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Toddler with a PSC — Parallax Forums

Toddler with a PSC

Brian CraigBrian Craig Posts: 4
edited 2007-01-19 04:23 in Robotics
Well, for my first post, I need to express my frustration at trying to integrate the PSC with my Toddler.· This combination seemed like it would be a good way to offload some processing, but so far it has just given me a headache.· I think my first mistake was to ignore this line in the PSC·manual:
The power source for the servos must be a separate power source from that of the Board of Education Rev C.

I·assume that since the standard setup for the Toddler powers two servos, that the addition of the PSC would be trivial, so I attempted to power the PSC from the Vin·terminal on the toddler PCB (6 V).· With this configuration·(and the three conductor cable connected to Vdd, Vss & Pin 5), I could ONLY run the version and baud rate sample programs successfully with the PSC switch in the OFF position.· When I turn the PSC switch ON the PSC no longer reponds, though the green LED does continue to indicate communication is taking place.
Is my problem really the power source?· I really didn't want to add another 4 batteries to my toddler.· There's not alot of real-estate there, given that it has to balance itself.
Any advice would be appreciated.

Comments

  • PARPAR Posts: 285
    edited 2007-01-10 13:31
    Brian Craig said...
    ...
    The power source for the servos must be a separate power source from that of the Board of Education Rev C.
    I·assume that since the standard setup for the Toddler powers two servos, that the addition of the PSC would be trivial, so I attempted to power the PSC from the Vin·terminal on the toddler PCB (6 V).· With this configuration·(and the three conductor cable connected to Vdd, Vss & Pin 5), I could ONLY run the version and baud rate sample programs successfully with the PSC switch in the OFF position.· When I turn the PSC switch ON the PSC no longer reponds, though the green LED does continue to indicate communication is taking place.
    Is my problem really the power source?·...
    Do you mean, "...so I attempted to power the SERVOS from the Vin terminal..." ?
    I.e., which connector(s) on the Toddler board did you connect with which pins on the PSC board, in order to power the servos? (You stated, "Vin" of the Toddler, but what about the ground also?)
    My question begs the issue of "power source for the servos must be a separate power source from that of the Board of Education Rev C." Parallax may have specific reasons (e.g., exposure to voltage drops, noise, power capacity limitations?) why the sources must be "separate".
    PAR
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2007-01-10 16:42
    VIN on the Toddler Board is 6V typically, however the PSC requires a 5V power source. The 6V source can damage it. Please move the power connection for the logic to VDD. You could power the servos from the VIN supply.

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    Chris Savage
    Parallax Tech Support
  • Brian CraigBrian Craig Posts: 4
    edited 2007-01-10 17:46
    I misspoke (typed).· I connected VIN to the·positive (+)·screw terminal and·VSS to the negative (-) screw terminal, thereby powering the servos from 6VDC.· On the cable, I connected VSS to black, VDD to red and P5 to white, so the PSC itself is powered by 5VDC.

    Once the servos are powered on, the PSC seem to ignore all serial commands.
  • PARPAR Posts: 285
    edited 2007-01-10 18:49
    Brian Craig said...
    I misspoke (typed).· I connected VIN to the·positive (+)·screw terminal and·VSS to the negative (-) screw terminal, thereby powering the servos from 6VDC.· On the cable, I connected VSS to black, VDD to red and P5 to white, so the PSC itself is powered by 5VDC.

    Once the servos are powered on, the PSC seem to ignore all serial commands.

    OK. How about connecting the + and - (servo powering) terminals to a separate power source, to see if your PSC works (sends commands to the servos) then --as a test only.

    If it doesn't work then either, it suggests that it's not the source(s) of power as the problem.

    PAR

    ·
  • ZootZoot Posts: 2,227
    edited 2007-01-11 13:17
    You've probably got a ground loop.... the Vin (-) and Vss terminals on the PSC are connected on the board. If you connect the same ground to both sides, the PSC will probably not operate correctly (I've been through this with the PSC -- this is actually a problem to grapple with on any single power source project).

    I would connect the PSC as follows and see if you get better results:

    - Vin (if not higher than 6v) or Vdd to the V+ terminal on the PSC (terminal block)
    - gnd to the V- terminal on the PSC (terminal block)
    - Vdd to the +5v header (servo header)
    - Stamp pin to the signal header (servo header)
    - do NOT connect logic ground (Vss) to the servo header

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    When the going gets weird, the weird turn pro. -- HST
  • Brian CraigBrian Craig Posts: 4
    edited 2007-01-12 02:32
    Zoot said...
    You've probably got a ground loop....I would connect the PSC as follows and see if you get better results:

    - Vin (if not higher than 6v) or Vdd to the V+ terminal on the PSC (terminal block)
    - gnd to the V- terminal on the PSC (terminal block)
    - Vdd to the +5v header (servo header)
    - Stamp pin to the signal header (servo header)
    - do NOT connect logic ground (Vss) to the servo header

    Thanks for the suggestion, Zoot, alas this did not solve my problem.· I have the exact same behavior in this configuration.· I will be trying an independent power source next, but that will have to wait for a trip to Radio Shack (maybe this weekend).
  • ZootZoot Posts: 2,227
    edited 2007-01-12 03:03
    Somebody said...
    You've probably got a ground loop....

    Sorry. I probably should have written "possibly".

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    When the going gets weird, the weird turn pro. -- HST
  • hackman127hackman127 Posts: 34
    edited 2007-01-16 04:22
    Don't meen to hyjack the thread, but I am also having a problem with this, I believe. The way I have my PSC (USB) set up is dependant on having a common ground between the servo power and the computer. The computer itself controls other operations on the board that is utilizing the signal from the PSC, so there HAS to be a common ground. Is there a way around this?

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    Luke

    The perversity of nature is nowhere better demonstrated by the fact that, when
    exposed to the same atmosphere, bread becomes hard while crackers become soft.
  • hackman127hackman127 Posts: 34
    edited 2007-01-16 05:47
    So after some reconsideration and re-evaluating my design, I have found that I did in fact have a ground loop problem, but it was not with there being an independent ground for the servo power and the USB power. I did find that they do in fact have a common ground on the board, and I found that my ground loop problem was in my power supply. Ooops!

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    Luke

    The perversity of nature is nowhere better demonstrated by the fact that, when
    exposed to the same atmosphere, bread becomes hard while crackers become soft.
  • Brian CraigBrian Craig Posts: 4
    edited 2007-01-19 04:23
    I guess the moral of the story is to follow the instructions.· I was able to get my PSC working by adding an independent power source (an additional 4 AA batteries).· It seems to work fine, but now I have to find a place to put the additional battery pack.

    I am fairly certain that my problem is basically a lack of current.· I will be experimenting with other configurations, and will report back my findings in this thread.· Thanks to all those that posted suggestions!
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