Selecting Temperature Units (Bit 7); Determining Open-Circuit Response (Bits 6 And 5) - Spectrum Controls 1769 Series User Manual

Chapter 3: Module, Data, Status, and Channel Configuration

3.2.6 Selecting Temperature Units (Bit 7)

The module supports two different linearized/scaled ranges for thermocouples,
degrees Celsius (°C) and degrees Fahrenheit (°F). Bit 7 is ignored for millivolt
input types, or when raw/proportional, scaled-for-PID, or percent data formats
are used.
If you are using engineering units x 1 data format and degrees Fahrenheit
WARNING
temperature units, thermocouple types B and C cannot achieve full-scale
temperature with 16-bit signed numerical representation. An over-range
error will occur for the configured channel if it tries to represent the full-
scale value. The maximum representable temperature is 3276.7 °F.

3.2.7 Determining Open-Circuit Response (Bits 6 and 5)

An open-circuit condition occurs when an input device or its extension wire is
physically separated or open. This can happen if the wire is cut or disconnected
from the terminal block.
If either CJC sensor is removed from the module terminal block, its open-
NOTE
circuit bit is set (1) and the module continues to calculate thermocouple
readings at reduced accuracy. If an open CJC circuit is detected at power-
up, the module uses 25 °C as the sensed temperature at that location. If an
open CJC circuit is detected during normal operation, the last valid CJC
reading is used. An input channel configured for millivolt input is not
affected by CJC open-circuit conditions. See Open-Circuit Detection for
additional details.
Bits 6 and 5 define the state of the channel data word when an open-circuit
condition is detected for the corresponding channel. The module overrides the
actual input data depending on the option that you specify when it detects an
open circuit. The open-circuit options are explained in the table below.
Table 3-4 (Open-Circuit Response)
Response Option
Upscale
Downscale
Last State
Zero
Definition
Sets the input data value to full upper scale value of
channel data word. The full-scale value is determined by
the selected input type and data format.
Sets the input data value to full lower scale value of
channel data word. The low scale value is determined by
the selected input type and data format.
Sets the input data value to the last input value prior to
the detection of the open-circuit.
Sets the input data value to 0 to force the channel data
word to 0.
User's Manual Pub. 0300244-02 Rev. A
3-7