I. Definition and Basic Characteristics of Pulses

1. What is a pulse?

A pulse (Pulse) typically refers to a brief, pulsating electrical impulse (voltage or current) commonly used in electronic technology, resembling a heartbeat. Its main characteristics include waveform, amplitude, width, and repetition frequency. A pulse is a signal that occurs for a short duration within the entire signal cycle, with most of the cycle being signal-free, much like a human pulse. Pulses are used for synchronization, triggering, or controlling multiple test devices in testing. Pulses are also used for clock generation or radar testing. To describe a pulse and make it reproducible, a set of parameters is defined.

II. Pulse Signals in Digital Systems

1. On/Off Signals

In digital systems, all transmitted signals are on/off, meaning there are only two types of electrical signals. This type of electrical signal is called a pulse signal (Pulse Signal), which is the basic electrical signal in all digital systems. It generally refers to digital signals, which have a signal present for half of the cycle. Signals within a computer are pulse signals, also known as digital signals.

III. Waveforms and Classification of Pulse Signals

1. Characteristics of Discrete Waveforms

Pulse signals are discrete signals with various shapes. Compared to ordinary analog signals (such as sine waves), their waveforms are discontinuous along the Y-axis (with obvious gaps between waveforms) but exhibit a certain degree of periodicity. The most common pulse wave is the rectangular wave (also known as the square wave).

2. Multiple Applications

Pulse signals can be used to represent information or as carriers, such as in pulse coding modulation (PCM) and pulse width modulation (PWM) in pulse modulation, as well as serving as clock signals for various digital circuits and high-performance chips.

IV. Period and Frequency of Pulse Signals

1. Basic Period Characteristics

In electronics, pulse signals are continuously emitted at a fixed voltage amplitude and time interval. The time interval between pulse signals is called the period; the number of pulses generated per unit time (e.g., 1 second) is called the frequency.

2. Frequency Units and Conversion

Frequency is the measurement term describing the number of pulses appearing in a unit of time for periodic cyclic signals (including pulse signals). The standard unit of measurement for frequency is Hz (hertz). The system clock in a computer is a typical example of a pulse signal generator with highly precise and stable frequency. Frequency is denoted by “f” in mathematical expressions, with corresponding units including: Hz (hertz), kHz (kilohertz), MHz (megahertz), and GHz (gigahertz). The conversion relationships are: 1 GHz = 1,000 MHz, 1 MHz = 1,000 kHz, 1 kHz = 1,000 Hz. The time units for calculating the period of a pulse signal and their corresponding conversion relationships are: s (second), ms (millisecond), μs (microsecond), ns (nanosecond), where: 1 s = 1,000 ms, 1 ms = 1,000 μs, 1 μs = 1,000 ns.

V. Pulse Signal Edge, Level, and Period Parameters

1. Rising Edge and Falling Edge

The edge of a pulse signal where the voltage transitions from low to high is called the rising edge, and the edge where the voltage transitions from high to low is called the falling edge. Some references also refer to these as the leading edge and trailing edge, respectively. The lower voltage is called the low level, and the higher voltage is called the high level.

2. Relationship Between Period and Frequency

Assuming the pulse signal period is T and the pulse width is t1, the following basic concepts apply.

Frequency f: The number of times the pulse signal period changes per second, i.e., f = 1 / T. The smaller the period, the higher the frequency.

VI. Pulse Frequency Processing in PLC Systems

1. PLC Scan Period Limitations

With the concept of frequency, we now discuss the maximum frequency of switch signals input to the PLC input terminal. Based on the scanning and PLC lag knowledge from the previous chapter, the PLC scan cycle is primarily determined by the length of the user program. Assuming a scan cycle of 20 ms, and considering the response delay of the input filter is 10ms, the PLC's scanning cycle becomes 30ms. If the change in the input signal is less than 30ms, according to the scanning principle, the PLC may not detect it at all. This means the pulse width of the input signal must be greater than 30ms, thereby limiting the frequency of the input signal.

2. Duty Cycle Example

Assuming the input signal is a pulse signal with a duty cycle of 50%, its period is T = 2t1 = 60ms. Therefore, the frequency of the input signal must not exceed 1/60ms = 16.6Hz. This is sufficient for general industrial control applications such as buttons and ordinary switches, but it is inadequate for real-time control applications requiring high I/O response speeds.

3. Optimization Solutions for High-Speed PLCs

For applications requiring high-speed response, PLCs from different manufacturers have implemented various measures in both software and hardware to enhance I/O response speed. The FX2N series features a high-speed counting module, providing 8 high-speed input terminals (X0–X7), with an RC filter time of just 0.5 μs. Software-wise, it employs real-time I/O data refresh, interrupt-based data transmission, and digitally adjustable filters via programmable instructions. As a result, the frequency of input signals that can be processed has significantly increased, reaching up to 20 kHz.

VII. Duty Cycle and Logic Definition of Pulse Signals

1. Concept of Duty Cycle

Duty cycle: The percentage ratio of pulse width t1 to period T. It is expressed as t1/T %. The duty cycle represents the proportion of the period occupied by the pulse. A higher duty cycle indicates that the pulse width is closer to the period T and that the average value of the pulse signal is larger.

2. Definition of Positive and Negative Logic

Positive logic and negative logic: A pulse signal has only two states: high level and low level, corresponding to the two logic states “1” and “0” in digital circuits. However, whether the high level represents “1” or the low level represents “1” can be set arbitrarily. If the high level is set to “1” and the low level to “0,” it is called positive logic; if the low level is set to “1” and the high level to “0,” it is called negative logic. Generally, unless otherwise specified, we use positive logic.

3. Actual Level Definitions

In actual circuits, there are no strict specifications for what constitutes a high level or a low level in volts. For example, in TTL circuits, the high level is approximately 3V, and the low level is approximately 0.5V; in CMOS circuits, the high level is 3–18V or 7–15V, and the low level is 0V.

VIII. Comprehensive Significance and Multi-Field Applications of Pulse Signals

1. Feature Summary

A pulse signal refers to a short-duration signal with large amplitude changes, typically containing high-frequency components and complex waveforms. This type of signal has applications in many fields and holds significant importance in areas such as radio communication, control engineering, and medicine.

2. Application Examples

In communication systems, pulse signals are used in digital communication, radar systems, etc.;

In the medical field, pulse signals can be used for medical diagnosis, treatment, and research;

In chemistry and biology, pulse signals are often used for detecting molecular dynamics and analyzing reaction processes;

In automatic control, pulse signals are frequently used for controlling motors, pneumatic, and hydraulic systems.

In summary, due to their characteristics of high amplitude, wide bandwidth, and short duration, pulse signals hold a highly significant position in modern technology.