It looks like you have a great start on your application. As a place to start, I would recommend that you take a look at this resource that gives a general overview of PID with LabVIEW. There are many resources on the internet that can explain the correct PID algorithms to use with your system. Essentially, you should already have all the pieces except for the precise algorithms to use to create the PID control system. A few resources to look at are here and here. If you are still unable to build your application, you may consider taking advantage of National Instruments' investment into this area and purchase the PID toolkit which will save you considerable development time. Have a great day and good luck!
If you are worried that your DIO-400 is not functioning correctly, I would output a signal using Measurement & Automation explorer. If you go under My Systems > Data Neighbordhood > FPName > cFP-DIO-400 @X, you should be able to double click on the value column for one of the channels and output a signal. Once you have verified the functionality of your FieldPoint module, I would use highlight execution within your application to help you determine where the main issue with your application is. Please let me know once you've done this and let me know if you're still having problems.
Thanks for the replies Jim and Ravens Fan. I'm going re-think how I go about doing this program and re-work what I've done.
A DO module can only be on or off, 100% or 0%. A PWM would be better in that you could set it for percentages in between. I just don't know how you would connect it to your heat tape. I would assume that the heat tape would require a higher current than you could pass through a module. You would have to compare the requirements for your heating system to the specifications for whichever module you would use.
One possibility to still allow you to use a DO module (which I would assume you are using a relay to switch on and off the higher current of the heat tape), would be to set up a loop. Let's say you want 50%. Then have a loop run so the relay is on for 1 second and off for 1 second. For 33%, 1 sec on 2 sec off. As long as the total cycle time is not so long that the tape heats up too much during the On time and you see wide fluctuation in your temp reading, but not too short where the extra cycles causes problems with wearing out a relay. With a total cycle time that works well, you could have a ratio of on to off times of the relay that would effectively give you reduced power through the heat tape.