RTD swap to thermocouple, question

Hi people, I have another scenario and I'm looking for answers. The situation is an extruder and die that has electrical heater control done with a PLC5. The PID loops for these heaters were referred to by RTD's. Recently, one of our techs replaced the RTD's with thermocouples. He failed to startup and checkout the machine, so I am left with this. We have a bit (b3 example)that is labeled as a "Zone 1 alarm RTD open heating. This bit is the output to a rung that looks like this. A normally open fault enable bit, which leads to an N17:0/0. This N17:0/0 is labeled as "zone 1 underrange". In parallel with that bit is another bit, normally open N17:0/8. This bit has no description. After these two integer bits in parallel with each other comes the output. What I am having trouble comprehending is this. I search the N17:0/8 or 0/0, and the only place is shows up is in this rung. WHen viewed in binary under the data table, I can see that n17:0/8 is set to 1, and 0/0 is set to 0. I know that this number is being written in there somehow, because I cannot simply force them to the opposite states. When I change the value, they are changed back again. I see where various RTD's are wired to inputs that are labeled as N17 words, but the prints show nothing in the column where the word N17:0 would be. Therefore, I am perplexed as to where this information is coming from. My understanding is that with the old RTD's we used this word, and now with thermocouples we will not need them. But before I take them out, I'd like to have a better understanding why they were there. Any input will help, sometimes if I jog my brain I can figure things out, but I have hit a road block. Thanks in advance.

Russ

Where the old RTD was plug.It is in a RTD module in your PLC?
You cannot plug a thermocouple in a RTD module.

The RTD module generate volt in the RTD. The RTD is look like a variable resistor.

The Thermocouple generate volt by itself. You must having a Thermocouple module in your PLC.

RTD and Thermocouple works very differently.

Russ,

When you said,

but the prints show nothing in the column where the word N17:0 would be. Therefore, I am perplexed as to where this information is coming from.

Click to expand...

Quick answer: It’s probably (almost certainly) coming from the input module via a Block Transfer Read.

When you said,

My understanding is that with the old RTD's we used this word, and now with thermocouples we will not need them.

Click to expand...

I wouldn’t bet on that. If the thermocouple lead ever breaks, how do you want your control system to react? The bits you are referring to appear (based on your post) to be the under-range and over-range signals that would let your controller know that something is wrong in the field. Your desire to learn more about these bits is a very good one.

Moving right along,

Are you aware that for the PLC-5 family (not the SLC-500 family of processors) analog signals (examples: those from thermocouple modules and RTD modules) must be handled through Block Transfer Reads and Block Transfer Writes? Reading between the lines, I’m guessing that you’re a little confused with how these work. We can definitely help, but we can do a much better job if you give us some more information up front.

What is the catalog number of the module you are using? (example: 1771-IFE/C, 1771-IXE, 1771-IR/D, etc.). Let’s hope that the dude who replaced your RTD’s with thermocouples also replaced the module to match. Like Mr. Allen Bradley said, RTD’s and thermocouples cannot simply be interchanged. They must be connected to different modules. Or are you using thermocouple transmitters which convert the millivolt signal from the thermocouple into a 4-20 milliamp signal for a 1771-IFE type input module? Maybe the dude replaced the RTD’s and their matching transmitters with thermocouples and their matching transmitters. Once you give us the catalog number of the input module, we’ll be in a better position to proceed - but the more you can tell us about how the signals are wired, the better off we’ll all be.

Next, find the Block Transfer Read associated with this module. What is the “Data File” entry for this Block Transfer Read? (example: N17:0).

In the same Block Transfer Read, what is the “Length” entry? (example: 12).

In the same Block Transfer Read, what is the “Control Block” entry? (example: BT11:0).

Next, find the Block Transfer Write associated with this module. What is the “Data File” entry for this Block Transfer Write? (example: N17:100).

In the same Block Transfer Write, what is the “Length” entry? (example: 37).

In the same Block Transfer Write, what is the “Control Block” entry? (example: BT11:1).

You may be able to save time by giving us the contents of the “configuration” for this module. To do this (using the examples given above) start at N17:100 (first word of “data file” for Block Transfer Write) and give us the values of the 37 words (“length” entry for Block Transfer Write) of data. (Example: N17:100=1280, N17:101= 1234, etc., etc.).

If this sounds complicated - well, it could certainly get that way. But I think I know what’s puzzling you - and this is some of the ground we’ll have to cover in order to answer your questions.

More info

Ron and others:

We are using a 1771-WF card. The biggest problem I have is that I cannot find the BTR or BTW for these. Would these be found in the PID ladder typically, or alarms, or somewhere else. I looked under I/O configuration and did notice a possible problem. These racks are all communicating via control net. When looking under I/O config, it tells me that several of these racks are not mapped. They are shown as node 1, 2 3, etc, but none of them are mapped. Am I correct in thinking that these must be mapped to an appropriate N file?

