New SCXI 1102 gives grounding errors, old 1102 on same chassis doesn't.

[Will_Crowe]

Dear all,

I have a 4 slot SCXI chassis. Recently we added a second 1102 thermocouple card to this. All the thermocouples on this new card (1 year old) show errors (probably due to grounding) when connected to a particular piece of machinery. However, these errors do NOT arise for thermocouples attached to another older (4 years old) 1102 card on the SAME chassis. I can't see any difference between the cards or the connector blocks.

Has anyone seen anything like this before?

Also, if anyone replies to this query, how do I then reply to them. I couldn't respond to the generous help that I receieved from this forum the last time.

Will.

[Seamus_C]

Hi Will

It is quite difficult to say why you are getting these bad readings.  It could be due to a number of reasons. 

Are you using the same Type Thermocouple with both modules?

Have you got your Cold Junction Compensation (CJC) set up?

Have you got any Open Circuit channels on your module?

See   

http://digital.ni.com/public.nsf/websearch/91262E2F72A608A9862564F3007B11F6?OpenDocument



Have you set up your second module correctly.  I would recommend checking all your settings in MAX, and then carrying out tests to try and find the problem. See the following tutorial on setting up the 1102 to make sure it is correctly set up. 

http://zone.ni.com/devzone/conceptd.nsf/webmain/5f569e1eeb62edb68625686600601f68

I hope this helps.  Please reply back and let me know how you get on.  To reply click the reply link above my name.

Seamus

[Will_Crowe]

Hi Seamus,

Just to update you on the problem. I found that the resistor bank (pull-up resistors to allow open thermocouple detection) had 10ohm resistors on the low voltage side, whereas on our other 1303 connector block, 10mega-ohm resistors were used. This seemed to be causing the problem. I have read a little bit about this, but I'm still not sure what the difference is, or why the factory default is 10ohm, whereas the reccomended value for thermocouple sensing is 10 mega-ohm.

Barry