Automation Direct DL405 User Manual page 167

8–18
F4–08THM–n 8-Channel Thermocouple Input
Temperature and
Digital Value
Conversions
Millivolt and
Digital Value
Conversions
SP1
LDF X20
K12
BCD
LDF X34
K4
OUTX
V3000
X1
LD
V3000
MUL
K1000
DIV
K4095
OUT
V3100
Since the thermocouple devices are non-linear, it is much easier to rely on
published standards for conversion information. The National Institute of
Standards and Technology (NIST) publishes conversion tables that show how each
temperature corresponds to an equivalent signal level.
Sometimes it is helpful to be able to quickly convert between the signal levels and
the digital values. This is especially useful during machine startup or
troubleshooting. The following table provides formulas to make this conversion
easier.
mV Range If you know the digital
value ...
0 to 25 mV
0 to 50 mV
0 to 100 mV
For example, if you are using a
–2 (100mV) version and you have
measured the signal as 30 mV, you
would use the following formula to
determine the digital value that should
be stored in the register location that
contains the data.
Loads the first 12 bits of the channel data word into the accumulator.
The X address depends on the I/O configuration.
Since we are going to perform some math operations in BCD, this
instruction converts the data format.
This LDF instruction loads the three channel indicator bits, plus the
MSB, into the accumulator. The channel data from the first LDF
instruction is pushed into the stack. X34 = X20 + 14.
The OUTX instruction stores the channel data to an address that
starts at V3000 plus the channel offset. For example, if channel two
was being read, the data would be stored in V3001.
When X1 is on, channel 1 data is loaded into the accumulator.
Multiplies the accumulator data by 1000 (to start the conversion).
Divides the accumulator data by 4095.
Stores the result in location V3100.
If you know the analog
signal level ...
A + 25D
4095
A + 50D
4095
A + 100D
4095
D + 4095
A
25
D + 4095
A
50
D + 4095
A
100
D + 4095
A
100
D + 4095
(30)
100
D + (40.95) (30)
D + 1229