1.11 User-defined TypesFor a typical thermistor, the relationship between resistance andtemperature can be expressed by the Steinhart–Hart equation1/T = A + B lnRT + C (lnRT)3where RT is the resistance in ohms at temperature T in degreesKelvin (K = °C + 273.15). The values of A, B, and C are calledSteinhart Coefficients. The error of the equation is less than +/-0.01°C in a 100°C span.i-7005 and M-7005 support user-defined types by specifying theSteinhart coefficients using the @AASxTttC(data) command, seeSection 2.30. The data sent is a 32-bit hexadecimal value inIEEE-754 standard format:Bits Description31 (sign) 0 = positive, 1 = negative30-23 (exponent) The exponent base is 2. The actual exponent iscalculated by subtracting 127 from the storedvalue.22-00 (mantissa) The mantissa is expressed as 1.f where f is thefractional part and is stored in this field.Example:For the hexadecimal value C3694000h1. Bit 31is 1, indicating a negative number.2. The value of bits 30-23 is 10000110 binary or 134 decimal.The exponent is 7, 134-127.3. The mantissa is 1.11010010100000000000000 binary.4. Adjust the mantissa for the exponent. The adjusted mantissais 11101001.0100000000000000 binary. That is 233.25decimal.5. Considering the sign bit. The floating-point number ofC3694000h is –233.25.The following methods can be used to obtain the SteinhartCoefficients of a thermistor.i-7005/M-7005 User’s Manual, Rev: B1.8 7MH-018-B18 23
