OOPIC Forum

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Saturday, August 4, 2007

Re: [oopic] Re: a bit off topic maybe - XBee units... ground ? grrr

I always connect the ground probes - In any of my descriptions up to
this point (as hard to decipher they appear to be for anyone but
myself) I have done this ...

"input to the TX XBee and the output of the RX XBee", I assume you
were comparing signals on the 2 different modules at the same time."

yep, exactly that ...

"This will only work properly with the probe gnds connected to the
individual boards, which will electrically connect the gnds of the 2
boards together."

That was my original point - It doesn't appear to do this ... I had to
physically connect them at the circuit - a bit of wire from one
breadboard to the other for it to work... - After further fiddling and
swapping the probes from input A to B I think one of the probes is at
'fault' (either that or I am)...

I have put both probes on the same pin of the RX (receiving XBee) and
connected the gnds to the battery power '-ve' that is powering the
same unit - I get the same signal on both scope probes (as expected) -
yet when I disconnect the ground on the 'fluke' brand probe both
signals go whacky (independent of whatever Input it is in) - if I
disconnect the 'phillips' brand probe both signals are still good
(regardless of it now being an 'antenna')

- I reckon however that further discussion is just going to get me
confused or at least you a little unfairly fatigued until I read up on
my scope basics ... At first I thought it was the XBee's but its
really my misuse or misunderstanding of scopes (or a potential fault
in the probe, unlikely though)

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[oopic] Re: a bit off topic maybe - XBee units... ground ? grrr

--- In oopic@yahoogroups.com, "Nick Mulder" <nick.mulder@...> wrote:
>
> Yep,
>
> Ok - I think you are both correct - connecting up my DMM I get the
> expected results ... (although not quite as visually as i would
like)
> - derivatives huh, interesting and I understand what you are saying
> but for now I'll put it down to silly me and my interpretations of
> visual data translated into text, like you said you really have to
be
> here...
>
> I think I need to go to scope school
>
> ... I am using a Phillips PM3050 here and have never had the manual
or
> any instruction, just sort of picking up bits of enlightened 'oh! ok
> thats what that does' here and there ...
>
> Its has two probes which was helpful as I could look at the input to
> the TX XBee and the output of the RX XBee and see what was happening
> to the pulse (a slight phase shift in case you're interested which
can
> be easily accounted for in the mechanical side of the wotsit I'm
> building) ...
>
> So when the ground potentials of both trace's are equal I get good
> scopin results - TX or RX gnd - by using the RX ground I could then
> confirm what the DMM had done already anyhoo...
>
> When using the two separate ground potentials for both traces the TX
> trace that was using a benchtop power supply gave an ok scope (same
> gnd as the AC for the scope hmmm) yet the battery powered one went
> screwy ... Yet using the battery ground on both traces gave ok
> results on both traces (likewise both on the TX/benchtop as already
> discussed).
>
> There is likely some sort of lesson to be learned from this ?
>

Yeah, first off, I'm not sure what you just said. Secondly, whenever
you connect a scope probe to anything you also MUST connect the gnd
wire of the probe. Otherwise it's just an antenna.

Where you indicated above "input to the TX XBee and the output of the
RX XBee", I assume you were comparing signals on the 2 different
modules at the same time. This will only work properly with the probe
gnds connected to the individual boards, which will electrically
connect the gnds of the 2 boards together.

> I often jump into things a little fast and then get caught up on
> relatively simple 'problems' that in reality is just the gear doing
as
> it should ...
>
> Thanks as always for being my online tutors ! I hope I'm not
taking advantage
>
> in case anyone is interested:
>
> http://www.maxstream.net/blog/2007/07/03/filming-dog-races/
>
> Really need to get them going with serial transmission proper and
not
> this line-passing business - still got to learn to crawl
beforehand...
>
> cheers!
> nick
>

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Re: [oopic] Re: a bit off topic maybe - XBee units... ground ? grrr

Yep,

Ok - I think you are both correct - connecting up my DMM I get the
expected results ... (although not quite as visually as i would like)
- derivatives huh, interesting and I understand what you are saying
but for now I'll put it down to silly me and my interpretations of
visual data translated into text, like you said you really have to be
here...

I think I need to go to scope school

... I am using a Phillips PM3050 here and have never had the manual or
any instruction, just sort of picking up bits of enlightened 'oh! ok
thats what that does' here and there ...

Its has two probes which was helpful as I could look at the input to
the TX XBee and the output of the RX XBee and see what was happening
to the pulse (a slight phase shift in case you're interested which can
be easily accounted for in the mechanical side of the wotsit I'm
building) ...

So when the ground potentials of both trace's are equal I get good
scopin results - TX or RX gnd - by using the RX ground I could then
confirm what the DMM had done already anyhoo...

When using the two separate ground potentials for both traces the TX
trace that was using a benchtop power supply gave an ok scope (same
gnd as the AC for the scope hmmm) yet the battery powered one went
screwy ... Yet using the battery ground on both traces gave ok
results on both traces (likewise both on the TX/benchtop as already
discussed).

There is likely some sort of lesson to be learned from this ?

I often jump into things a little fast and then get caught up on
relatively simple 'problems' that in reality is just the gear doing as
it should ...

Thanks as always for being my online tutors ! I hope I'm not taking advantage

in case anyone is interested:

http://www.maxstream.net/blog/2007/07/03/filming-dog-races/

Really need to get them going with serial transmission proper and not
this line-passing business - still got to learn to crawl beforehand...

cheers!
nick

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[oopic] Re: a bit off topic maybe - XBee units... ground ? grrr

--- In oopic@yahoogroups.com, "rtstofer" <rstofer@...> wrote:
>

Yeah, from the description, like Richard said, it sounds like the
scope ground is not set properly, at least in one case, and sounds
like the scope is set to AC coupling in the other. Scope gnd needs to
be hooked directly to the board being probed.

