PCI 6514 outputs - random OFF state values

[MogaRaghu]

I have a PCI6514 with all its 32 output pins connected to 24V dc Finder relays . These relays have a coil rating of 0.5W and thus draw about 20mA each which is well under the rating of 6514 which is 75mA. And each relay has its own freewheel diode module, to supress inductive kickback. 

The problem that I notice is this :

- When all the bits of a whole port say 4.0 to 4.7 are OFF, the voltage on each port pin is 4V. (Why this is so high instead of near Zero beats me )

- Now I switch ON 4.0, 4.1, 4.2 and 4.3 and the voltage on each port pin is 24V. ( the Vcc that I supply externally )

- After some minutes I switch OFF 4.0. But this pin now holds anywhere between 6 to 10V, instead of returing to 4V, preventing the  Finder relay from dropping out.

Issue noticed on two new PCI 6514 cards. Not sure if these cards need to get back to NI or is it "normal"  that they behave like this ??

[satish_21]

Hi Raghu,

I assume you didn't find the same scenario with no Finder relays connected. If you haven't tried it out, I advice you to try that out. If the PCI shows appropriate voltage readings at pins w/o the relays, then there is no problem with the PCI card. I guess the Maximum Repetitive Reverse Voltage of the freewheel diode module should be the cause for this behaviour. Also try adding 1N4148 diode in parallel with the freewheel diode module. 1N4148 has a 100V Maximum Repetitive Reverse Voltage and it should avoid the backward emf, if at all that was the problem. When you connect the diode, make sure it is in reverse polarity.  

[MogaRaghu]

Hi Satish,

I only hope it was that simple.  Load or no load , the outputs still behave funny.

Following is the observation :

P4.0 to P4.3 port lines Switched ON. ( All Finder relays disconnected from these port lines )

Measure P4.0 line after switching it OFF              = 10.1 V

Measure P4.0 after switching P4.1 also OFF         =  8.2  V

Measure P4.0 after switching P4.2 also OFF         =  7.5  V

Measure P4.0 after switching P4.3 also OFF         =  6.1  V

Now connect back the Finder relays to the four port lines. Repeat above measurement.

Result : Measured voltages same as above but  about 0.2 to 0.3V less in each case.

[satish_21]

Hi Raghu,

In  that case, I can think of only one possible reason. Have you connected your external Vcc supply ground with the PCI ground? If not, connect the ground and try measuring the values. If it's still the same, probably it's better to contact NI support.

[MogaRaghu · Accepted solution]

The PCI-6514 card has 20 terminals to which you are supposed to connect the 24V gnd connection.

As ill luck would have it, 24V Gnd connections to 16 terminals were made leaving out 34, 46, 84 and 96. And these precisley are the  COM terminals for the DO stages.

Once the above terminals were connected to 24V gnd, the card is working fine. I get 0.0V when the port line is switched OFF.

[satish_21]

Hi Raghu,

Glad that you rectified your problem with my help. But I guess the 20 pins for ground will be internally shorted. I am not sure though.

[MogaRaghu]

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@satish_21 wrote:

 ..... But I guess the 20 pins for ground will be internally shorted. I am not sure though.

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No, not all of them are shorted internally. Had they been I would not have suffered like this....

[satish_21]

What you say must be right. Else, you shouldn't have got this problem.