"Potential-Free Contact" and system integration

[zubenelgenubi]

Greetings,

I am designing a system (B) which must receive a signal from an outside system (A). When the signal is sent by system A, system B needs to toggle a PLC pin from TTL high (5V) to TTL low (0V).

The supplier of system A has told us that they will provide a "potential-free contact". They have also told us that this could be NO or NC.

Since system B needs to toggle a PLC pin, my questions are:

-How can we receive this signal from system A & toggle the PLC pin?

TIA

Charlie

 

[jedft]

Not sure if I completely understand your question but it sounds like you have a 5V power supply somwhere supplying an input on a PLC, and a dry contact on another system.

If you use a NC contact on system A, put your 5V through the contact and into the pin on PLC B, then when system A changes state your PLC pin will go to 0V.

Pardon the poor ASCII:

------------ PLC pin
|
---
\ A
---
|
------------ 5v

 

[Pandiani]

If they provide a potential free contact, that means it's just it, a contact. Your system will provide +24V that (in case contact closed) will be returned to your system. This is pretty common. This way, your system (B) sees whole system (A) as just one contact. You can use DI channel on your PLC on system B, and then make additional logic in PLC program of system B that will toggle a bit. Just search this forum for a toggle bit.

Regards,

Pandiani

 

[Clay B.]

Another name commonly used for potential free contact is a "dry" contact.

 

[halosome]

In this case you don't need a PLC. Even the resister in the drawing is not necessary.

 

[zubenelgenubi]

This is beginning to make more sense. I had assumed that the 2 wires which system A supplies would be 5V and 0V, but now I am understanding that System B supplies the 5V to one of the terminals supplied by System A and that the closing of the NO contact then supplies the voltage.

The Aha moment!

So, please help me understand now the thing labeled '320K' in halosomes drawing. Is this some kind of protection?

TIA

 

[Tharon]

So, please help me understand now the thing labeled '320K' in halosomes drawing. Is this some kind of protection?

TIA

It is a 320kohm resistor. It's there so when you close the contacts on System A you don't create a short circuit between your 5V and your 0V.

 

[keithkyll]

The 320K pulls the output low when the contact is open. Not needed for a PLC input.
Pontentional free = Dry Contact = pushbutton switch = toggle switch = contacts on a relay = 2 dangling bare wires that get shorted together.

 

[JERA]

Last time I made a signal exchange to machine with 5V signal, I used a Beckhoff bus terminal.

5V signals from the machine were sent to a KL1124 digital input terminal.
5V Signals to the machine were sent from a KL2124 digital output terminal.

For communication to the PLC where used a Profibus Bus Coupler.
This gives a “clean” interface.

 

[panic mode]

first of all if this is going to be interfacing to TTL system, 320k is way too much. TTL I/O have low impedance (few hundred ohms at best).

next thing is that all TTL inputs are sink. in fact if you leave TTL input floating, it will be considered "high" or logic "1".

so if you are bringing in dry contact, tie it between input and 0VDC. you may add pull-up resistor such as 1k from input to +5V. this helps when input is sensitive to noise.

if the potential free contact is solid state (output from optocoupler) then pay attention to polarity. TTL inputs expect to be driven by NPN open collector (which is typical TTL output stage).