Pulse counting and signal conversion (conditioning)

Hello All,


I have a late model yokogawa magmeter with an open-collector output for the pulse output function. The current PLC's configuration is an S7-300 and two SM321 16 x 120vac D.I. cards and does not include any counter modules or inputs.

I have been asked to proprose a way to use the existing h/w to count the output pulses (2 hz. MAX as limited by magmeter controller).

Initially I thought of using a DC controlled solid-state relay as the interposing function to "convert" the open-collector output (via a 24vdc P.S.) to a 120vac output for the input to the SM321.

Is it possible to use the SM321 in this way?

Can an SS relay be used this way?

Are there better ways to "convert" the open-collector output to 120 vac out?

Von,

I guess that it is obvious from your post that you don't have and DC inputs available. The part of you solid state relay scheme that I would want to research carefully is the input functionality. I have used a few over the years and they usually wanted a dry contact closure. I would expect the there are devices available that would respond to you open collector situation.
I would want to verify that the open collector is NPN (usual situation). That would imply that it be used as a "sinking" device, ie..provides the return to negative ground for a device powered from a positive source.

You could also use a 24 VDC relay which is grounded by your open collector based pulse. This is not as sexy as the SSR, but I'm sure that it will work for a while! If you do, I would be certain to use noise supression techniques on the relay coil (in this DC case, a reversed biased diode). I would probably consider a reed relay for this type of situation (low current, many operations).

Best Regards,

Bob A.

I can't say much about the SM321, but what you may need to be concerned with is loading of the SSR. You may need to add a resistor across the SM321 input to insure the SSR to shut off. As for using a relay, a reed type would be best as Bob A. recommends. It would need to handle the constant switching and frequency. I don't remember any specs for an SSR but I think it should handle 2 Hz.
Any good reason you can't add a card that would be ready to accept the OC output?

Thanks all,


I just wanted to test my understanding of this scenario and you have helped to do so. I too would like to use a counting module but just wanted some "second opinions" before I recommend it.
Thanks again...

RWW

At less than 2HZ would not bother using a counter card.

A SSR interface should be fine if the scan of the plc is slow < 0.5 of the required pulse time then there are functions in S7 to update I/O imidiate or updat in a timed OB (10ms) so you could guarantee catching all the pulses.

Another option is to talk to Yokogawa about an output module for the magmeter that converts the pulse inside the meter into an analog output. Most magmeters that I have seen have this feature installed at the factory, because of the difficulty of accurately counting the raw pulses.

You would have to add an analog input module to your S7-300.