S5 and Thermocouples

  • Thread starter Mario Castro Zamora
  • Start date

Thread Starter

Mario Castro Zamora

Hi all,
I have a S5-135U and I want to connect some TC type E without the compensation box. How could understand the input values, or anybody knows
how works the "integration measure method"?

I'm not very familiar with the S5-135U, but if your not going to use an input made for TCs, then the next best approach is to use a linearizing transmitter with a 4-20mA input. Then you simply scale the input to the T/C range. You'll also dodge the cold junction error issue that way.

But if you insisit on linearizing the input yourself, then use the following polynomial for directly converting the mV input:

V is input in millivolts.

V(17.0225 + V(-.220972 + V(5.489x10^-3 + V(-5.76699x10^-5))))

The result is in deg C and should give you better than 1 degree accuracy so far as the calculation in concerned, however you won't have any cold junction compensation which can add significantly to your error. The polynomial is fairly math intensive and I dont know about the capabilities of the S5-135U. The type E T/C curve is relatively linear except at the lower end so you might find it more practical to use a 5 or 10 break point look up table and use a simple y=mx+b equation, with m and b taken from the table. You can download the data for your table from mega.com, then decide on the best break points for your table.

Donald Pittendrigh

Hi All

The Math below is quite acheivable with the 135U
CPU but a far easier road if you have a Profibus interface on the system, is to use the S7 range
of ET200M Profibus station with S7300 analog input
cards, the Thermocouples can be coupled directly to the card, compensation can be done externally or internally and types N, E, J and K are permitted. The linearisation is done by the card.

If you can use PT100's instead they can be coupled
directly to the S5 analog input cards,

Donald Pittendrigh

Daniel Chartier

Hello, Mario;
In the world of Siemens S5, you normally use specialised FBs (function blocks) to read an analog value; If you want to read the input value directly, follow the following instructions, copied from the Siemens Web page
( www.ad.siemens.de, Products and support, Support )

Automation Systems > SIMATIC S5 > Central modules/Compact devices > S5-115U/H/F

"Reading in an analog value without using the integrated function block FB 250 (CPU 941-945)
This entry is available in the following languages:

Entry ID: 150777
Date: 12/01/1997

How can I read in an analog value without using the function block FB 250?

You can also read out the analog value without the FB 250. After loading the analog value, you must extract the Ü-bit, F-bit and X-bit by shifting the input word 3 positions to the right. If the analog value is negative, then you must
set the sign bits after the shift.

Sample program: reading in an analog value with fixed-point representation
(unipolar or bipolar).
:L PW 128 Load the analog value
:T MW200 Store in flag word 200 (scratch flag)
:UN M 200.7 Query whether analog value is positive
:SRW 3 Shift 3 times (extract Ü-bit, F-bit and X-bit)
:SPB =M001 If positive, jump to M001
:L KH F000 Load mask for sign bits
:OW Set sign bit
M001 :T MW 128 Transfer value to target area

Set the method of representation to complement on two (fixed-point representation) on the analog input module.

Print preview

Contact Display navigation


© Siemens AG, 1998, 2002
All information provided on this page is for informative purposes only and is not binding. We reserve the right to make changes at any time without notice. Any liability or warranty for the accuracy of the information is explicitly

Meeuwis Bouw

Consider using an S5 temperature control card IP244. It comes with the right function blocks to connect up to 13 thermocouples and a PT100 for cold junction compensation. It will fit in a 115 or 135 or 150 PLC.
Good luck,
Meeuwis Bouw
Superrfos Packaging