A digital, or binary, signal can have only two possible specified levels or states ; an ‘on’ state, in which the signal is at its highest level, and an ‘off’ state, in which the signal is at its lowest level.
Exaples:- the output voltage signal of a transistor-to-transistor logic (TTL), Control devices, such as relays, and indicators such as LEDs,
DAQ hardware would only be required to convert the signal level to a digital form for processing by the computer using an analog-to-digital converter (ADC). Low speed A/D boards would be capable of measuring this class of signal.
A transducer is a device that converts one form of energy or physical quantity into another , in accordance with some defined relationship.
In data acquisition systems, transducers sense physical phenomena and provide electrical signals that the system can accept. For example, thermocouples, resistive temperature detectors (RTDs), thermistors, and IC sensors convert temperature into an analog voltage signal, while flow transducers produce digital pulse trains whose frequency depends on the speed of flow.
Active transducers convert non-electrical energy into an electrical output signal. They do not require external excitation to operate. Thermocouples are an example of an active transducer.
Passive transducers change an electrical network value, such as resistance, inductance or capacitance, according to changes in the physical quantity being measured. Strain gauges (resistive change to stress) and LVDTs (inductance change to displacement) are two examples of this.
Cut-off frequency > This is the transition frequency at which the filter takes effect. It may be the high-pass cut-off or the low-pass cut-off frequency and is usually defined as the frequency at which the normalized gain drops 3 dB below unity.
Roll-off > This is the slope of the amplitude versus the frequency graph at the region of the cut-off frequency. This characteristic distinguishes an ideal filter from a practical (non-ideal) filter. The roll-off is usually measured on a logarithmic scale in units of decibels (dB).
Interrupts are the mechanism by which the CPU of a computer can attend to important events such as keystrokes or characters arriving at the COM port only when they occur. This allows the CPU to execute a program and only service such I/O devices as needed
Microprocessor controls data transfers within the PC (using the IN (port) and OUT (port) instructions.
In many I/O interfacing applications and certainly in data acquisition systems, it is often necessary to transfer data to or from an interface at data rates higher than those possible using simple programmed I/O loops.