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Ever use the Thermocuple or RTD card in the ET200SP remote IO lineup? Keep reading so I can maybe save you some time on your next temperature sensing project. That is of course if that involves the et200sp line of io cards. Examples of the devices are below.
6ES71936BP400DA1
SIMATIC ET 200SP, BaseUnit BU15-P16+A0+12D/T, BU type A1, Push-in terminals, with 2x 5 add-on terminals, New load group, WxH: 15 mm x 141 mm, with temperature acquisition
SIMATIC ET 200SP, Analog input module, AI 4xRTD/TC High Feature, suitable for BU type A0, A1, Color code CC00, channel diagnostics, 16 bit, +/-0.1%, 2-/3-/4-wire
Which Base Units can you use for the IO modules of the ET200SP? Great question! Just incase you haven't seen this, there is a document that details every configuration possible. This FAQ that is found on the Siemens Industry Online Support Website describes well what all the base units adders mean. ID# 108894595
So it started out by asking the customer if they required RTD's or Thermocouples. They said RTD's and then quickly changed to thermocouples after an order was placed. I think here the biggest issue was not knowing the customer required the use of the Thermocouples at all. So luckily the card was able to accommodate the Type J thermocouples. Well, So I thought. Turns out the card is capable but in order to design with the TC portion of the card, you do need a special base unit.
When I configured the channel and went online, It read the highest value possible raw value at 27648. This is an error code and does say the cause in the manual for the AI card. This error was indicating that I didn't have the required base unit (BU) to include the Temperature Coefficient offset from the base unit as I had actually configured it for. There were a couple of settings there I definitely gave it the old "college try" to see if any of could have helped.
Nothing worked until I found out I needed a different base unit, one which has a temperature sensor built right in. This sensor helps define the temperature offset required for an accurate reading. To be fair the manual did default to telling me this a few different ways.
So why do I need this specific base unit for thermocouples? Turns out you may be okay without it, but keep reading.
Simply put the temperature of the surrounding environment directly affects the reading of a thermocouple so to offset this difference a localized temperature reading is required. Thankfully for this High Feature IO card, there were two options to tackle the configuration method.
I advised to just continue with purchasing an ~A1 base unit, due to being an OEM builder. I chose to use the base unit which can be configured in TIA Portal to allow the thermocouple coefficient to be maintained by the "Internal reference junction". This setting was found under the properties and configuration of the base unit with the device view of the TIA Portal software configuration.
It was fortunate for me that the required ~A1 base unit was sorted ahead of time, and did not cost production any downtime.
I will say at least if you find yourself in a pinch, thankfully a second alternative was to supply the reference temperature via a channel of the analog input module. This again would provide the corresponding temperature coefficient for offset within the use of a configured thermocouple or RTD on the analog input card. I found this to be a bit useful for the just in case situation like above. However, using this method does consume a precious analog input channel. We all know these also come with a price in a different way.
TL;DR The base unit with temperature acquisition built-in is cheaper than using an IO channel on the RTD/TC AI card for the ET200SP. Do consider buying the type A1 Base unit just in case the customer has a change of mind in the last minute of design. Better safe than sorry. The feature can be used or not, the difference in base unit price is minimal.
Base Unit here includes 4-wire landing pad for powering the a device.
Many configurations are possible, plan on speaking with your local Siemens specialist.
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