Issue with S7-1200 Type K Thermocouple

[sonia kiangala]

Dear experts,

I am using a Thermocouple K type and connected to a SM 1231 signal board (6ES7 231-5QD32-0XB0) controlled by a S7-1200 PLC.
In my hardware configuration I have set the my signal board to Thermocouple, Type K, degree Celsius. I was supposed to read my temperature on the IW inputs already in degree celsius and divide them by 10 according to the manual to get the final value.
But I am getting a constant value 65534 which I found out to the double of 32767 the value one obtain when invalid data is received on the channel. I am at room temperature. At a certain pointing time it seemed to work fine and gave me right temperatures. Suddenly it changed again and since I am only reading these strange values 65534 on my two connected channels. I am suspecting the wiring but I can't find where the mistake is exactly.
Can you please assist me with this? As I am busy commissioning a plant and this is really stopping me.

Your assistance, will be much appreciated!

Kind Regards,
Sonia

 

[danw]

I don't know Siemens but a full scale, upscale value typically indicates thermocouple burn-out, which is when a thermocouple analog input is an open circuit.

Driving the indicated/reported temperature upscale (or downscale, depending on configuration) is a visible flag that something is wrong, and when the thermocouple temperature is an input to a heating control loop using reverse action, the control output will drive to zero to avoid putting energy into control loop whose process variable is an unknown.

I'd recheck the wiring.

 

[BryanG]

Have you used thermocouple wiring and thermocouple connectors between the thermocouple and the 6ES7 231-5QD32-0XB0? And is your thermocouple of the type 'isolated'?

 

[Lare]

https://www.google.fi/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwjQo9igp9jPAhVI3SwKHfP2CbQQFggrMAE&url=https%3A%2F%2Fsupport.industry.siemens.com%2Fdokumentation%2FPDFTopicDownload.topicPDF.aspx%3FDocVersionId%3D62121591435%26TopicId%3D61290143627%26Lang%3Den&usg=AFQjCNHNYw80WcDiAhGXttPQvoBX4VxENg&bvm=bv.135475266,d.bGg&cad=rja

https://www.google.fi/url?sa=t&rct=...sg=AFQjCNHP-izYhjTLx41dX2Q4591L78kyDQ&cad=rja

I would check wiring.

Make also sure that you have same 2,3 or 4 point actual wiring that is selected for RTD channel.
Not sure if there is configurable setting for this on 1200, but some Siemens AI cards have it and input is not working if wiring and setting not match exactly.


p.s 65534, can it be -2 (0,2'C)?

https://www.google.fi/url?sa=t&rct=...sg=AFQjCNHP-izYhjTLx41dX2Q4591L78kyDQ&cad=rja

 

[rootboy]

Sounds like the thermocouple opened up on you, you should check your continuity with a meter.