Description
1-4
SIMATIC 505–2556 Installation and Operation Guide
1.3 Thermocouple Input to Digital Conversion
The
following equations may be used to calculate the digital word in decimal
format which will result from a particular thermocouple input:
Thermocouple Mode, Digital W
ord (WX) = Degrees X10
Millivolt Mode, Digital W
ord (WX) = Millivolts X100
As an example, the following figure illustrates the effects of a change in
input level going from 0
_
to 102.4
_
F in the Thermocouple Input Mode.
Figure 1-3
Example Change in Input Level
When data format is selected as SCALE the full temperature range of the
thermocouple is scaled as an unsigned integer from 0–32000. The following
formula may be used to calculate the scaled integer value.
Scaled Integer = (measured temp – min temp)
÷
(max temp – min temp) x
32000
For example the scaled integer offset at 0
_
C for a T
ype J thermocouple is:
Scaled integer = 0 – (–210)
÷ (760 – (–210)) x 32000 = 6928
Thermocouple inputs exceeding the ANSI standard of 760 degrees C for
T
ype J, or 1768 degrees C for T
ype R and S, and 1000 degrees C for T
ype E
will cause the overrange bit to be set and the maximum temperature for
that thermocouple type to be returned. Open thermocouples report
temperatures that are out of the allowable range. This condition may occur
due to failure of the thermocouple or due to the thermocouple wire being cut
or disconnected. The SIMATIC 505–2556 will report an open thermocouple
condition within 200 milliseconds.
NOTE:
The SIMA
TIC 505–2556 uses the least significant bit (16) to indicate
an open thermocouple. The value of this bit is set to 1 when this condition
occurs. An open thermocouple condition will report within 200 milliseconds.
Engineering Units
Scale Units
Ef
fect of
Out-of-Range Input
Signals
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