24 VDC pulse output chip/circuit

Thread Starter

Mr.Green

Joined Feb 2, 2021
7
I am looking for a 24 VDC pulse output chip/circuit, designed output 3 signals to the receiver; 1 Operating Normally, 2 Alarm sensed, 3 Alarm Resolved. Any pulse train different than these 3 forms will be interpreted as a communications failure by PLC.
I am looking to send this pulse to a low cost micro PLC with approx 14 inputs,
have it interpret the pulses and send an email alarm if pulse format 2 or 3 are received.
Something tells me the chip/circuit exists already.
Please advise, thanks. Mr. Green
 

Thread Starter

Mr.Green

Joined Feb 2, 2021
7
No, don't care, I was thinking 4 bit binary, but then thought if they are coded TEXT:Binary, it may be easier for the PLC to convert Binary:Text. Unless there is a better, less costly way to do it.
 

Thread Starter

Mr.Green

Joined Feb 2, 2021
7
"What signal determines which of the three outputs are sent? " this is another project, but since you asked, it is a compare circuit with a reset input. Any resistance over a specific ohm value = OK, anything below=alarm, if the reset button is pressed we want the reset pulse sent.
 

crutschow

Joined Mar 14, 2008
27,198
If you are familiar with micros such as the Arduino, that would be a relatively easy way to go.

If not a parallel-in serial-out shift-register with some associated circuitry, including a clock and load logic, may work.
 
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schmitt trigger

Joined Jul 12, 2010
438
I am sure you will receive a dozen different recommendations, each with its pros and cons.
Here are my two cents;
I would use three different frequencies, one for each state.
A complete loss of received frequency would indicate a comm loss.
 

djsfantasi

Joined Apr 11, 2010
7,689
No, don't care, I was thinking 4 bit binary, but then thought if they are coded TEXT:Binary, it may be easier for the PLC to convert Binary:Text. Unless there is a better, less costly way to do it.
I think this conclusion comes in part from a misunderstanding of internal representation. There is no functional difference from binary and text formats. At least not how the TS differentiated them. A given value triggers a given result. There is little difference if the value is 0001 or “A”. The optimal code is the same.

Or did I miss the TS point?
 

AnalogKid

Joined Aug 1, 2013
9,249
One oscillator circuit could send three different continuous square waves, so you don't have to both amplitude and frequency-detect the signals.

1 kHz = all OK

3 kHz = alarm tripped

5 kHz = Reset pressed.

OR -

One oscillator circuit that runs at a constant frequency, but with three different mark/space ratios (duty cycles):

50% = all OK

25% =alarm tripped

75% = Reset pressed.

ak
 
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Thread Starter

Mr.Green

Joined Feb 2, 2021
7
An oscillator circuit could send three different continuous square waves, so you don't have to both amplitude and frequency-detect the signals.

1 kHz = all OK

3 kHz = alarm tripped

5 kHz = Reset pressed.

Or something like that.

ak
hmm will consider that, was thinking 5 VDC for ease of converting to test for ease of alarm messaging. Thanks Mr. G
 

crutschow

Joined Mar 14, 2008
27,198
It would see more difficult to change the 555 duty-cycle with a control signal then to change the frequency, unless you use two 555's.
 

DickCappels

Joined Aug 21, 2008
7,427
You can switch duty cycles easily by switching
In additional capacitors. Decoding can be as simple as a low pass filter and comparators.
 

AnalogKid

Joined Aug 1, 2013
9,249
<PWM> Decoding can be as simple as a low pass filter and comparators.
Or using one input signal phase to gate a hardware or software counter. PLC's love counters.

For three frequencies, I was thinking of one permanently connected capacitor in parallel with two caps that are switched to GND with NPN transistors.

ak
 

Thread Starter

Mr.Green

Joined Feb 2, 2021
7
I am wondering if this can be even simpler using 24VDC relay. IF OK, 0VDC output, if Alarm 24 VDC output. There is a comm check bit sent every 60 seconds, 24VDC is present for 1/60 second (Hz) followed by 0 VDC to confirm comms.
 
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