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Multiple Propeller Project Board USB communication interface questions — Parallax Forums

Multiple Propeller Project Board USB communication interface questions

dbritta1dbritta1 Posts: 36
edited 2013-03-02 15:47 in Propeller 1
Project has 4 Propeller USB boards seperated 1 to 8 feet in noisy environment.
Prefered wiring is heavy duty servo - 3 conductor twisted 22ga.
Will use "Full-Duplex Serial Driver" object to communicate between boards.

' Serial Comm Settings
BAUDMODE = %0000
BAUDRATE = 115_200
TIMEOUT = 10

QUESTION 1) What are appropriate series resistor values for Propeller IO pins?
QUESTION 1.1) Should resistors be placed at both ends of lines, or only at tx or rx pins?
QUESTION 1.2) Should by-pass capacitors be used, if so what value and where placed?

Comments

  • kwinnkwinn Posts: 8,697
    edited 2013-02-24 11:22
    How will these 4 boards be connected to the cable? In parallel (bus), point to point, star, or some other method?

    It sounds like you intend to connect all 4 on a bus without using any drivers but posting a wiring diagram would help.

    QUESTION 1) What are appropriate series resistor values for Propeller IO pins?

    A 91 ohm resistor on each serial I/O pin is enough to avoid damage to the pins for a worst case programming error (1 output high, 3 outputs low or vice-versa).

    QUESTION 1.1) Should resistors be placed at both ends of lines, or only at tx or rx pins?

    For twisted pair wiring a 120 ohm resistor is placed at each end of the cable but if you do this along with the 91 ohm series resistors the signal voltage may not be high enough to be detected.

    QUESTION 1.2) Should by-pass capacitors be used, if so what value and where placed?

    No bypass capacitors are needed on the communications bus.
  • dbritta1dbritta1 Posts: 36
    edited 2013-03-02 05:44
    kwinn,
    Thank you for information.
    Configuration is master comm board to multiple slave boards, seperate cable from master to each slave, not daisy chain.
    Master comm board handles com bus and wireless com.
    Slave boards handle sensors and effectors.

    QUESTION 2) Would either a pull-up or pull-down resisitor on com RX IO pins be appropriate to handle cable disconnect fault?
    QUESTION 2.1) If so, what would be appropriate resistor value, and on which side of 120 ohm series resistor should it be placed?
  • kwinnkwinn Posts: 8,697
    edited 2013-03-02 09:02
    dbritta1 wrote: »
    kwinn,
    Thank you for information.
    Configuration is master comm board to multiple slave boards, seperate cable from master to each slave, not daisy chain.
    Master comm board handles com bus and wireless com.
    Slave boards handle sensors and effectors.

    QUESTION 2) Would either a pull-up or pull-down resisitor on com RX IO pins be appropriate to handle cable disconnect fault?
    QUESTION 2.1) If so, what would be appropriate resistor value, and on which side of 120 ohm series resistor should it be placed?

    I would place a 91 ohm resistor between the TX pin and the cable and a 470 ohm pulldown resistor on the RX end. The 91 ohm series resistor will provide short circuit protection and the 470 ohm pulldown provides cable disconnect detection and some noise immunity. You may have to change the 470 ohms to a higher value (560 to 1000 ohms) resistor for reliable detection of the low to high and high to low transitions. If this does not work you may have to use line drivers/receivers, and I would suggest RS485 or CANbus drivers if that is necessary.
  • dbritta1dbritta1 Posts: 36
    edited 2013-03-02 15:47
    kwinn,
    Thank you for expert advice!
    Hopefully, drivers will not be necessary.

    Suggested circuit leaves RX pin unprotected, in case of cable or programming error, would a series resistor on RX pin be appropriate?

    What would be impact on recommendations?
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