How To Sense Temperature with a Do-more PLC from AutomationDirect

Reading temperature with a Do-more PLC is easy. In fact this is all the code you need right here. Don't see any? Me neither - there isn't any. You don't have to write a single line of code to get temperature values with a Do-more PLC! You'll have to write code to USE the temperature, of course, but once you configure the temperature modules, the temperature values just appear ready for you to use. Easy. In this example we have a Do-more PLC connected to 2 thermocouples, 2 RTDs, and 3 different types temperature transmitters, a Sensor Head Mounted Transmitter, an integral transmitter, and a fixed DIN rail mounted transmitter. Here's what it looks like on the test rack- Here's the Do-more PLC, 2 thermocouples, 2 RTDs, and 3 different types temperature transmitters. They are wired exactly like the block diagram shows. The sensor wire shields are grounded only at the sensor end - if you ground the shield at both ends you just created another current path which can cause ground loops and actually make things worse than having no ground at all! So be careful to only ground sensor shields at one end. The thermocouples use the special thermocouple DIN rail terminal blocks - these ensure continuity of the special thermocouple wire by clamping the wires together instead of connecting them through a terminal block which would create unwanted thermocouple junctions. To configure the thermocouple module, you just move some jumpers. Select the number of channels you are using with these first two jumpers. We're using two channels, so we only want a jumper on terminal 1 - exactly like you see here. The thermocouple module supports all kinds of sensors and can even be used as a 5V or 156mV analog input! We have Type J Thermocouples, so it looks like we need all 4 jumpers installed - and we do, so we are good to go. The last two jumpers select the units - degrees C, degrees F, and you can select whether you want the result to be signed or 2's complement - which some systems require. We have this one is set for 2's complement, degrees F. We're only using 2 of the four channels here. Since these channels measure extremely small voltages, it is easy for them to pick up stray signals from electrical noise when not connected - so make sure you jumper any un-used channels to keep your system as electrically clean as possible. That's it for the thermocouple module, let's look at the RTD module. The RTD module uses the same jumper settings for the number of channels, no jumpers is one channel, this jumper adds a channel, and this jumper adds two channels. We're using two channels total, so we need to add one by putting a jumper in this slot here. The RTD module supports all of these RTDs, and we are using Pt100's so we need these two jumpers installed, just like you see here. The units selection is just like the thermocouple module, again we have it set for 2's complement Degrees F. Finally, we're using an analog input card to read the 4-20mA signals from three temperature transmitters and it has a similar jumper selection setup. We're using three channels, so we need the jumper that adds 2 channels. Perfect. Well, the modules are all configured, so let's look at the software. As we mentioned earlier, you don't have to write any code to get the temperature values once the modules are configured. The cool thing about the Do-more Designer Software is when you connect to the Do-more PLC, it automatically identifies all the modules you have plugged in and assigns memory locations for the results. We see that under system configuration, I/O Mappings. Here we see the analog card had 8 status bits we can read and 8 analog values we can read. The Thermocouple and RTD modules are the same except they have 4 status bits and 4 analog values, one for each channel. All of the details about these modules status bits and mappings and configuration is in the Do-more Hardware manual, which is free to download. That's important don't go looking in the individual module's specs or manuals those are maintained for the older legacy processors, not the Do-more. Everything you need to know about connecting a module to a Do-more is in the Do-more hardware manual. Here's a Dataview showing us all of the Thermocouple and RTD values. These values have an implied decimal point. If we put a cup of cold water on one of the probes and hot water on another, we see the values change. Exactly what we expect. If we add the temperature transmitter values to the dataview, we can see that the results aren't in degrees F. Remember, the analog card takes 4-20mA in, and converts that to a number in the range of 0-4095. Wouldn't it be great if we could get the corresponding number in degrees F? Well, The cool thing about the Do-more is it has a scale function that does exactly that. We just specify the input range and the range we want that to correspond to, in this case 0 to 100 degrees F and put that result into a Real memory type and we are done! Now if we watch these values in the dataview, we get actual temperature values with decimal point and everything which makes writing your ladder code much easier. I have a C-more HMI monitoring all of those values. If we swap the probes, we see the results graphically. Perfect. And if I disconnect one of the wires, the indicator that is monitoring that status bit provided by the temperature modules lights up so I instantly know there is an issue with the wiring of the sensor. Awesome. And again, we didn't have to write one line of ladder code to get these temperature values though we did add a scale function to makes things a little easier to work with. If you have any questions or need any help, please don't hesitate to contact AutomationDirect's free award winning tech support during regular business hours. They will be happy to help. And don't forget the forums there are lots of folks there that love to share their years of experience. Just don't post support questions there, AutomationDirect's support staff doesn't monitor the forums on a regular basis.