The Universal Sensor Port

Description of Terminals

Sensor Power (PW)

This terminal can provide regulated DC power for sensors and transducers. It is use in preference to the sensor excitation available from the Analog Terminals when:

1. There is a sensor warm-up time requirement, or

2. More power is needed than available via the Analog Terminals

The Sensor Power is programmable, its setup being associated with the current schedule:

The Sensor Power Terminal takes about 200ms to stabilize under load, however most transducers take from 1s to 30s to for a valid reading. This stabilizing delay is controlled by the associated schedule. Top

Power Return (RT)

The Power Return terminal is the negative connection for Sensor Power. It is connected to ground via a precision 2Ω shunt resistor that is used for monitoring the sensor's current. It is not essencial to use the Power Return terminal - the Ground terminals can also be used - however for some sensor types, measuring the return current can assist in compensating for lead resistance. Top

Ground 1 and Ground 2 (G1, G2)

The Ground 1 and Ground 2 Terminals are identical and connected. They provide a return path for Sensor Power, and can be used as a shared terminal for ground referenced measurements. Internally the Ground terminals of the Sensor Port are sensed close to the terminals to eliminate errors due to possible ground currents (internally all measurements are differential). Top

Analog 1 - 8 (A1 - A8)

The eight Analog Terminals have identical functionality, are interchangeable and can often be shared between sensors. It is the Analog Terminals that provide the Universal Sensor Port with the flexibility and power to support just about every type of sensor without additional plug-ins or signal conditioning modules.

Each Analog Terminal can at any instant in time be one or more of the following:

• Positive input
• Negative input
• Excitation output
• Excitation return
• IEEE 1451.4 TEDS Class 1 and 2 interface
• RS232 and RS485 transmit positive
• RS232 and RS485 receive positive
• RS485 receive negative

This is achieved by utilizing four parallel multiplexers as shown in the diagram below:

Analog Terminal Functionality

Additional points, not shown, are measured by the analog system to enhance precision and functionality. These include:

• Excitation current
• Sensor Power voltage (one per Port)
• Sensor Port temperature (one per Port)
• Nominal 11V input offset voltage for ICP type sensors
• Various references for automatic calibration

Smart sensors with RS232 or RS485 are also supported via the Sensor Port's Analog Terminals.

The Sensor Port's Smart Sensor Communications Support

As can be seen the Port's multiplexers are used to direct the communications signals where required. This multiplexed communications scheme is disruptive to other Port activities for the duration of the communications.

Sensors with IEEE 1451.4 TEDS can be connected via the Analog Terminals also. The TEDS reading (and writing) for both Class 1 and Class 2 devices is supported. The simplified internal signal routing is shown in the following diagram:

Sensor Port Support for IEEE 1451.4 TEDS

The TEDS is accessed via a 1-Wire interface and routed to the microprocessor. The Sensor Port has its own TEDS that stores the calibration of the Port's temperature sensor, the return current shunt and the Sensor Power voltage gain. Top