Rockwell Automation Publication 1756-UM540E-EN-P - December 2017 75
Temperature-sensing Analog Modules Chapter 4
When enabled, Synchronized Sampling provides each module a synchronized
starting point for its respective input scans. Because the RPI values are the same,
the inputs on the modules are sampled at the same rate and the same time.
To see where to enable Synchronized Sampling, see page 131
.
Cold Junction Compensation
When using the module with a thermocouple input type, the channel must
account for the thermoelectric effect of a junction of the thermocouple field
wires and the screw terminals of an RTB or IFM.
The junction at which temperature is measured is the hot junction. The junction
where the thermocouple wire interfaces with copper is the cold junction. The
module always measures and reports the cold junction temperature in
°C. The
transition from thermocouple wire to copper typically happens either on the
module screw terminal itself or at an IFM.
The thermoelectric effect alters the input signal and must be compensated for to
measure temperatures accurately. To accurately compensate the input signal from
your module, you must use a cold junction compensation (CJC) sensor to
account for the increased voltage.
Remember the following when compensating the input signal from your module:
• CJC is optional and can be disabled.
• The module uses two channels for CJC. When using an RTB, you must
connect CJC sensors at RTB terminals 1, 2, 35, and 36.
• Differences exist between using an RTB or IFM to connect wiring to the
module. They are described in the rest of this section.
CJC sensors are only required with use of the Thermocouple input type and
when channel wiring is connected via an RTB.
If you are using an IFM to connecting wiring to a channel using the
Thermocouple input type, you do not need to use CJC sensors.
CJC sensors do not come with the module. You must order CJC sensors,
product catalog number 1756-CJC, separately from the module for CJC sensors
which attach directly to the module's screw terminals. Catalog number
1756-CJC includes two CJC sensors.
To order CJC sensors, contact your local Allen-Bradley distributor or Rockwell
Automation sales representative.
If you use CJC, you must connect CJC sensors to both channels, that
is, terminals 1, 2, 35, and 36.
You cannot use CJC and connect a CJC sensor to only one channel.
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