Banner Sure Cross DX80 Instruction Manual page 26

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Sure Cross Radio Networks and Host Controller Systems
When using a 4 to 20 mA sensor with a 0 to 20 mA input, the sensor uses the 4 to 20 mA section of the total range. Using a
4 to 20 mA with a 0 to 20 mA input allows you to determine when you have an error condition with the sensor. A normal
input reading between 4 and 20 mA indicates a functioning sensor whereas a value below 4 mA indicates an error condition,
such as a broken wire or loose connection. Some Sure Cross devices allow you to configure the analog inputs and outputs
to use either 0 to 20 mA or 4 to 20 mA.
Signed Numbers
Temperature values are stored in Modbus registers as two's complement signed numbers. Using two's complement allows
negative numbers to be stored in Modbus registers.
Although not technically a sign bit, the most significant bit (MSB) indicates a negative number when the value is set to one
(1). When the most significant bit is zero (0), the value is greater than or equal to zero.
Modbus register values of 32768 through 65535 (decimal) represent negative temperatures. These numbers in binary form
are: 1000 0000 0000 0000 through 1111 1111 1111 1111.
To convert to a negative temperature value from a Modbus register value, first convert the value from the two's complement
number. To convert from a two's complement number in binary form, invert all the bits (0 changes to 1, 1 changes to a 0),
then add 1. Convert this binary value to a decimal value and divide by either 20 (high resolution mode) or 2 (low resolution
mode) to calculate the negative temperature.
Register Value
Two's Complement Value
0000 0000 0000 0101
0000 0000 0000 0100
0000 0000 0000 0011
0000 0000 0000 0010
0000 0000 0000 0001
0000 0000 0000 0000
1111 1111 1111 1111
1111 1111 1111 1110
1111 1111 1111 1101
1111 1111 1111 1100
1111 1111 1111 1011
1111 1100 0001 1000
Mapping Unit Types
Using signed or unsigned value types allows the user to generically map any input to any output. The signed and unsigned
values read the null and span parameters to create the linear translation between one scale and another. The output type is
set to mA or V.
Output = (Fullscale/Span)(InputValue − Null) + Offset
Output Scale
0 to 20 mA
4 to 20 mA
0 to 10 V
Fullscale. Defined in the table; the output range.
Span. The total range of values mapped to the output.
Null. The starting point for the output scale.
Input Value. The value mapped to the output.
Offset. Defined in the table; the starting output value.
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Converted Decimal
Decimal Value
5
4
3
2
1
0
65535
65534
65533
65532
65531
64536
Fullscale (range)
20 mA
16 mA
10 V
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Calculated Temperature
(Converted Decimal ÷ 20)
5
4
3
2
1
0
-1
-2
-3
-4
-5
-1000
0.25
0.20
0.15
0.10
0.05
0
-0.05
-0.10
-0.15
-.20
-0.25
-50
Offset
0 mA
4 mA
0 V