NI9266 open loop

[udtwmc1]

I am using an NI9266 4-20mA output card to drive pneumatic positioners (I2P) via intrinsically safe galvanic isolation barriers (the actuators are located in a hazardous area). 

My controller is a cRIO 9035 and I’m running labVIEW 2019 ( I don’t have the VI’s to show right now, but I think this is a hardware issue anyway …. 

the I2P positioners are powered from the 4-20mA current loop.

I set a minimum 4mA so the positioners are always powered however some positioners don’t power up and I’ve noticed the 9266 output channels are not driving a current despite my request to drive 4mA. The only way around this I’ve found so far is to manually disconnect and reconnect the current loop to my load which kick starts the 9266 card to turn its output on/ the positioners then power up 

the NI datasheet says … “The NI-9266 provides channel-based indicators for open current loop condition or if the loop is out of regulation. This could be the result of a wire disconnect or a violation of the maximum load or maximum compliance voltage”. 
and is a safety feature designed to prevent actuators operating on first power up.

I think this is the problem but I don’t understand what the root cause is (my intrinsically safe barrier maybe??) and how to get around it without manually disconnecting/reconnecting the loop wires every time! 

has anyone had a similar issue and know what I need to do? 

many thanks 

[santo_13]

Can you share more information about your 'intrinsically safe barrier'?

Basically, 9266 can drive up to 600 ohm load resistance beyond which it will detect as open and shutdown. Based on this, the culprit could be your isolation which may not present a < 600 ohm load to the 9266.

[udtwmc1]

See attached datasheet. it is a dual channel isolating driver: model D5020D by GMI international

From what I take from the datasheet, the NI9226 card sees <125ohm (2.5Vdrop @20mA... would you agree with this?)  ....

"Input: 4 to 20 mA with ≤ 2.5 V voltage drop, reverse polarity protected in normal operation, ≥ 5 KΩ impedance (≈ 2 mA sinking from 10 to 30 Vdc) when fault condition detected."

I;m not sure what the >5Kohm impedence when fault detection means - eg if the fielsd device is faulted (off say) does it turn its input (what the NI9226 sees) to 5kOhm?? i still dont inderstand why i can simply disconnect/reconnect the loop and it magiaclly starts working, seems a bit odd

Thanks for your time!

[santo_13]

As per the documentation, it is possible that there is some fault on the output side of the isolation that is replicated in the input side.

Do you observer any fault led on the isolator?

[udtwmc1]

No fault on the isolation barrier although I suspect it is this barrier causing the issue. The 9266 card clears it’s fault flags when I disconnect/reconnect loop wires. I’ve made a work around for now (not ideal) but I’ve got the system up and running.

thanks for your input