Frequency output from a PLC.


Thread Starter


I am using a FX2N PLC with transistor output (Source mode). I am using it to generate variable frequency up to 15kHz. The rise time of the pulse is 0s. My problem is with long fall time of the pulse. At high frequency the pulse dose not reach 0v before another one is generated.

I have added a pull-up resistor to increase current up to 200mA, but the fall time of the pulse has not improved as much as I like.

Any one have any ideas on how this signal can be "squared up"?

Any help will be appreciated. Thank you.

Curt Wuollet

A pullup isn't going to help with fall time so I assume you are using a pulldown, to ground. If not, try that. The current decay is probably more a factor of your load than the output although it wouldn't surprise me if they had a capacitor to limit the fall time to make driving coils a little easier. Your solution is a driver that can both sink and source current. Try the output by itself with say, a 50 ohm _pulldown_ resistor.
If the speed is adequate, you can make a simple buffer with a pair of complimentary transistors. IF that doesn't scare you off, mail me for a drawing. Otherwise what you a looking for is a
unity gain power buffer. I know National semi makes such a device. It is also possible to use one of my favorites, a 555 timer chip available at any Radio shack for a buck or less. We don't do any timing but it has high impedance inputs and can drive at least 100 ma. both sourcing and sinking with reasonable Tr and Tf. Just hook up power and ground (it works with 24V) tie 2 and 6 together and use them as the input. output is on pin 3. It does invert but that's easy to handle with most PLC's. If the speed with just a 50 ohm pull down is inadequate, the 555 trick may still work as it has built in comparators, a discussion of which is beyond the scope of this email. But the thresholds can be diddled somewhat. Mail me for details if interested.


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Thank you for your reply. I am using a 130ohm(15W) resistor connected across the output of the PLC and 0V. The output of the PLC is also connected to load(it is 1.5kohm resistor in series with a opto-isolator and then to 0v.).
I can not use 50ohm resistor since the max rated current for this PLC output is 300mA.
I like the 555 timer idea I will give it a try. Do you think it will be reliable for years?
I am also instrested in your complimentary transistors idea. If you would please email me drawings to "[email protected]", mailto:[email protected] . Thank you.
The opto coupler in the PLC plays a part also.
As the light switches from bright to off,the transistor does not switch equally and the 'on resistance keed increasing. with frequency. The tie up resistor will not help. And it is not possible to get a clean square wave output more tha 5 KHz.As the freuency goes up, the output voltage will fall. Use a schmitt trigger or some other electronics to shape the pulse into order.
Or else you have to use a dedicated pulse output card for the PLC which can go up to 25-35 KHZ.