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Pololu Jrk 21v3 interface with Propeller questions — Parallax Forums

Pololu Jrk 21v3 interface with Propeller questions

dbritta1dbritta1 Posts: 36
edited 2013-03-02 15:51 in Propeller 1
Propeller IO is 3.3v and Jrk IO is "TTL"

"If the jrk is in serial input mode, then its serial receive line, RX
can receive bytes when connected to a logic-level 0 to 4.0 - 5.0 V or TTL non-inverted serial signal.
The jrk provides logic-level (0 to 5 V) serial output on its serial transmit line, TX.
The data format is 8 data bits, one stop bit, with no parity, which is often expressed as 8-N-1."

Byte communication issues are addressed in following:
http://forums.parallax.com/showthread.php/123918-Serial-coms

Single Propeller Project Board USB is used to control multiple Jrk 21v3 motor controllers.
Wiring distance between boards is 2 to 8 feet in noisy environment.

QUESION 1) What are interface circut requirements to connect Propeller Project Board USB to Pololu Jrk 21v3?
QUESTION 1.1) Are logic level converters required, or can passive components be used?
QUESTION 1.2) If level converters are required, what would be appropriate circut?
QUESTION 1.3) If only resistors and capacitors are required, what are appropriate values and where should they be placed?

Comments

  • dbritta1dbritta1 Posts: 36
    edited 2013-03-02 06:15
    Please help!

    I have built many multi-voltage systems, typically 24v,12v, 7.2v, 6v and 4.8v mixes, but this is first time interfacing Propeller to Pololu logic levels.
    Have used Jrk controllers in open-loop PWM mode, however need to com with Jrk to create closed-loop haptic servos.
    Propeller docs are very good, Pololu docs good for software com protocol and IO pin assignments, but do not reveal hardware IO circuit.

    Does anyone know the interface circuit requirements between Propeller and Pololu Jrk IO pins?
  • Bill HenningBill Henning Posts: 6,445
    edited 2013-03-02 06:50
    Put a 4.7K resistor in series with the jrk's output; you don't need one for its input, however if you do add one, you will protect against accidental rx/tx reversal.

    There is a quad serial object in Obex.

    Make sure you use pins in the range of P0..P27 so you don't interfere with the USB/serial chip and the I2C bus; also stay clear of the VGA and uSD pins if you have populated them on your board.
  • dbritta1dbritta1 Posts: 36
    edited 2013-03-02 07:37
    Bill,
    Thanks for info.
    Also, would like to handle cable faults and floating input pins.

    In addition to 4.7k series resistor on Propeller RX pin, would a pull-up or pull-down resistor be appropriate?
    If so, what value?

    Also, considering series resistor to protect Propeller TX pin what value would be appropriate?
  • Mike GreenMike Green Posts: 23,101
    edited 2013-03-02 09:07
    A pullup resistor, perhaps 100K would protect against floating input including an open cable. Use the same (4.7K) resistor value to protect the Propeller's Tx pin. This assumes that the two devices (Propeller and Pololu) are close to each other so there's not a lot of capacitance on the line between them. I'd actually recommend a smaller resistor, maybe 2.7K. The issue here is that 5V devices feeding a 3.3V device input will cause the protective diode in the 3.3V device to conduct and the current through the diode has to be kept under 0.5mA. That's 5V - 3.3V - 0.7V (Vf of diode) = 1V plus 20% worst case = 1.2V. 1.2V / 0.5mA = 2.4K. I'd use a 2.7K resistor.
  • dbritta1dbritta1 Posts: 36
    edited 2013-03-02 15:51
    Mike,
    Thank you for expert advice!
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