in the I/O pin circuitry to the +3.3V power bus on the Propeller and raise the voltage on the bus. That may burn out the protective diode and, if it doesn't burn out fast enough, the excess voltage will damage the Propeller chip further.

The protective diode is rated for around 500uA. With +5V applied, you need to drop around 1V in an external resistor to prevent damage. That's around 2K. People have used resistor values from 1K to 10K in this sort of situation.

03-22-2009, 12:41 PM
thanks for the replys guys. i installed 15k resistors(thats all i had)·in series so hopefully that will work

03-22-2009, 09:39 PM
Mike, mon... tanks for d'advice... me dread look de bomb!


"They may have computers, and other weapons of mass destruction." - Janet Reno

03-25-2009, 03:42 PM
So if I have a 15V input, it will be safe with a 27kohm resistor ?!

(15-3,3)/0,0005 = 23400

03-27-2009, 11:51 PM
nohab said...
So if I have a 15V input, it will be safe with a 27kohm resistor ?!

(15-3,3)/0,0005 = 23400
Yes, the formula is correct. Consider that 500uA is the absolute max limit (look at prop datasheet) and I suggest you a safety band of at least the tolerance of the resistor

03-27-2009, 11:54 PM
Thanks for your reply, sounds reasonable with a safety margin, I'll add at least 10% (probably more)