Sensor connections, Rtd's and resistors – Sensoray 518 User Manual

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Sensor Connections

All sensors are connected to the 518 by means of connector P1. Optionally, sensors may be
connected to the Sensoray model 7409TC screw termination board, which in turn connects to
518 connector P1. Sensors may be electrically connected and disconnected from the coprocessor
or 7409TC without removing the 518 or power from the PC104. The following discussions
assume the presence of a 7409TC termination board.

Each sensor has as many as five connections to a channel circuit. Two connections are used for
all sensor types: V+ and V-. These two signals are the positive and negative differential voltage
sense inputs.

In addition to the sense terminals, all resistive sensors require connection to the I+ and I-
terminals. The I+ and I- terminals provide positive and negative excitation for resistive sensors.
The fifth lead — labeled S for shield —connects to a the cable shield (if present). The following
sections describe how to connect sensors to a 7409TC for proper operation.

RTD's and Resistors

RTD’s and resistors can be connected to the 7409TC in three possible configurations: two-wire,
three-wire, and four-wire. In the following discussions, RTD is used interchangeably with
resistor.

The simplest configuration is the two-wire circuit. As the name implies, this configuration
requires only two wires. The V+ and I+ terminals are shorted together, and V- and I- are shorted
together at the 7409TC. This scheme conserves wire, but introduces error as a result of the
voltage difference between the V/I junctions and RTD. There are two possible solutions to this
problem: use short wires, or use more than two wires.

By using three wires, it is possible to reduce RTD cable loss errors by 50 percent. Instead of
shorting V- and I- together at the 7409TC, separate wires can be run from each of these terminals
out to the RTD. The wires are then shorted together at the RTD. In this situation, the high
impedance V- terminal is able to detect the voltage at the RTD before any voltage loss can occur.