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ctx32
08-11-2008, 01:39 PM
Is there anyway to bypass the 6v. power for the other 3 servos?
I'd like to have a central power connection for the ground and the power.
Will the signal still travel through the controller by the white wire without the standard
power connection?

Also,
What is the longest USB line that can be used between the computer and the controller?
Thanks
Calvin

Alex
08-11-2008, 02:18 PM
Yep, you can power the servos directly (ie. bypassing the board) and the control signal will still go to the servos.

Here's a link that talks about it a bit:

http://forums.trossenrobotics.com/showthread.php?p=4110&highlight=servo+power#post4110

Alex
08-11-2008, 02:23 PM
Whoops, didn't see your second question:)

USB cables shouldn't exceed ~15ft. I'm sure Jodie or someone can elaborate more on this from a hardware perspective.

In the past, if we've needed longer distances than this, we'd use USB Repeaters, or USB-RJ45 extenders:

http://www.trossenrobotics.com/usb-serial-cables-hubs.aspx

However, depending on what you're doing, I'd be careful. Having an R2D2 rolling around off of a >15ft USB cable can be quite dangerous vs. a simple RFID reader app which has no movement and is only transmitting a small amount of data.

ScuD
08-11-2008, 02:34 PM
Max size of the cable depends on the type of USB used.
Low-speed, high-speed, full-speed.

Either way, the reasons for keeping the wires short remain the same.
Each conductor has a capacitance. No way around it. Period.
So if you're transmitting a high-speed digital signal over this wire, each voltage transition will 'charge' this capacitance, creating a slope in the transition.
The faster the signal, the more the slope starts playing a role. If the time between the transitions becomes shorter, but the capacitance remains the same, it will shorten the time the signal is actually at it's proper level.

Plus, each conductor picks up stray signals.
The power lines in your house, an overhead fluorescent light, the WIFI router, etc etc which increases the noise on the signal, lowering the SNR (signal-to-noise ratio).
The longer the wire, the more surface to absorb these stray signals.

ctx32
08-11-2008, 02:45 PM
My USB lines will probly be about 5 feet in the legs at most.
So hopefully this is'nt a problem.

On the servo power:
I see from the other post it's.
Signal wire from the servo controller to the servo - Ground wire from the controller to the servo tied into the power supply ground - Power (+) from the power supply to the servo only.

Would a large 6v. battery (lawnmower size or UPS size) be enough to run say 8 servos without damaging them?
Calvin

Alex
08-11-2008, 03:45 PM
so long as the battery's voltage is within range of the servos, then you're good. If you're servo's specs say "max of 6v", don't hook them up to anything over a 6v battery.

Adrenalynn
08-11-2008, 09:56 PM
Yup - as Alex noted - you can separate the red and black wire out of the servo connector and just run the yellow (signal) with power coming from elsewhere.

The USB->RJ45 that Alex mentioned are USB Baluns. They use twisted pair to lower the noise and cancel some of the charge ScuD mentions - they also use a balanced signal much like commercial microphones. I've run USB Baluns out 300+ feet - further at USB 1.1. In fact, we have some in the office rated to 25km!!!

Alex
08-12-2008, 08:23 AM
Just to add to Jodie's reply, Phidgets are 1.1 USB devices, so you should be able to run the cable out pretty far:)