10 DeviceNet Base RTD and Thermocouple CompactBlock LDX I/O
Publication 1790-IN011C-EN-P - January 2015
When using a 3-wire configuration, the block compensates for resistance error due to lead
wire length. For example, in a 3-wire configuration, the block reads the resistance due to
the length of the wires and assumes that the resistance of the other wire is equal. If the
resistance of the individual lead wires are much different, an error may exist. The closer the
resistance values are to each other, the greater the amount of error is eliminated.
To ensure that the lead values match as closely as possible:
•
keep lead resistance as small as possible and less than 25
•
use quality cable that has a small tolerance impedance rating
•
use a heavy-gauge lead wire which has less resistance per foot
RTD Wiring Configurations
Three configurations of RTDs can be connected to the blocks:
•
2-wire RTD, which is composed of an RTD EXC (excitation) lead wire and a
RTN (return) wire
•
3-wire RTD, which is composed of a Sense and 2 RTD lead wires (RTD EXC
and RTN)
•
4-wire RTD, which is composed of a Sense and 2 RTD lead wires (RTD EXC
and RTN). The second sense wire of a 4-wire RTD is left open.
IMPORTANT
The RTD block requires three wires to compensate for lead resistance error.
We recommend that you do not use 2-wire RTDs if long cable runs are
required, as it reduces the accuracy of the system.
However, if a 2-wire configuration is required, reduce the effect of the lead
wire resistance by using a lower-gauge wire for the cable (for example, use
AWG #16 instead of AWG #24). The block’s terminal strip accepts
AWG #14 gauge wire.
IMPORTANT
To ensure temperature or resistance value accuracy, the resistance
difference of the cable lead wires must be equal to or less than 0.01
.