> --- In oopic@yahoogroups.com, "Nick Mulder" <nick.mulder@> wrote:
> >
> > Hiya,
> >
> > I've set up two XBee pro units to do digital line passing - so
the pin
> > states are mirrored between the remote TX unit and the base RX
unit..
> >
> > In setting up the units I had them on the same power supply and
ground
> > potential and had them working fine passing a %50 cycle 25Hz
digital
> > pulse (3.3v) between themselves - Well, the purpose of having
wireless
> > is so you don't have to use the same power and ground potential
isn't
> > it...
> >
> > So I set the RX unit up on its own battery power supply and sent
it on
> > its merry way to the other end of the desk all by its
lonesome ... Now
> > on the scope its giving a sine-ish wave output on its output pin
(the
> > mirrored one) at about 20Hz-ish when idle and when I send my 25Hz
> > pulse its the same 20Hz-ish wave with little spikes where there
should
> > be a digital transition from high to low or vice-versa ...
> >
> > However ... this glitch disappears if I connect the units to the
same
> > ground potential.
> >
> > If I connect the two grounds by a 10MOhm resistor I get a kind of
> > 'half-problem' where I get my digital pulse at 25Hz but modulated
by
> > the 20Hz-or-so pulse ...
> >
> >
> > What am i doing wrong ? I have used the XBee's for line passing
> > successfully beforehand - no idea where this glitch is coming from
> >
> > prob something really beginners ?
> >
> > nick
> >
>
> It's nearly impossible to help with something like this without
being
> able to actually see what is going on. But, it sounds like a scope
> probe grounding issue to me.
>
> The scope chassis and probe ground lead are connected to safety
> ground. If you probe the signal without connecting the ground lead
> you can get strange results. If you do connect the lead, you are
> changing the ground reference for the device but that's the nature
of
> using a scope.
>
> That you are getting spikes at the rising and falling edge of the
> pulses makes me believe you are seeing the derivative of the pulse.
> This occurs when you have a capacitor between the signal and the
scope
> input or when the scope is set to AC coupling. Not connecting the
> probe ground can give the same results.
>
> Richard
>

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[oopic] Re: a bit off topic maybe - XBee units... ground ? grrr

--- In oopic@yahoogroups.com, "Nick Mulder" <nick.mulder@...> wrote:
>
> Hiya,
>
> I've set up two XBee pro units to do digital line passing - so the pin
> states are mirrored between the remote TX unit and the base RX unit..
>
> In setting up the units I had them on the same power supply and ground
> potential and had them working fine passing a %50 cycle 25Hz digital
> pulse (3.3v) between themselves - Well, the purpose of having wireless
> is so you don't have to use the same power and ground potential isn't
> it...
>
> So I set the RX unit up on its own battery power supply and sent it on
> its merry way to the other end of the desk all by its lonesome ... Now
> on the scope its giving a sine-ish wave output on its output pin (the
> mirrored one) at about 20Hz-ish when idle and when I send my 25Hz
> pulse its the same 20Hz-ish wave with little spikes where there should
> be a digital transition from high to low or vice-versa ...
>
> However ... this glitch disappears if I connect the units to the same
> ground potential.
>
> If I connect the two grounds by a 10MOhm resistor I get a kind of
> 'half-problem' where I get my digital pulse at 25Hz but modulated by
> the 20Hz-or-so pulse ...
>
>
> What am i doing wrong ? I have used the XBee's for line passing
> successfully beforehand - no idea where this glitch is coming from
>
> prob something really beginners ?
>
> nick
>

It's nearly impossible to help with something like this without being
able to actually see what is going on. But, it sounds like a scope
probe grounding issue to me.

The scope chassis and probe ground lead are connected to safety
ground. If you probe the signal without connecting the ground lead
you can get strange results. If you do connect the lead, you are
changing the ground reference for the device but that's the nature of
using a scope.

That you are getting spikes at the rising and falling edge of the
pulses makes me believe you are seeing the derivative of the pulse.
This occurs when you have a capacitor between the signal and the scope
input or when the scope is set to AC coupling. Not connecting the
probe ground can give the same results.

Richard

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Friday, August 3, 2007

[oopic] a bit off topic maybe - XBee units... ground ? grrr

Hiya,

I've set up two XBee pro units to do digital line passing - so the pin
states are mirrored between the remote TX unit and the base RX unit..

In setting up the units I had them on the same power supply and ground
potential and had them working fine passing a %50 cycle 25Hz digital
pulse (3.3v) between themselves - Well, the purpose of having wireless
is so you don't have to use the same power and ground potential isn't
it...

So I set the RX unit up on its own battery power supply and sent it on
its merry way to the other end of the desk all by its lonesome ... Now
on the scope its giving a sine-ish wave output on its output pin (the
mirrored one) at about 20Hz-ish when idle and when I send my 25Hz
pulse its the same 20Hz-ish wave with little spikes where there should
be a digital transition from high to low or vice-versa ...

However ... this glitch disappears if I connect the units to the same
ground potential.

If I connect the two grounds by a 10MOhm resistor I get a kind of
'half-problem' where I get my digital pulse at 25Hz but modulated by
the 20Hz-or-so pulse ...

What am i doing wrong ? I have used the XBee's for line passing
successfully beforehand - no idea where this glitch is coming from

prob something really beginners ?

nick

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