Time Duration or Pulse Train input to analog output

It is possible to use an S7 series PLC to convert a pulse train input (-24VDC to 0 "decreasing"on one set of contacts and 0 to +24VDC "increasing" on another) and convert this input into a 4-20 mA output? The application is a measurex gauging system moisture control output in time duration or pulse train. The Foxboro converter my customer once used is obsolete.

Jpesh

As you said "It IS possible ... "
I'm not quite clear about the 'set of contacts' - how many in a set?

For a pulse-train input I would have expected you need to measure the frequency of the input pulses on a single contact. The lower and upper range of the frequency being transmitted in this pulse-train will then equate to the 4mA and 20mA of your analogue output. Simple arithmetic should provide the link between the two.

However, can you clarify the 'sets' of contacts and also give some indication of the frequency range you're expecting to deal with?

Regards

Ken.

Ken M said:

Jpesh

As you said "It IS possible ... "
I'm not quite clear about the 'set of contacts' - how many in a set?

For a pulse-train input I would have expected you need to measure the frequency of the input pulses on a single contact. The lower and upper range of the frequency being transmitted in this pulse-train will then equate to the 4mA and 20mA of your analogue output. Simple arithmetic should provide the link between the two.

However, can you clarify the 'sets' of contacts and also give some indication of the frequency range you're expecting to deal with?

Regards

Ken.

Click to expand...

Thanks Ken. The current obsolete converter I am seeking to replace with a PLC (Siemens or A-B) has the following terminations/wires: +24VDC power, Return or common, Increasing, Decreasing. Minimum time unit can be selected as 2 msec, 10 msec, 20 msec, 50 msec or 100 msec. Deeply appreciate your help.

JPesh

Sorry to come back with more questions.
This existing converter (I get all confused when people talk about 'current'. Do they mean 'present' or do they mean 'ouch!'?) has what seems to be 4 terminals. Have you any indication of which are the inputs to the converter and which are the outputs from it? Eaxctly what is it converting to what?
Presumably you have to attach the 24VDC supply to it, but are the Increasing and Decreasing signals generated by the converter and output from it? And are these the pulse trains? etc

Regards

Ken

Thanks for staying with me on this one. Two inputs to the existing converter are possible. One is a time duration input and the other is a pulse train. It appears that it may be easier to configure a PLC to handle a pulse train input rather than a time duration or pulse width signal. The application is a Measurex gauging system on a paper machine measuring moisture. The Measurex system outputs a pulse train to the converter to drive a steam valve to control moisture. It is either an increasing or decreasing signal (thus the inc and dec contacts). If the pulse train is on the INC contact, the converter then outputs an analog signal to drive a steam valve open in proportion to the number of pulses input to the converter. If the pulse train is negative, energizing the DEC contact, the 4-20mA drives the valve proportionately closed. The existing now-obsolete converter actually outputs a 3-15 psi pneumatic signal instead of a 4-20mA but we are hoping to use a PLC to convert the pulse train to 4-20mA to drive the valve. You are correct that a 24VDC power supply is present.

I would probably put a MLX 1200 on there and wire the increasing signal to an input and the decreasing signal to an input. Install an analog output module on the MLX. When measurex says increase I would ramp the 4-20 signal up based on a timer set in the PLC and ramp down with a decrease signal....
Hmmm... Where have I heard this before.