Pulse train using a FP-CTR-502

[Martien Janssen]

Hai,

I am trying to generate a continuous pulse train of varying duty cycle using
a Fieldpoint Counter module 502. I have got a description how to do this by
using the FP explorer (This description is included in the NI documentation
of the FP counter module) . I have created a Labview program which generates
the necessary instructions for the FP module and it all works. However, to
my big surprise I only get certain duty cycles where the off-period length
must be a multiple of the on-period length. This limits the duty cycle to
discrete values: 50 %, 33 %, 25 %, 20 %, 16.6 % etc. This seems to be by
design, though there is no warning whatsoever in the NI instructions.

Does anyone have a proper idea (or possibly code) how to get a varying
duty
cycle pulse train from such FP counter module ?

Thanks
Martien

[Sean_C.]

Hello Martien,

Thank you for contacting National Instruments.

The behavior you are observing is expected. This is the only to vary the duty cycle of a pulse train with this module.

You might want to consider using the (c)FP-PWM-520 which has 12-bit duty cycle resolution (4,096 discreet values from 0 to 100 percent).

You could also use the (c)FP-PG-522. This module is limited in the resolution of the duty cycle as well. To set the duty cycle, you specify the on time and off time of the pulse.

Please let me know if you have further questions.

Sincerely,

Sean C.
National Instruments
Applications Engineer

[Martien Janssen]

Dear Sean,

Thanks for your reply. I am disappointed that in the description of the
example for pulse width modulation, I could not see any warning or hint for
this effect to occur. I confess, that it took me also quite some time to
understand the example properly and to figure out this problem.

I am aware of the PWM-520 and I guess I will have to go for it.
Nevertheless, since I have already built in the 502 in an application and I
have some space limitations, I am still puzzled why it is not possible to
run with varying duty cycles with the 502 module. After all, the unit has 8
counters, 4 inputs and 4 output channels, all of which can be linked to each
other. Therefore I could imagine that a first counter counts the internal
clock (e.g. 1 kHz) f
or the duration of the off-period. Then it could trigger
a second counter for the duration of the on-period. I can't figure out how,
but I could imagine that with proper uses of output channels and input
gating (or even external wiring from outputs to inputs) it could be done ?
The bottom line is, are you 100 % sure that it is not possible with the 502
? Than I guess I will not try to loose my time on it.

Thanks again,

Martien






"Sean C." wrote in message
news:50650000000500000076C30100-1079395200000@exchange.ni.com...
> Hello Martien,
>
> Thank you for contacting National Instruments.
>
> The behavior you are observing is expected. This is the only to vary
> the duty cycle of a pulse train with this module.
>
> You might want to consider using the (c)FP-PWM-520 which has 12-bit
> duty cycle resolution (4,096 discreet values from 0 to 100 percent).
>
> You could also use the (c)FP-PG-522. This module is limited in the
> resolution of the duty cycle as well. To set
the duty cycle, you
> specify the on time and off time of the pulse.
>
> Please let me know if you have further questions.
>
> Sincerely,
>
> Sean C.
> National Instruments
> Applications Engineer

[JaredA]

Hello Martien,

Sean is correct on this one. The operating instructions for the (c)FP-CTR-502 also clarify this.

Post back if you have any other questions or comments.

Jared A

[Martien Janssen]

Thanks Jared and Sean,

I agree with both of you. It does not seem possible with the counter unit.
My only point of criticism is that the documentation qouted by Jared below
does simply mention that you can create a pulse train with varying duty
cycle. It does not mention that this duty cycle is seriously restricted. If
you dig into the formula's you can indeed find this limitation, but an early
warning could be helpful for users who need pulse width modulation. I am now
planning to order the PWM-520 which should do the job.

Thanks again,

Martien

"JaredA" wrote in message
news:5065000000050000006AC40100-1079395200000@exchange.ni.com...
> Hello Martien,
>
> Sean is correct on this one. The operating instructions for
the
https://www.ni.com/docs/en-US/bundle/323324b/resource/323324b.pdf
> also clarify this.
>
> Post back if you have any other questions or comments.
>
> Jared A