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ROC827 Instruction Manual
Issued Mar-06 Input/Output Modules 4-15
The RTD input module works by supplying a small consistent current to
the RTD probe and measuring the voltage drop across it. Based on the
voltage curve of the RTD, the ROC827 firmware converts the signal to
temperature.
The RTD input module monitors the temperature signal from a resistance
temperature detector (RTD) sensor or probe. A two-channel 16-bit RTD
module is available. The RTD module isolation includes the power supply
connections.
The RTD modules draw power for the active circuitry from lines on the
backplane.
It may be more convenient to perform calibration before connecting the
field wiring. However, if the field wiring between the ROC827 and the
RTD probe is long enough to add a significant resistance, then perform
calibration in a manner that considers this.
4.9.1 Connecting the RTD Wiring
Temperature can be input through the Resistance Temperature Detector
(RTD) probe and circuitry. An RTD temperature probe mounts directly to
the piping using a thermowell. Protect RTD wires either by a metal sheath
or by conduit connected to a liquid-tight conduit fitting. The RTD wires
connect to the four screw terminals designated “RTD” on the RTD
module.
The ROC827 provides terminations for a four-wire 100-ohm platinum
RTD with a DIN 43760 curve. The RTD has an alpha equal to 0.00385 or
0.00392Ω/Ω°C. You can use a two-wire or three-wire RTD probe instead
of a four-wire probe, but they may produce measurement errors due to
signal loss on the wiring.
Wiring between the RTD probe and the ROC827 must be shielded wire,
with the shield grounded only at one end to prevent ground loops. Ground
loops cause RTD input signal errors.
Table 4-1. RTD Signal Routing
Signal Terminal Designation
CH 1 (REF) 1 Constant Current +
CH 1 (+) 2 V+ RTD
CH 1 (–) 3 V– RTD
CH 1 (RET) 4 Constant Current –
Not Connected 5
N
CH 2 (REF) 6 Constant Current +
CH 2 (+) 7 V+ RTD
CH 2 (–) 8 V– RTD
CH 2 (RET) 9 Constant Current –
Not Connected 10 N/A