High-Speed Counting?

[ZOMGVTEK 0]

Does anyone know how to do high speed counting on Mitsubishi FX3? I'm reading the documentation, and don't have a clue. Trying to get it to count a ~5v pulse output from a fan tachometer output, and output the result in RPM. I can measure the pulse with a meter, but can't get the PLC to see it. I'm a bit of a NOOB and have about a day of experience though...

I tried the SPD command, and it never reads anything. Is it possible the output from the fan is too wimpy?

Edited 1 Nov 2021 by ZOMGVTEK

[glavanov 40]

5V signal do't have the current to put the PLC input to ON condition:

[panic mode 246]

as stated, you need to make sure interface is electrically viable. 

but it is unclear what the fan output really is and what is this "5V"... in fact it is also unclear what the polarity of your PLC inputs is as obviously from the screenshot, they can be PNP or NPN. 

normally fan can be powered by some voltage source such as 5V or 12V or 24V for example. 

in my experience the pulse output tends to be the NPN open collector type and usually it is 30V tolerant. 

if that is also your case, you can connect it directly to your PLC.

if the fan output is an open collector type (And 30V tolerant), supply voltage of the fan itself and voltage at the output do not need to be the same. in this case fan can operate on 5V or 1V for example while output is connected to 24V input.

if the fan is open collector NPN but PLC inputs are configured for PNP, you can still use direct connection by adding pullup resistor. this need to be chosen carefully to match both fan output and plc input specs. 1.5k 0.5W would probably do just fine.

if the fan output is not 30V tolerant or it is not an open collector or it is not referenced to 24V supply that PLC uses for I/O, you need to add intermediate circuit as an interface. this could be an optocoupler (with 30V tolerant output) and with one or two resistors. 

on the fan oupput use something like 330 -560 Ohm 1/4w resistor in series with optocoupler input (essentially an IR LED). connect oprocoupler output to PLC input. optionally insert some current limiting resistor in series with the optocoupler output to protect it in case you make wiring mistake.

note that optocouplers are polarized devices so it matters how you connect them...

if you do not know fan output current rating, it is good idea to assume it is low. in that case optocoupler with higher CTR would be nice (Darlington) 

 

[ZOMGVTEK 0]

Thanks guys! Yep, turns out the PLC is PNP input, the fans output is NPN open collector, and the PLC has crazy high input impedance, requiring more like 12VDC to register.

It's working now! PLC temperature controllers are way more complicated than I would have guessed...

https://www.sanyodenki.com/archive/document/product/cooling/technical_material_en.pdf

Edited 2 Nov 2021 by ZOMGVTEK