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Inter propeller communication hardware — Parallax Forums

Inter propeller communication hardware

agimuhingagimuhing Posts: 39
edited 2011-01-26 14:51 in Propeller 1
What do you need to hardwire two props together?

Do you need protection resistors between the two props or is directly hooking the IO pins together okay?

This is all taking place on a breadboard

Comments

  • JonnyMacJonnyMac Posts: 9,208
    edited 2011-01-25 19:04
    Putting a resistor between pins will keep an accident (one pin driven low, the other driven high) from doing possible damage.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2011-01-25 20:34
    If you want your Propellers to be more than a few feet apart, you might want to use a chip to communicate with RS-232 levels or use a RS-485 (I think those are the right numbers) chip.

    JonnyMac wrote an article a while back about how to use RS-485 communication. I now use it (RS-485) to communicate with my (Propeller based) sprinkler controller in the garage (from my lab in the next room). From what I've read, RS-485 is more resistant to electronic noise than some other protocols. Anyway, I think it's pretty cool and surprisingly easy (and inexpensive).
  • JonnyMacJonnyMac Posts: 9,208
    edited 2011-01-26 09:48
    with my (Propeller based) sprinkler controller

    That's interesting -- I used to work for The Toro Company as Sr. Product Manager for sprinkler controllers.
  • agimuhingagimuhing Posts: 39
    edited 2011-01-26 09:53
    JonnyMac wrote: »
    Putting a resistor between pins will keep an accident (one pin driven low, the other driven high) from doing possible damage.

    Which value of resistor should I use?



    And the Prop chips are about an inch apart
  • Duane DegnDuane Degn Posts: 10,588
    edited 2011-01-26 10:18
    An 100 ohm resistor is enough to keep it save. It would limit the current to about 33mA.

    I think you could also use higher values (I'd think 10K would still work).

    I think higher reistor values will limit the maximum communication speed (baud).

    Duane
  • JonnyMacJonnyMac Posts: 9,208
    edited 2011-01-26 10:55
    The resistor size will only limit current, not transmission speed. I typically use 2.2K (I have them for interfacing 5v circuits to the 3.3v Propeller).
  • Duane DegnDuane Degn Posts: 10,588
    edited 2011-01-26 14:51
    Jon,
    The resistor size will only limit current, not transmission speed.

    Wouldn't the resistor effect transmission speed some? I've been reading the book Serial Port Complete. It mentions several times that higher capacitance wire limits the slew rate (and the bit rate). I couldn't find any mention in the book to indicate a series resistor would slow things down but I'd think the wire would act as a RC circuit. The transmit driver has to charge the (very small) capacitor (wire). I'd think a greater resistance before the wire would make charging the wire take longer. So wouldn't using a large valued resistor cause problems for baud rates above a couple of Mbps?

    I know from experience the Prop can communicate fine at 1 Mbps through a 10k ohm resistor. I'm not sure (if my theory is correct) at what values a resistor would limit the communication rate.

    I did find a statement in Serial Port Complete that casts doubt on my theory. On page 217:
    A line's series resistance has little effect on the characteristic impedance at high frequencies, . . .

    It does say "little effect" not "no effect" so maybe it matters some?

    I've been wondering about this for a while. I saw a post on this forum not too long ago that stated (the post was about a Prop to Prop communication object) if one were to remove the resistors between the two Props, the communication rate could be increased.

    BTW, I had recently read that you had worked for Toro. I thought it kind of funny that I used what I learned from you to add RS-485 communication on my homemade sprinkler control board. So you (indirectly) helped design my sprinkler controller. Thanks.

    Duane
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