I don't think this scenario is possible if the resistance between pairs is
nominal. When the cable is disconnected from the controller, the motors are
completely isolated from one another (provided there are no shorts in the
cable.) So, if one member of a pair was connected to one motor, and the
other member of the pair was connected to another motor, the resistance
would be infinite.
FYI, each SteppIR motor has four wires, one pair for each of the two
windings. If a pair is accidentally reversed, the motor will run backwards.
If both pairs are reversed, the motor will run normally. If the pairs are
crossed, the driver board will be damaged. None of these miswiring scenarios
is likely because it was reported that the individual motors are operating
If the resistance checks were not done properly, and some pairs are really
showing infinite resistance, then the scenario you suggest may come into
play. But I don't know if a motor would move if only one pair was energized.
SteppIR motors with H-bridges are complicated beasts.
BTW, it's relatively easy to wire a SteppIR cable, but you have to be very
careful when identifying the pairs in 16-conductor cable. Seven of the pairs
have a black wire. The only way to tell which is which is by the color of
the wire with which it is twisted. The twists come undone very easily when
the jacket and shield are removed, so SteppIR puts tiny rubber bands at the
base of each pair to make sure you can keep them together. I had SteppIR
make short 15' pigtails to run from my controller to the utility box with
suppressors on the side of the house, and bought bulk cable for the rest of
the runs to my stack, each of which is interrupted by suppressors at the
base of the tower. When I stripped each cable, I immediately put a little
strip of electrical tape around the base of each pair.
I'll have one more comment to make about the problem in another post...
73, Dick WC1M
> -----Original Message-----
> From: Robert Chudek - K0RC [mailto:email@example.com]
> Sent: Monday, June 11, 2007 9:53 PM
> To: TowerTalk@contesting.com
> Subject: Re: [TowerTalk] SteppIR cable length
> In an earlier message, Jerry, K3BZ wrote:
> "Although it might not pinpoint exactly why, it would at least indicate
> source of the problem is the cable. Since we have several customers
> successfully using even longer cables, I think that might be sufficient
> justify cable replacement."
> While I was out planting trees today, this problem was continuously
> running through my mind... why does running 3 EHU's work fine, but
> adding the 4th create a problem? After I was finished outside, I
> reviewed the message thread for clues. Then I saw the above message
> from Jerry stating that even "longer" cables have worked fine at other
> installations. He didn't say how long, but his statement basically
> rules out this 500 foot run as being the first installation with this
> length of cable and creating some unforeseen engineering problem. I
> hereby retract my original statement regarding engineering this problem
> at the factory!
> So... while digging in the dirt, a lightbulb of logic snapped on. I
> really believe there is indeed a "mis-wired" twisted pair in that 500
> foot cable. Here's my logic:
> The problem only occurs when the final EHU is installed. For
> simplification, lets say wires 5&6 (a pair) run EHU 3 and wires 7&8
> (another pair) run EHU 4. If the cable is cross-wired it would use
> wires 5&7 for EHU 3, this is most likely working "good enough" even
> though those two wires are NOT grouped into a "pair". But when current
> is applied to wires 6&8 (not a pair) to run EHU 4, now you have
> contention within the two legitimate twisted pairs. The pulses driving
> EHU 3 will be coupling with the pulses driving EHU 4 and vis-a-vis.
> Like Jerry originally stated, I also highly suspect the cable run. Not
> necessarily the cable, the conduit, or the length, but the high
> probability of a cross-wired pair of conductors.
> 73 de Bob - KØRC in MN
> P.S. This is all theory on my end... I have never bought, installed,
> used, or even seen a SteppIR in person. But my next antenna will be a
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