I cannot tell you exactly how these new thermocouples are wired, other than to say they do not appear to be any different than anything I have seen before. I do not have updated prints to help with, so I cannot even tell you for sure. I printed off the manual to the analog card we are using and will try to decipher some more. If this helps you to understand more of my situation I am open to more suggestions. Hopefully I have answered some of your questions. I will post more as I find out more.

Russ

The 1771-WF is the wiring arm, not the module. That part is used with the 1771-IR RTD module. If you were using a 1771-IXE thermocouple module, you would have a 1771-WI wiring arm and if you were using a 1771-IFE analogue input module, you would have a 1771-WG wiring arm.

So, it would seem that your problem is that you have thermocouples connected to a module that works with RTD's.

my mistake

Sorry, my brain is getting a little goofy. To set the record straight, yes, the 1771-wi is the wiring arm for the old card, which was for the RTD's. I wrote down the info for both and wrote the wrong one on here. The new thermocouples are on a 1794-IT8. This is what the prints are calling them. I did not look on the rack to verify as the line is currently running. I apologize for the glich. An example here is as follows,

Extruder Zone 1 thermocouple input is N17:200. This word is where the input is written to, correct? Now N17:208 is where the overrange and underrange bits are located. I'm just confused on how those bits are set. I still have not found a BTR or BTW anywhere for these. THanks for the help thus far.

Russ

Understanding a little more

Ok men, I feel like I'm getting a grasp on this stuff. What we have is a 1794 Flex I/O module. This is where the thermocouples are wired in. The address for these inputs are N17:200 on up. There were 6 zones, therefore the 6 intputs were words N17:200 to N17:205. Now, after reading some of the Flex I/O maunal, I understand now that the 9th word is where the overrange and underrange bits are. That would make the over under word N17:208. I also see that the first 8 bits of this word are for the underrange, and the following 8 bits of word N17:208 are for the overrange. After reading this, a lot of what I see is falling into place. My only question now is, where are the BTR and BTW typically found in a program. I still have failed to find them. The system is now functioning, as I found the correct bit for Zone 1 (N17:208/0 and N17:208/15). As a test, I unplugged the thermocouple to zone 1, and the overrange bit N17:208/8 went true. Therefore, somehow, somewhere, I know that they are reading the info from this card. I just need to find out where. This is not a matter of understanding any longer. I just want to see the entire process from start to finish to 100% grasp everything being done. Therefore, my only remaining question is, is there an easy way to find the BTR for this card? I am also curious as to where/how the card is configured to write the input data to the desired N file. Thanks to everyone for the help thus far.

Russ

again

Boy, I'm beginning to feel like I'm talking to myself. The question now is this. I have done a search of the complete ladder for a BTR instruction, and there isn't a single one. Can anyone shed some light on how this information is getting transferred to the main processor? I thought based on some of Ron's info that these were needed, yet this system seems to work without them. Any light on the subject?

In an earlier post you mentioned that the I/O is on ControlNet. With this, what you can loosely consider an I/O scan is configured with another software package called RSNetworks. This package produces a configuration file which is linked into the PLC project file and defines frequency and destination of the data transfers. Block transfers are not used.

Unfortunately, I have no practical experience with this configuration, so the above pretty well exhausts my knowledge on the subject.
beerchug

that's what I thought

Gerry,

Thanks for the info. I thought that's probably how it was done. We have RSNetworx on the computer we use to talk to this batch of PLC's. Unfortunately we do not have an offline copy of that configuration at our leisure, nor do we have the appropriate driver/card needed to interface it. There is one guy in the plant that has access to that, at least that is what I have been told. Anyways, I now have a decent grasp on what they were trying to accomplish it, and how and why it didn't work before. That is pretty much what I was hoping to learn. Thanks to everyone, hopefully someone else can follow the posts here and get an idea of what is being done as well.

Russ

Russ,

When I said that Block Transfers were essential - I didn't know at the time that you were using Flex-I/O and ControlNet. As Gerry said, with ControlNet you don't need the Block Transfers. Sorry if I misled you - but I think you’re beginning to see how important it is to give the system details up front.

I wish that I had time to work through the rest of your problem but my work schedule is going to keep me tied up for a day or two at least.

Good luck.

Conclusion

Ron,

You're misleading was not due to error on your part. Because I was working without updated prints, it took me awhile to find out exactly where the TC's were wired to. After a little digging, I found out that they were wired not to the main PLC rack, but to a remote I/O rack. The system was functional as of yesterday, but the remaining questions have been answered. I understand now how the tech messed up, simply by using the incorrect over and underrange bits. On the whole, your input was very helpful, as was everyone elses. Thanks to you and the whole situation in general, I now have a much better understanding of the process. I do know now as well that the Input information is mapped thru RSNetworx. I would like to go thru this whole process sometime and actually install something new, but for now this will have to do. Thanks again for everyone's help.

Russ