NI9219 - 4-20mA Cabling

[Mivil]

Hello, I'm using a NI9219 card with a cDAQ-9174 and I got a cabling problem... My 4-20mA signal is read by a external meter before the signal is acquired by the NI-9219 card (refer to sketch). My setup is working fine except for one thing. When I power off the cDAQ-9174, my external meter cannot acquire a signal (the loop look cut???). How I can configure the NI-9219 card to be able to read my 4-20mA signal with my external meter as it is possible with a standard PLC? I don't want to use a signal isolator module to create two separate loops. Regards, Jean-Marc

[SnowMule]

Does it need to be automated?  A cheapo SPST across the input will do the job...

My bet is the input impedance of those terminals goes high so you don't accidentally short something out if you were to reconnect it to something else.

[AnalogKid2DigitalMan]

I think SnowMule hit the nail on the head.

Another option would be to set the 9219 to voltage mode and put a resistor across the input terminals. Then scale the voltage reading to the associated current using Ohm's Law.

-AK2DM

[SnowMule]

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@AnalogKid2DigitalMan wrote:

Another option would be to set the 9219 to voltage mode and put a resistor across the input terminals. Then scale the voltage reading to the associated current using Ohm's Law.

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Then the meter would be off since the additional resistor would be in the current loop.  You'd need an extremely low value resistor, and read extremely small voltages across it.

[Mivil]

Where is it write that the output is high impedence when the power is off? I did'nt find the information in the user manual...

[Mivil]

http://www.ni.com/pdf/manuals/374473e.pdf

Input impedance

Voltage and Digital In modes

(±60 V, ±15 V, ±4 V) .................. 1 MΩ

Current mode .............................. < 40 Ω

All other modes .......................... >1 GΩ

Is it possible to set a hardware configuration to stay in 'current mode' when the card power is off???

[AnalogKid2DigitalMan]

SnowMule:

Current loops can have multiple readout devices in series as long as the total voltage drop is below the compliance voltage of the source power supply.

-AK2DM

[Cablejon]

Use a power diode across the two terminals of you cDAQ device. take special care to keep the orientation of the diode in the direction of the loop. when you device is on current will flow through your device and when it is off it will flow through the diode keeping the loop intact.  

[K C]

Hi,

It depends on the specification of the 4-20mA source. Most devices I know can drive a 500-750 Ohm resistance. The resistance of the 9219 input is < 40 Ohm. So it is low but not a fixed value.

With a normal industrial 4-20 mA current source you can add an additional resistor in the loop of 250 Ohm and measure the voltage across this resistor (0-5V).

For the current loops I know this is common practice. e.g. to read communication data (HART) from the device.

Kees