Allen-Bradley MicroLogix 1200, User Manual, Page: 39 / 144

Publication 1762-UM002A-EN-P - July 2002
Module Data, Status, and Channel Configuration 3-9
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.
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.
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 on page 3-10.
IMPORTANT
If you are using engineering units x 1 data format
and degrees Fahrenheit 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.
TIP
If the CJC sensor is removed from the module
terminal block, its open-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
on page 4-4 for additional details.
AB Parts