Emerson ROC827 Connecting the RTD Wiring, RTD Signal Routing, Signal Terminal Designation

ROC827 Instruction Manual

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

Issued Mar-06 Input/Output Modules 4-15