PCI-6514 card DO port behaviour. LOW is as high as 7V

[MogaRaghu]

I am facing a funny issue with the PCI-6514 card installed on a WIN10 PC. 

SETUP :
1. The DO section is causing the problem
2. I am using the entire bank of P4 / P6 and P5.0 to P5.2
3. The DO pins directly drive a Phoenix DC relay ( 19 of them ). The relay draws 7mA when operating at 24V dc.
4. The VCC pins for P4/P5/P6/P7 are all connected to 24V dc power supply.
5. The P4/P5/P6/P7 .COM  pins are all tied together and connected to the 24V dc supply GND.
6. Each DO port pin connects directly to an isolation relay coil and other end of relay coil is grounded. No flywheel diode is used as I think the PCI-6514 has it built in.

PROBLEM :
1. When only 1 DO port is used it swings between 3.5V ( LOW ) and 23V ( HIGH )
2. But when many DO ports are driven, the LOW value of each un-driven port rises much above 3.5V and almost reaches 6.9V dc.  Like for instance if I drive all ports of P6  HIGH and then drive port P4.7  LOW , it does not drop to 3.5V but stays at 6.9V.  Due to this the connected isolation relay does not drop and this creates havoc with the machine Auto Sequence.
3. I also did some trial to further get a grip on the issue.

• All 19 DO pins are at low level and now all measure 3.5V
• I now clamp the volt meter to any one of them ...say P4.7
• I now start firing other ports in P6 and automatically the LOW voltage of P4.7 starts climbing as below :
• P6.0 ON / P4.7 Volt = 3.7
• P6.1 ON / P4.7 Volt = 3.9
• P6.2 ON / P4.7 Volt = 4.2
• So on till  P6.3 to P6.7 plus P5.0 to P5.2 are all ON. Now P4.7 measures 6.9V

Just wanted to know if this is the way the card will behave ? I think it should not. The LOW value should be a fixed one and the HIGH will depend on the VCC connected to the card. 

[crossrulz]

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@MogaRaghu wrote:

No flywheel diode is used as I think the PCI-6514 has it built in. 

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How sure are you about that?  I am not seeing anything in the data sheet about it.  Also, it looks like the PCI-6514 is really just a relay connection between the output and the port's VCC.  In other words, it goes to an Open when "off".  So any voltage that cannot be drained will be left out there.  So add the flyback diodes and I think that should fix your problem.

[MogaRaghu]

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@crossrulz wrote:

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@MogaRaghu wrote:

No flywheel diode is used as I think the PCI-6514 has it built in. 

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How sure are you about that?  I am not seeing anything in the data sheet about it.  Also, it looks like the PCI-6514 is really just a relay connection between the output and the port's VCC.  In other words, it goes to an Open when "off".  So any voltage that cannot be drained will be left out there.  So add the flyback diodes and I think that should fix your problem.

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I guess this block diagram should clarify my point. the relay comes right across the diode between the port pin Px.x and Px.COM.