Bar (Customer) asked a question.
Hello,
I have a sourcing output from a standard output (Not High Speed) on the BRX and using the PWMOUT command. The source Voltage is 5V. I measure 5V from a neighboring active output (not PWM) to com/System ground. COM on the 5V power supply is connected to the system ground.
If you notice on the scope that the pulse does not drop to 0V but somewhere around 1V. Is this to be expected? Thanks!
I don't know about "expected" but I wouldn't be very surprised.What kind of load do you have on the output? if it is only a 10meg Ohm scope probe I would be at all surprised, the specs for a sourcing BRX output show a minimum output current of 0.1ma, I might try a 1K resistor to ground. If driving a device with an optoisolator input the LED will have a voltage drop that might well be around a volt or more.
@Tinker (Customer) I think you are correct about the optoisolator. When disconnected from the load, the pulse drops to 0V. Thank for your help on this!
As a side note, I don't think the standard output is great for typical PWM use. (Fortunately the high speed outputs are available). I started down this scope testing because I was seeing ~45% duty cycle on a Fluke meter even though the input was 40%. Measured by the scope its 42%. I am guessing this is because the measured pulse width is wider due to the trailing edge response time of <2ms and the pulse width is the time difference between 50% amplitude at rising and falling edges. Perhaps the Fluke meter is measuring at less than 50% amplitude resulting in an even higher duty cycle than the scope. And curiously, the response time for the rising edge is listed as <5ms, yet the rising edge on the scope is sharper.
On a separate not, I think there is a discrepancy between the documentation too. The manual states ~100 Hz as the maximum switching frequency but the software allows up to 500hz for a standard output using the PWMOUT command. Perhaps being at the limit of switching frequency is a factor.
Anyway, just wanted to document these findings because there were not a lot of comments on PWM usage in this forum and someone in the future might find it helpful. Thanks again for the help.
I don't know about "expected" but I wouldn't be very surprised.What kind of load do you have on the output? if it is only a 10meg Ohm scope probe I would be at all surprised, the specs for a sourcing BRX output show a minimum output current of 0.1ma, I might try a 1K resistor to ground. If driving a device with an optoisolator input the LED will have a voltage drop that might well be around a volt or more.