TBX-68, will it accept a 4-20 mA current input rather than a voltage input?

[Craig Murray]

Hello, I'm very new to the whole DAQ thing but I have a little problem I'm facing. I need to measure and record a 4-20 mA signal from a pressure transmitter and I'm using the LabView with a TBX-68 terminal board, the only thing is I cannot figure out how to insert this board into the current loop. Is this board only intended as a voltage recorder? I know this maybe a very simple question to many but I would greatly appreciate the assistance.

[Christian_L]

Craig,

The TBX-68 is only the connector block (physical interface) to your data acquisition card or device. It doesn't actually perform any operation of its own. The data acquisition card is plugged into your computer (PCI, PXI, etc.) or connected to it using an external bus interface. Could you let us know what kind of data acquisition card you are using so that we can help you further.

Thank you.

Christian L
NI Consulting Services

[Craig Murray]

Sorry about that. The card is a PCI-6024E (soudn right?). When I'm creating a new virtual channel in Measurement and Automation Eplorer I'm inputing a 4-20mA signal but I'm also not sure if the Input Mode is differential, Refernce Single End or Nonrefenced single end. If there is any more information that is required please let me know.

Thanks
CM

[Christian_L]

Craig,

The PCI-6024 by itself can not measure current loops. It is only equiped to measure analog voltages. To measure your current signal you should add a high precision resistor to the loop and then measure the voltage across the resistor. For this you should setup your analog input channel to differential mode. Then you can connect the resistor across the + and - terminals of the analog input on the TBX-68 and connect the leads from the pressure transmitter to these terminals as well. The 6024E will measure the voltage drop across the resistor and you can easily calculate the current using I=V/R. The most common resistance to use for this purpose is 249 ohms, resulting in a 0-5V signal.

The following pages will provide more information and sample V
Is.

http://zone.ni.com/devzone/devzone.nsf/webcategories/7E5833CB6C840B3B86256C41006DB0F1?opendocument&node=200080_us
http://sine.ni.com/apps/we/niepd_web_display.display_epd4?p_guid=B45EACE3EAC356A4E034080020E74861&p_node=DZ52304&p_source=external

Christian L
NI Consulting Services

[Craig Murray]

Thanks Christian for the quick responses. I will give it a try.

Thanks again
CM