1 khz PWM output

[Trevor]

Hi there,

 

I am working on a project that will use a V350-35-TRA22 Unitronics PLC. My current problem is to get a 6V 1 kHz PWM signal on one off the HS outputs. The spec sheet indicates an output frequency range of 5-200kHz, but i can't get an output higher than 100 Hz ( cycle time value = 4, duty cycle = 500 ). I have tried to set the cycle time to 0.4 (to get 1 kHz) but it seems that I can't enter a fractional value.

 

Any help with regards to this problem will be appreciated.

 

Thanking you in advance,

Trevor Pieterse

B.tech Electrical

University of Johannesburg

[Joe Tauser]

The PWM "frequency' value for an HSO output on the PLC and the PWM function block are different.  I know this is confusing.

 

Check the Help on configuring an HSO output - Search tab->HSO->Configuring a High-Speed Output.

 

On a dedicated HSO, the frequency MI is the actual frequency.  For a 1 kHz signal enter 1000 into the MI value.

 

For a PWM block, the Cycle Time is in units of 2.5 ms.  The best you can do with a PWM block is 400 Hz.

 

I'm assuming you have a 6V power supply connected through a 2.2K pull-up resistor to the NPN HSO output of the PLC to get your desired signal level.  Don't forget to connect the 6V power supply common to the PLC's power supply common.

 

Joe T.

[Trevor]

Mr T

 

I got the HSO all set and running but I've noticed that the output frequency is not constant (+/- 5 Hz deviation). Could this be a result of noise?

 

I'm struggling to set up a HSC for frequency measurement. The signal being measured is a 2V p-p square wave that won't exceed 3 kHz.

I understand that the voltage must be greater than 3V to be read as a logic '1'. For testing purposes I set up a 5V p-p on the HSC0 (pin 15) with the signal's ground connected to the PLC common but no frequency output is generated in the code.

 

Any recommendations?  

 

Regards

Trevor

 

P.S.

Your assumption was correct.

[Joe Tauser]

The HSO's frequency stability is not specified.  +/- 5 Hz may be the best it can do.  Did you verify this with an oscilloscope? 

 

Go back and closely examine the specs on the V350 high speed input - the 3 V you mention for a logical 1 is for the NPN input and the voltage must be below that.  You'll have to get the input above 20.4 V to get the logical 0 or the PLC will never see it.

 

I would suggest the following level shifter-

 

 

Power it with 24V instead of 12V and run the output to the PLC input.  You may have to fiddle with the 1K value to get the transistor to switch with your 2V signal.

 

Joe T.