SCXI-1124 current source open circuit behavior?

[Darrow]

We have an SCXI-1124 utilizing the internal loop supply delivering 1ma into an resistive load that may be any fixed value between 75 and 4725 Ohms.  An SCXI-1125 measures the resulting voltage at or near the 1124 output.  If through some hardware failure, the load is not connected and the 1124 is presented with an open loop what voltage will 1125 measure at the 1124 output?  I see in the datasheet and the user manual the 1124 has open loop protection for the current output but no explination on it's behavior in a fault condition.

Thanks,

Darrow

[hoffiz]

Hi Darrow,

With an open circuit situation nothing will happened to the SCXI-1125 either to the SCXI-1124, first because the “high impedance” input of the SCXI-1125 will not let current flow between the inputs of the SCXI-1124. In the case of and short circuit current will flow directly through the short so no current will flow to the SCXI-1125 again due to the "high impedance". In the case of a short circuit the specs of the SCXI-1124 states that the card is protected. For the information of how the open and short circuit protection works it will be better if I can know why do you need this information? What is your concern in the case of a short or open an circuit? Why do you need to know the fault behavior? What is the scenario you are trying to avoid?

[Darrow]

Hi Jaime,

We are using unique resistor pairs in our fixtures so the test system can identify which fixture is in use before beginning a test.  My fear is with the 1125 hardwired to the 1124 outputs, in the case of an open loop, the voltage at the output of the 1124 will be measured by the 1125 and that voltage used by the system to generate the wrong fixture serial number.  (I had not thought about a short but I suspect that would cause a similar error but result in a different range of wrong serial numbers.)  The problem is very intermittent but the evidence I have from the few times this event has been recorded in the test system database points to the open loop condition and a repeatable open circuit voltage at the 1124 output.

Thanks,

Darrow

[hoffiz]

Hi Darrow

I have set up a SCXI-1124 with an SCXI-1325 going through a resistor and on the other side I have a SCXI-1125 with a SCXI-1327 to read the voltage drop.  After testing this configuration and reading different resistors I did simulate a short and an open in both cases the voltage measured was the maximum voltage in my case it was max 10 V.  

Can I ask you in those cases where you have an open, what did you read? Is there a chance you can filter this situation programmatically? Why can’t you tell the SCXI-1125 when to read? Is this supposed to be an automated test? 

 Is there the possibility for you to use a range of resistors that will drop the voltage in a range that you can control, for example lets say you generate a current of 5mA you maximum input in the SCXI-1125 is set to 15 V if you can use resistors that will give you only voltages between 2 and 13 Volts you can programmatically discard values like 15V that will show and open.

I hope this help