Problems with USB-6218 wiring diagram in DAQmx

[MrDomino]

I'm using MAX to set up thermocouple measurement tasks for a portion of our Diesel engine DAQ. It's a pretty simple portion of DAQ - read in temperature measurements from thermocouples and then plot them. The problem I'm running in to is with the wiring diagram. Looking at the USB-6218, it's labelled for analog inputs but when I pull up the pinout in the Measurement and Automation Explorer (picture is attached), it shows that the pins are labeled differently.

For example, say I choose to have a thermocouple (two wires) in analog input 0 (ai0). The wiring diagram shows that I should connect the two thermocouple wires to channels 15 and 16 which are AI0 and AI8, respectively in the pinout diagram. Then say I assign another thermocouple to analog input 8 (ai8), it tells me to write up the two thermocouple wires to channels 16 and 28 which are AI8 and AI GND, respectively on the pinout diagram. This is fine except for the fact that both ai0 and ai8 are using the same channels on the DAQ board.

I get the same thing when I select ai1 -> CH 17 (AI 1) and CH 18 (AI 9) and then ai9 -> CH 18 (AI 9) and CH 28 (AI GND). Channel 18 (analog input 9) is used for both thermocouples.

Any idea as to why LabVIEW is overlapping analog inputs?

[Dennis_Knutson]

Differential measurements require two inputs. AI0 is paired with AI8, AI1 with AI9, etc. With differential, you can only use half of the available channels. If you assigned one to AI0, you should not use the second channel of the pair. In other words, don't try to assign AI8 if you have already chosen differential. Your choosing it probably caused a switch to single ended measurement. With single ended measurements, you can use all of the analog in channels.

[MrDomino]

Is there any way to specify which differential I want to use? It's doing very weird pairings on its own (AI0, AI8) which aren't that close so it's going to be a huge pain to try and wire in a bunch of thermocouples.

[Dennis_Knutson · Accepted solution]

No, you can't change the pairings and as the image shows, ai0 and ai8 are right next to each other. Can't be any closer.

[MrDomino]

All of the sudden it became clear what is going on. Thanks for the help!