Siemens SIMATIC S7-1200

Technical specifications
A.14 Analog signal boards (SBs)
S7-1200 Programmable controller
1520 System Manual, V4.2, 09/2016, A5E02486680-AK
Technical data
SB 1231 AI 1x16 bit Thermocouple
Cable length (meters)
100 m to sensor max.
Wire resistance 100 Ω max.
Table A- 224 Diagnostics
Technical data
SB 1231 AI 1 x 16 bit Thermocouple
Overflow/underflow
1
Yes
Wire break
2, 3
Yes
1
The overflow and underflow diagnostic alarm information will be reported in the analog data values even if the alarms
are disabled in the module configuration.
2
When wire break alarm is disabled and an open wire condition exists in the sensor wiring, the module may report ran-
dom values.
3
The module performs wire break testing every 6 seconds, which extends the update time by 9 ms for each enable
channel once every 6 seconds.
The SM 1231 Thermocouple (TC) analog signal module measures the value of voltage
connected to the module inputs.
The SB 1231 Thermocouple analog signal board measures the value of voltage connected
to the signal board inputs. The temperature measurement type can be either
"Thermocouple" or "Voltage".
● "Thermocouple": The value will be reported in degrees multiplied by ten (for example,
25.3 degrees will be reported as decimal 253).
● "Voltage": The nominal range full scale value will be decimal 27648.
A.14.4.2
Basic operation for a thermocouple
Thermocouples are formed whenever two dissimilar metals are electrically bonded to each
other. A voltage is generated that is proportional to the junction temperature. This voltage is
small; one microvolt could represent many degrees. Measuring the voltage from a
thermocouple, compensating for extra junctions, and then linearizing the result forms the
basis of temperature measurement using thermocouples.
When you connect a thermocouple to the SM 1231 Thermocouple module, the two dissimilar
metal wires are attached to the module at the module signal connector. The place where the
two dissimilar wires are attached to each other forms the sensor thermocouple.
Two more thermocouples are formed where the two dissimilar wires are attached to the
signal connector. The connector temperature causes a voltage that adds to the voltage from
the sensor thermocouple. If this voltage is not corrected, then the temperature reported will
deviate from the sensor temperature.
Cold junction compensation is used to compensate for the connector thermocouple.
Thermocouple tables are based on a reference junction temperature, usually zero degrees
Celsius. The cold junction compensation compensates the connector to zero degrees
Celsius. The cold junction compensation restores the voltage added by the connector
thermocouples. The temperature of the module is measured internally, and then converted to
a value to be added to the sensor conversion. The corrected sensor conversion is then
linearized using the thermocouple tables.