DPRG mailing list archives: [DPRG] Serial Woes

Sat Feb 3 18:16:48 CST 2007

Hey Eric,

>When I attach the servo controller to the serial adapter, the level
of the servo pulses going
> to my MD22 drops to about 2.5V and is really noisy.

Ding!  Ding!  Ding!  Hear that alarm bell going off?

I think you're right about the ground problem.  There have been lots
of times that I've pulled hair out trying to debug a digital problem
that turned out to be (usually) an open ground connection somewhere.
(Like a chip doesn't have a ground - but it will KIND of work).

The alarm bell is "noisy signal". We tend to poke around with an
o'scope probe looking at voltages, but we seldom check grounds.  If
you can, only ground the RS232 line at one end or the other.

 Make sure you've got good grounds at all communication chips.  It
might be a ground loop, but at these short lengths, I bet something
isn't earthed.

-Jeff

-Jeff

On 2/3/07, Eric Sumner <kd5bjo at gmail.com> wrote:
> > Does taht seem to amke sense?
>
> Yes, but I have some new data that demonstrates that there are some
> really funny interactions going on here.  When I attach the servo
> controller to the serial adapter, the level of the servo pulses going
> to my MD22 drops to about 2.5V and is really noisy.  The problem stays
> there until I disconnect the serial line and power-cycle the MD22.  I
> can't remember if I also have to reset the servo controller.  These
> symptoms also occur if I don't connect the serial line proper, but
> just the power lines.  I think I may have a ground loop through the
> serial port on the computer.  I'm going to try cutting the ground
> connection between the servo controller and the serial adapter and see
> what happens.  If that doesn't work, I'll be all out of ideas.
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