using scxi 1303 and 1100 for pressure measurements

[mgreenwood]

Hello,

I am using the scxi 1303 and 1100 to measure voltage inputs. Specifcally I have  8 voltage measurments at 0-10V and 9 pressure measurements at 4-20mA converted to 1-5V signals with a 250ohm resistor. 

My question now arrises regarding the pull up resistors. I do not have any resistors inserted (as the manual leads me to believe they arn't necessary). My problem now arrises from a difference in the voltage across the resistor (same as pressure transmitter display) and the voltage displayed by Labview.

For example:

Actual voltage = 1.47 V

Labview Display = 1.18 V

Does the 1303/1100 setup require the 10ohm resistors or do you think that something else is going on?

Thank you and forgive me if this has been addressed!

[fromm8]

Hello mgreenwood

You say you do not have any resistor inserted, does it means you do not have the Rpullup (Rp5,Rp6,Rp7,Rp8) and the Rbias (Rp1,Rp2,Rp3,Rp4) in your accesory 1303?

If we check the manual, figure 1, we do no have those resistors? Are we connecting the channel directly?

Are youn connecting the CH+ where the current goes into the resistance and the CH- where the current is coming out from the resistance?

Regards

Frank R.

[mgreenwood]

I do not have any resistors installed (i.e. Rpullup (Rp5,Rp6,Rp7,Rp8) and the Rbias (Rp1,Rp2,Rp3,Rp4) resistor slots are empty). My pressure transducers (PT) and other voltage signals are grounded near where they are located.

 My PTs are wired as described on OMEGA.com. I attached a picture of the layout.

[fromm8]

Hello mgreenwood

The connections seems to be fine, but what about the grounding. Is the negative reference of your power supply connected to ground? 

How exactly are you measuring the actual voltage?

Is the same behavior for different channels?

Regards

Frank R.

[fromm8]

Hello mgreenwood

The connections seems to be fine, but what about the grounding. Is the negative reference of your power supply connected to ground? 

How exactly are you measuring the actual voltage?

Is the same behavior for different channels?

Regards

Frank R.

[mgreenwood]

Yes the negative reference of the power supply is grounded.

I am measuring the actual voltage using a multimeter across the 250ohm resistor. I then compare that value to the mA signal reported by each PT and convert it to voltage. Those two point of comparison always agree. I then have also checked the voltage across the +/- ports of each channel to determine if the voltage was still correct at that point. Every channel has correct readings.

Next, I compare the actual value to Labview output. And this is where voltages go wrong. To determine what was going on I went ahead and took the leads of one PT and sent it to another 1303. That reading was correct. I then tried different channels/removed channels etc. Regardless of the channel, outputs were off.

I then tried to simplify my problem and just looked at 1 channel. I removed all leads. I then attached a 9V battery to CH0. When I ran my program all the channels that were empty displayed voltages around that of CH0 (not exact but close). I then tried different channels... same result. Now I thought that was pretty weird. So I changed my program so it only read CH0 and CH1. I then hooked the 9V to CH0 and a 1.5V to CH1. (At all times I checked voltages with the voltmeter). When I ran my program both of the values in the channels were wrong!!! the CH0 read low and CH1 was up to 2.5V.

At this point I think there is something wrong with my terminal block/module. They aren't new so I will see if there are some others laying around that I can appropriate.

If you have any insights I would be all ears... well eyes 🙂 If replacing the module/terminal block works I will be sure to post that.

Thanks for your help in trying to get the problem nailed down. Have a nice day/night.

Scott

[fromm8]

Hello mgreenwood

According to your 2nd paragraph, if another 1303 works fine, this is a detail that you need to confirm. If both do not have resistors and they are tested under the same conditions you should be expecting the same result. If this is not the case, well you could think in a HW issue. You should test both accessories and multiple channels to confirm their status.

Regards

Frank R.

[mgreenwood · Accepted solution]

Good news... After much testing, time, and tears I have fixed the problem. I was indeed having a hardware issure. The SCXI 1100 module I was using was broken in some manner and has been replaced with a functional SCXI 1102.

Thanks for helping me think of troubleshooting ideas!!!