NI 4350 S1/S2 Switches (TBX-68T)

[ArchivedPointOnePa]

I'm using the TBX-68T terminal block with an old NI-4350 USB device for analog input from a pair of thermistors.  The TBX-68T has a pair of switches labeled "S1 S2", which are for the current source(s): (the 4350 has a 25uA source and the 4351 has an additional current source).

My question is, how are these switched wired?.....(i.e. why do I need to throw both switches ON to get the 25uA current source on the 4350)?

 It works, but I may also have to use the 4351 and I want to know the details of how S1/S2 function.  Originally I thought S1 would be for the 25uA current source and S2 would be for the 2nd current source in the 4351, but I had to through both S1 & S2 to get the 25uA supply.  

I've checked the online NI PDF documents for the 435x and 4351 and I've not been able to find this detail.

[Brooks_C]

Hello PointOnePa,

I will look into this to see if I can find out why both switches have to be used to access the current excitation.  However I believe that on the 4351 you will still only be able to access one of the current excitation values.  Which ever excitation value you select in software will be available if S1 and S2 are switched on.  I do not believe there is a way to access both excitation levels at the same time.  I will do a bit of research and see if I can find any evidence to the contrary.

Are you trying to access both excitation levels at the same time or just trying to figure out how to access the 1mA level externally?  If you just want to get the 1mA level then you just need to select this in software and turn S1 and S2 to on.

Let me know what you're looking for and I'll let you know what I find.

Cheers,

[ArchivedPointOnePa]

>Are you trying to access both excitation levels at the same time or

>just trying to figure out how to access the 1mA level externally? 

  No, I'm just using the 25 uA current source and it is working for two thermistors in series.

  I was checking the circuit and was curious as to the function of S1 & S2.

  I know this is old hardware (not suported by MAX under Windows Vista).

Actually I'm debugging an old installation that has 2 thermistors (in series using the 25uA current source) and two Omega pressure sensors (PX01 series with 4-20mA output, and a 12-30Vdc supply).  The board has been having problems so I was checking into the details, including the function of S1/S2.  I think the problem was a power supply for the pressure transducer that was unreferenced to analog ground.

 (but I was still curious about S1/S2).

[Brooks_C]

Hello PointOnePa,

It looks like finding out the exact connections these two switches make is going to be difficult.  However, I think that their function is pretty clear--that switching both on allows access to the excitation current externally.  If you're having trouble with some aspect of the board I would ba happy to troubleshoot that with you, but I think it is unlikely that these switches have anything to do with it.

Can you post some details about what "problems" you've come across, and can help give you some direction as to what the issue may be.

Cheers,

[ArchivedPointOnePa]

Brook,

Thanks for checking into this.  I was just curious about the switches, since their are two switches to do one function.

 Regarding the other measurement issue, I've already resolved it using one of the NI documents that discusses measuring the voltage of a floating supply with a differential input ADC.  I referenced the floating supply to AIGND and that has resolved the problem.  Thanks for the followup.

[Brooks_C]

Hello PointOnePa,

It appears that the switches may just be for the + and - lines, but since its an old product I haven't been able to find anything confirming this.  The easiest thing to do may be to probe the continuity on the terminal block with a DMM to determine what they connect to.  I'd do this myself, but we don't have this particular terminal block on hand.

My guess is that there are just two single pole single throw switches rather than one double pole double throw switch to connect both the + and - excitation pins.

If you do decide to probe the continuity I would be great if you can post back with your results so other interested forum members can know as well.

Best Regards,