PLC Logo Problem: Keeping Output Q1 On After Power Loss

ixo65

For A)
M1 in the remote LOGO! to high.
Then disconnect the Ethernet cable from the remote LOGO!.
In the LOGO! 8.4, VR0.0 remains high, after about 10s it changes from high to low. Ethernet cable plugged back into the remote LOGO!, VR0.0 immediately to high.

For B)
M1 in the remote LOGO! to high, VR0.0 in the LOGO! 8.4 to high, then LOGO! switched to STOP and disconnect the Ethernet cable from the remote LOGO!.
Then switch the LOGO! 8.4 back to RUN, VR0.0 goes high and then after 1s to low.

ixo65, thank you very much for your time and effort. 

The retentive behaviour of the VR0.0 bit is not really helpful for this case. Or at least not in the direct way.

ixo65

I'm wondering how the remanence is supposed to work.
the block NI only has one input.

Let's assume, theoretically, that VRx is really remanent. VRx would be set via the only input and would remain set when the input signal goes low. How would it then be reset???

Yes, you are making a valid observation. 

Hi,

I have used the attached program for testing with a LOGO! 8.4:

I have set both bits to "1" in online test. Then I have done a power cycle at the LOGO! 8.4. Here the result after the power cycle and restart of the online test:

VR0.0 has still "1" signal, V0.0 "lost" "1" signal.

So remanenz via VR in a LOGO! 8.4 should work as long as no other device activilly change the state of VR0.0.

AutoGen_380038

Hi,

I have used the attached program for testing with a LOGO! 8.4:

I have set both bits to "1" in online test. Then I have done a power cycle at the LOGO! 8.4. Here the result after the power cycle and restart of the online test:

VR0.0 has still "1" signal, V0.0 "lost" "1" signal.

So remanenz via VR in a LOGO! 8.4 should work as long as no other device activilly change the state of VR0.0.

Thank you for volunteering your time to this topic.

In my humble opinion, changing the state of the bits in online test is not a fair simulation of the situation described by this topic. The way that ixo65 have done is better. 

Perhaps there is a way that a communication partner can reproduce the force on / force off that the online test uses. As a novice LOGO! user, it is unclear to me how a peer LOGO! or Modbus TCP device would do this.

There is no doubt for us that the VR bits have a retentive behaviour. What is not clear is how this can be utilized for the topic's benefit. Would you mind giving my summery of your client's request some thought..

The primary LOGO! receives a request from a communications partner what the output state should be. That output state should be restored when the LOGO! reboots. Assume it may take some time before the communications is restored.

ixo65

That's for me an unrealistic test.

For me, the alleged remanence is not what is meant by remanence.
I did another test with 2 real LOGO!s.

In the remote LOGO!, Q1 and Q9 are switched on by I1.
QB0 (Q1-Q8) is transferred to the VB0 of the LOGO! 8.4, QB1 (Q9-Q16) is transferred to the VRB0.
In the LOGO! 8.4, V0.0 switches on the Q1 output, VR0.0 switches on the Q2 output.
If I leave I1 of the remote LOGO! switched on and switch the voltage of the LOGO! 8.4 off and then back on again, both Q1 and Q2 of the LOGO! 8.4 are switched on.
What is the point of the alleged remanence if the transmission signal is not allowed to change?

 Hi,

as long as you active transfere I1 like in your test thats clear. After power ON your second LOGO! transfer I1 to your remote LOGO! 8.4 after the connection is established.

Can you try to not active transfer I1? Means, disconnect power of the remote LOGO! 8.4, unconnect the other LOGO! and power the remote LOGO! 8.4.

In this condition I1 is not active transfered. How is QB0 and QB1 after this sequence?

I believe this is a similar test like: Starting online test, via data table write "1" to V0.0 and VR0.0 (active transfer), then do a power cycle and check the state of V0.0 and VR0.0 without changes.. V0.0 has "0" signal, VR0.0 has "1" signal.