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propeller usb servo controller and cat5 — Parallax Forums

propeller usb servo controller and cat5

GumbaGumba Posts: 14
edited 2011-09-05 09:03 in Propeller 1
I'm trying to use the 'newer' USB servo controller board to run servos (or even one servo) via Cat5. Power is 6.6 v, 8 amps. Servo works when I connect it directly to the board, but not via the Cat5. Good, solid connections, voltmeter shows good current, and I've seen many, many posts on the web about this being done, at least through other servo controller boards. My Cat5 cable run is between 15 and 20 feet. I've seen posts for 75 feet or more.

Thinking it was a power problem I tried to use speaker wire to run positive and ground and the Cat5 just for signal. No joy.

Any idea if the propeller chip (or something else on the board) used in the Parallax USB servo controller would restrict how far the signal can travel along a thinner wire like a Cat5? Or a limit along wire of any size?

If so, anyone know the distance limit?

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2011-08-31 20:59
    I think the problem you're seeing is due to the signal buffer chips (TXB0108) used on the controller board. They're bidirectional and sometimes get confused when their load is unusual. Try a resistor in series between the controller board and the CAT5 cable. Try 2.2K for a start. I'm not sure what's the best value.
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2011-08-31 23:14
    Gumba,

    Are you sending power over that distance to the Servo, or are you powering the servo locally?

    I have achieved 10 feet, with the servo powered over the CAT-5 but you need to use a largish capacitor (1000uF) as close to the servo power as possible. You also need to buffer the signal to the servo with a 4.7K resistor in series with the signal leaving the Propeller.
  • GumbaGumba Posts: 14
    edited 2011-09-01 05:09
    Beau Schwabe (Parallax) wrote: »
    Gumba,

    Are you sending power over that distance to the Servo, or are you powering the servo locally?

    I have achieved 10 feet, with the servo powered over the CAT-5 but you need to use a largish capacitor (1000uF) as close to the servo power as possible. You also need to buffer the signal to the servo with a 4.7K resistor in series with the signal leaving the Propeller.

    Thank you both for the replys. I am powering servo through the Cat5, which is connected to a standard servo connector plugged into the board. The servo power source is plugged directly into the board as well, in the pos/neg inputs near the power switch.

    I've found that with certain brands of servos I've had to add a resistor to make it work at any wiring distance--other servos don't need this addition (for short runs anyway). I don't think I've used 4.7k, though. So, Mike, I have a resister in line, but near the servos, not between the controller and the 'start' of the Cat5 connection.

    Beau, what's the limit for volts on the 1000uf capacitor? And when you say that the capacitor needs to be as close to the power as possible, do you mean in-line between the power source and the servo power inputs that are on the board?
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2011-09-01 07:22
    Gumba,

    For a good rule of thumb, the capacitor voltage should be rated for at least twice the operating voltage.

    As far as connections... there are a couple of options. There is one where a series resistor on the signal line limits or attenuates any stray signals from entering the micro processor which will work in most cases. A second option creates a transmission line between point A and point B where the input load at point A closely matches the output load at point B. The second solution is probably best for noisy environments having the best noise immunity from the wire length becoming an antenna.
    Servo Attenuation Line.jpg
    1024 x 455 - 57K
    Servo Transmission Line.jpg
    1024 x 472 - 62K
  • GumbaGumba Posts: 14
    edited 2011-09-02 05:26
    Thanks. I will try this. In the meantime, do you know what is the maximum wiring distance (from board to servo, power to servo through the on-board inputs) that I can expect without adding these components?
  • GumbaGumba Posts: 14
    edited 2011-09-04 07:18
    To answer my own question, I've found by trying different size wires that you can get a reliable max run of about one foot prior to adding the capacitor and resistor recommended by Beau. I got a distance of two feet from one micro servo. In each case the only thing I got to work were servo extension wires---I got either jitters or no movement at all from Cat 5 and various gauges of wire up to 16 gauge speaker wire.
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2011-09-04 09:39
    Gumba,

    So how long have you been able to get with either of the circuit mods? -just curious.
  • GumbaGumba Posts: 14
    edited 2011-09-05 09:03
    Haven't tried them yet--trying to get a project finished...but will as soon as I get a chance. Will report back.
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