Rockwell Automation 1746-NT4 Series B,D17466.6.1 SLC 500 4-Channel Thermocouple/mV Input Module User Manual - Manual (Page 17)

Publication 1746-UM007C-EN-P - July 2004
Overview 1-5
Module Operation
The thermocouple module input circuitry consists of four differential
analog inputs multiplexed into a single analog-to-digital (A/D)
convertor. The mux circuitry also continuously samples the CJC A and
CJC B sensors and compensates for temperature changes at the cold
junction (terminal block). The figure on the following page shows a
block diagram for the analog input circuitry.
The A/D convertor reads the selected input signal and converts it to a
digital value. The multiplexer sequentially switches each input
channel to the module’s A/D convertor. Multiplexing provides an
economical means for a single A/D convertor to convert multiple
analog signals. However, it does affect the speed at which an input
signal can change and still be detected by the convertor.
Thermocouple Compatibility
The thermocouple module is fully compatible with all SLC 500 fixed
and modular controllers. It is compatible with all NBS MN-125
standard types J, K, T, E, R, S, and B thermocouple sensors and
extension wire; and with NBS MN-161, 14AWG, standard type N
thermocouple and extension wire. Refer to Appendix C for more
details.
The Series B (or higher) 1746-NT4 differential design allows for a
maximum channel-to-channel common-mode voltage
difference/separation of 2 volts. This means that if you are using an
NT4 with multiple grounded thermocouples with metallic sheaths or
exposed thermocouples with measuring junctions that make contact
with electrically conductive material, their ground potentials must be
within 2 volts. If this is not done, your temperature readings will be
inaccurate or the module could be damaged. If your grounded
thermocouple protective sheath is made of an electrically
non-conductive material such as ceramic, then the voltage separation
specification is not as important. Refer to Appendix D for an
explanation of grounded, ungrounded, and exposed thermocouples.
Use the analog common ( ANALOG COM ) terminal for applications that
have multiple grounded thermocouples. This analog common
terminal must be jumpered to either the (+) or (-) terminal of any
active channel which is connected to a grounded thermocouple. See
Wiring Considerations on page 3-8 for complete information on the
use of the
ANALOG COM terminal.