The Principle of Physical Layer of Digital Signal Transmission

There are many ways to implement the physical layer. Network devices offer a wide range of connectivity options. Certain networks are well-defined using the OSI model, where cables, bridges, industrial routers, serial servers, DTUs, and PCs can be easily identified. Sometimes only a handful of devices are linked together via some kind of proprietary network, or in a black-box fashion where a web service is bundled with the device.

The most common serial data exchange device is the serial server, which is an RS232, RS422, and RS485 device used to connect two or more devices together. All three interfaces use the terms data terminal equipment (DTE) and data communications equipment (DCE). A DTE is a component that wants to communicate with another component elsewhere, such as a PC that communicates with another PC. The DCE is the component that actually communicates or performs the generator and receiver functions discussed in the standard. A modem is a common example of DCE. Модуль связи CAN2.0

The interface between DTE and DCE can be classified into mechanical, electrical, functional, and procedural aspects. Mechanical specifications define the connector type and pin count. Electrical specifications define line voltages and waveforms, as well as failure modes and effects. Functional specifications include timing, data, control, and signal grounds, as well as which functional pins to use. The program interface specifies how to exchange signals.

RS485 is another serial data transmission method. Officially, it's EIA 485, or the Electronics Industry Association's (EIA) "Standard for Electrical Characteristics of Generators and Receivers Used in Balanced Digital Multipoint Systems." The standard defines a method of generating zero as a voltage pulse. Remember that for all the data processing, framing, grouping, routing, and addressing performed by the upper layers, it still boils down to pushing 1s and 0s on some physical medium.

The important information about RS485 is that it allows multiple receivers and generators and that the cable characteristics are specified in terms of signaling speed and length. A typical cable is a shielded twisted pair of copper wires, sufficient for typical signaling rates of 10 million bits per second (Mbps). The standard only defines the electrical characteristics of the waveform. Note that RS485 does not specify any media control capabilities - strictly depends on the device (usually a chip) connected to the generator. RS-485 is typically suitable for cables up to 2,000 feet long.

An example of a simple serial network might be a series of loggers connected via an RS-485 link to a PC that receives the data collected by each logger. The manufacturer sells an add-in card that installs in each recorder with wiring instructions. Each network card is daisy-chained to other network cables via a series of shielded twisted pair cables that end up on the network interface card in the PC. Aside from knowing the limitations of RS-485 (distance, shielding, data rates, etc.), it's not really necessary to know and understand the network layers in this arrangement.

In the title, the RS422 standard is TIA/EIA 422 B, "Electrical Characteristics of Balanced Voltage Digital Interface Circuits" developed by the Telecommunications Industry Association (in conjunction with EIA). Similar to RS485; the main difference is the rise time and voltage characteristics of the waveform. RS422 typically allows cable lengths up to 1.2 kilometers at up to 100,000 bits per second (kbps). At 10 million bps (Mbps), the cable length is limited to around 10 meters (Figure 4-3). In the presence of cable unbalance or high common-mode noise levels, the cable length can be further reduced to maintain the desired signaling rate. AC-DC двойной

RS232C is probably the most common form of serial data exchange. The TIA, together with the EIA again, formally calls it EIA/TIA 232 E, "Interfaces between data terminal equipment and data circuit terminating equipment using binary data exchange". The suffix "E" indicates a later version than the normal "C" version. The standard differs from RS422 and RS485 in that it defines both mechanical and electrical interfaces.

RS232 is suitable for signal rates up to 20 kbps and distances up to 50 feet. Zeros (spaces) and ones (marks) are measured based on the voltage difference from signal common (+3 V dc = 0, -3 V dc = 1). The most common mechanical interfaces are D-sub 9 and D-sub 25 connectors.

The switching circuits (pins) in RS232 devices are divided into four categories: signal common, data circuits (transmitted data, received data), control circuits (ie, request to send, clear to send, DCE ready, DTE ready), and timing circuit.

The above standards are all used in serial communication schemes designed for longer distances. There is a common parallel interface called the General Purpose Interface Bus (GPIB) or IEEE-488. Up to 15 devices can be interconnected, usually personal computers and scientific equipment. It provides high data signaling rates of up to 1 Mbps but is limited in length. The total allowable bus length is 20 meters and the distance between devices does not exceed 4 meters.

The IEEE-488 bus is a multipoint parallel interface with 24 lines accessible to all devices. These lines are divided into data lines, handshake lines, bus management lines, and ground lines. Communication is digital, and messages are sent one byte at a time. The connectors are 24-pin; devices on the bus use female sockets, while interconnect cables have matching male plugs. A typical cable will have male and female connectors to enable daisy-chaining between devices.

An example of an IEEE-488 implementation is a measurement system designed to evaluate the performance of chemical sample cells. The water tank performs sample conditioning (pressure, flow, and temperature control) and chemical analysis (pH, dissolved oxygen, and conductivity) of the water samples. The tank is equipped with a pressure sensor, resistance temperature detector (RTD), thermocouple, and reference junction. A 30-point scanner is used to multiplex data from all sensors. The scanner is connected to a desktop or laptop computer using the GPIB interface. Under IEEE-488, data can be efficiently and reliably acquired, stored, displayed, and reduced using an application on a PC.

The medium used to implement the physical layer is usually a set of copper wires. Unshielded twisted pair (UTP) cables are the most affordable. Compared to shielded twisted pair (STP), it is lightweight, easy to pull, easy to terminate, and takes up less cable tray space. However, it is more susceptible to electromagnetic interference (EMI).

STP is heavier and more difficult to manufacture, but it can greatly increase the signaling rate in a given transmission scheme. The twist cancels the magnetic field and current on a pair of conductors. Magnetic fields are created around other conductors carrying large currents and around large electric motors. There are various grades of copper cable available, with grade 5 being the best and most expensive. Grade 5 copper cable suitable for 100 Mbps applications has more twist per inch than lower-grade copper cable. More twists per inch mean more linear feet of copper wire used to make up the cable run, and more copper means more money.

Shielding provides a way to reflect or absorb the electric field around the cable. Shielding comes in many forms, from copper braid or copper mesh to aluminized mylar tape wrapped around each conductor, to twisted pairs.

As user applications demand higher and higher bandwidth, optical fibers are increasingly used. The term "bandwidth" technically refers to the difference between the highest and lowest frequencies of a transmission channel, measured in Hertz (Hz). More commonly, it represents the volume or amount of data that can be sent through a given circuit.

The standard bandwidth using fiber optic cables is 100 Mbps. When first introduced, optical fiber was considered only for special applications because it was expensive and difficult to use. The quest for greater bandwidth, combined with easier-to-use fiber, has made it more common in recent years. Provides tools and training for installing and troubleshooting fiber optics.

There are three basic types of fiber optic cables: multimode step index, multimode gradient index, and single mode. Multimode fibers are usually driven by LEDs at both ends of the cable, while single-mode fibers are usually driven by lasers. Single-mode fiber can achieve higher bandwidth than multimode fiber but is thinner (10 microns) and physically weaker than multimode fiber. The cost of equipment to transmit and receive single-mode fiber-optic signals is much higher (at least four times) than multi-mode signals.

A distinct advantage of fiber optic cables is noise immunity. Although fire ratings should be adhered to, fiber optic cables can be run freely through high-noise areas with impunity. Cables running through multiple spaces in the plant should meet the National Fire Protection Association (NFPA) Fire/Ventilation/Air Conditioning (HVAC) ventilation system rating.