Siemens Simatic M7-400 Reference Manual page 270

Analog Modules
Compensation of the Reference Junction Temperature
There are several options for you to choose from for acquiring the reference
junction temperature in order to obtain an absolute temperature value from the
difference in temperature between the reference junction and measuring point.
You can use internal or external compensation, depending on where you want the
reference junction to be.
The last column of the following table lists the feature you must set for the
"Reference Junction" parameter in STEP 7. The reference temperature value is a
separate parameter in STEP 7.
Table 5-44 Options for Compensation of the Reference Junction Temperature
Option
No compensation
(refer to Figure 5-18 for connection)
Internal compensation
(refer to Figure 5-18 for connection)
External compensation with a
compensating box in leads of an
individual thermocouple
(see Figure 5-19 for connection)
External compensation with a
resistance thermometer to obtain
the reference junction temperature
(recommended method)
(see Figure 5-20 for connection)
External compensation with a
resistance thermometer when
thermocouples with the same
reference junction are divided
amongst several modules
(see Figure 5-20 for connection)
Constant reference junction
temperature (thermometer, ice bath;
see Figure 5-18 for connection)
5-54
Explanation
When you want to acquire only the difference in
temperature between the measuring point and the
reference junction
If you employ internal compensation, the internal
temperature of the module is used for comparison
purposes.
You have already acquired and compensated the
reference junction temperature using a
compensating box, which you have looped into an
individual thermocouple.
No further processing is necessary owing to the
module.
You can acquire the reference temperature by
means of a resistance thermometer (pt 100) and
have it calculated by the module for any
thermocouple.
Use a resistance thermometer on a module that
measures the reference junction temperature. Read
in the climatic temperature value to the CPU and
transfer the value to the other modules using
SFC55.
If the reference junction temperature is constant
and known, you can specify this value in parameter
assignment in STEP 7.
S7-400, M7-400 Programmable Controllers Module Specifications
Ref.
Junction
None
Internal
None
RTD on
Channel 0
RTD on
Channel 0
Reference
temperature
value
A5E00069467-07