UMAXRTD8CO V2.1.1 Preliminary Documentation – May be Sub ect to Change 8-62
Depending on the value in ob ect $3000, the ob ects $3010, $3020 and $3030 Callendar-Van
Dusen Cons an A, B and C are automatically updated as necessary. The constants A, B, and C
for a standard sensor are stated in IEC751, and the values used by the scanner are listed below.
IEC 00385 3.908300 -5.77500 -4.18301
JIS 003916 3.974673 -5.89730 -4.35300
US 003902 3.960000 -5.93000 -4.30000
Legacy US 003920 3.984800 -5.87000 -4.00000
SAMA 003923 3.981531 -5.853116 -4.35453
Table 1 – Callendar-Van Dusen Cons an s for S andard RTD Coefficien s
Generally speaking, the Callendar-Van Dusen ob ects are treated as read-only variables. However,
should a “User Defined” coefficient be selected, these ob ects would become write-able in order to
allow for RTD sensors not listed in the above table to be connected.
Ob ects $6126 RTD Scaling Fac or and $6127 RTD Scaling Offse are used to convert the field
value to read-only ob ect $7130 RTD Inpu Process Value, which is mapped either to TPDO1 (1
to 4) or TPDO2 (5 to 8) by default.
The formula to convert the field value (FV) to process value (PV) is:
Process Value = (Field Value * Scaling Fac or) + Scaling Offse
While the FV is a real number, containing the temperature in °C, the PV is a 16-bit integer value.
The default scaling has been selected such that the PV will send the temperature with a resolution
of 0.0625 °C/bit and a offset of -273°C. [Scaling Factor = 16, Offset = 4368] Since the maximum
temperature the scanner can measure for a RTD is 1735°C, this means the range of the PV data
will be 0 to 32123 (-273°C to 1735°C.)
Alternatively, it may be desired to send the temperature in Fahrenheit with a 0.1°F resolution per
bit. In this case, the Scaling Factor would be set to 18, and the Offset to 320. Other scaling can be
selected as desired by the user.
In all cases, certain values will be ‘plugged’ into the PV ob ect to indicate various conditions.
Should the associated RTD be disabled by ob ect $6112, then the value in the PV will always be -1
(0xFFFF).
Alternatively, should the scanner detect an open circuit on the sensor, then the PV value will be set
to -512 (0xFE00). A short circuit on the sensor returns a PV value of -448 (0xFE40). Lastly, in the
unlikely case that the processor detects that the ADC converter has stopped working (i.e. no
longer sending updated data on every scan), then the controller will not continue to broadcast the
‘frozen’ data, but rather update the PV value to -384 (0xFE80) to indicate that there is a problem
with the measurement.
In both error conditions mentioned above, open circuit or frozen data, the associated ob ect $6150
RTD S a us will also be updated to reflect the problem. Other faults that the scanner can detect
and flag are determined by the values in ob ects $4000 Low Tempera ure Warning Threshold,
$4010 High Tempera ure Warning Threshold and $4020 High Tempera ure Shu down
Threshold.
