Voltages at AQs

I'm using an AM2 AQ module with a LOGO.

The two AQs send command voltages to the same device but not simultaneously. Essentially, they are connected to the same node. When AQ1 sends its command voltage (3V) to the node, the AQ2 output is OFF. When AQ2 sends its command voltage (5V), AQ1 is OFF.

Of course, since the two AQs are sending voltage to the same node, the contacts of both will "see" that node's voltage. When AQ2 is sending 5V to the node, AQ1, though not outputting any voltage, will "see" 5V at the AQ1 contacts. And vice versa.

Is this okay? 

I don't expect it to be a problem at the contacts, but I'm just not sure how the LOGO plc will handle seeing a voltage where it is not outputting a voltage? Will it see it at all? Does it even care?  Might the LOGO re-zero the output(s) or something I don't know about?

Thanks in advance. 

Hello Eaglewood,

... in addition, an external voltage at the analog output of the logo extension can destroy it.

ixo65

If I understand you correctly, then you output 2x a voltage through 2 analog outputs to an analog input.

Why?

If only one signal from the two is active at a time, you only need one output and switch different values to one AQ using the analog MUX.

You understand correctly. I wasn't aware of the MUX. This is my first time using the LOGO. I should have poked around a bit more at the analog offerings.

Thanks again!

Eaglewood

Though the MUX is the answer to my question, I am curious why applying a small voltage to the contacts could destroy the analog output. I'm not going to do what I was thinking of doing, but if I had I still wonder if there would have been a problem.

Consider that the analogs are active power sources. There is no situation where the analog source disconnects from its terminals. The analog actively (attempt) to keep the voltage/current to the required value. Even if it is 0V.

Look at the image below. Note the connection of the two batteries, resistor and voltmeter.

The voltage potential over the resistor (R1) is 4V. Now assume we make R1 = 0ohm. This will result in the two batteries 'fighting' each other. This can cause damage to the batteries. Heat will be involved too. The resulting voltage potential over the batteries will be neither 5V not 1V.

In the case of this playing out on the LOGO! -- this may damage the electronics.

qwazee

 

Eaglewood

Though the MUX is the answer to my question, I am curious why applying a small voltage to the contacts could destroy the analog output. I'm not going to do what I was thinking of doing, but if I had I still wonder if there would have been a problem.

Consider that the analogs are active power sources. There is no situation where the analog source disconnects from its terminals. The analog actively (attempt) to keep the voltage/current to the required value. Even if it is 0V.

Look at the image below. Note the connection of the two batteries, resistor and voltmeter.

The voltage potential over the resistor (R1) is 4V. Now assume we make R1 = 0ohm. This will result in the two batteries 'fighting' each other. This can cause damage to the batteries. Heat will be involved too. The resulting voltage potential over the batteries will be neither 5V not 1V.

In the case of this playing out on the LOGO! -- this may damage the electronics.

Eaglewood

Now I find myself in a similar situation with digital outputs. I would like to have two push buttons activating the same Q but for different time spans. But due to the LOGO connection rules I can't connect two inputs to the same output.

???

Take the two outputs from B001 and B002, link them with an OR block and put its output on a logo output (e.g. Q2).