There are many ways to implement the physical layer. Network equipment offers 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 there are just a few devices linked together through some kind of proprietary network, or a black box approach where the network service is tied to the device.
The most common serial data exchange device is a 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 somewhere else, such as a PC communicating with another PC. The DCE is the component that actually communicates or performs the generator and receiver functions discussed in the standard. Modems are a common example of DCE.
The interface between DTE and DCE can be classified according to mechanical, electrical, functional and process 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 the functional pins to be used. The program interface specifies how signals are exchanged.
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.” This standard defines a method for generating zeros as voltage pulses . Remember, for all the data processing, framing, grouping, routing, and addressing performed by the upper layers, it still comes down to pushing 1s and 0s on some physical medium.
The important thing to know about RS485 is that it allows multiple receivers and generators, and cable characteristics are specified in terms of signaling speed and length. Typical cables are shielded twisted pairs of copper wire, sufficient for typical signaling rates of 10 million bits per second (Mbps). This standard only defines the electrical characteristics of the waveform. Note that RS485 does not specify any media control capabilities – it depends strictly on the device (usually a chip) connected to the generator. RS-485 typically works with cable lengths up to 2,000 feet.
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 a plug-in card that is installed in every VCR and comes with wiring instructions. Each network card is daisy-chained to other network cables through a series of shielded twisted pair cables, which ultimately terminate at a network interface card in the PC. Aside from knowing the limitations of RS-485 (distance, shielding, data rates , etc.), there’s really no need to know and understand the network layers in this arrangement.
By 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 the EIA). Similar to RS485; the main difference is the rise time and voltage characteristics of the waveform. RS422 typically allows cable lengths of up to 1.2 kilometers and up to 100,000 bits per second (kbps). At 10 million bps (Mbps), the cable length is limited to about 10 meters (Figure 4-3). In the presence of cable imbalance or high common-mode noise levels, the cable length can be further reduced to maintain the desired signaling rate.
RS232C is probably the most common form of serial data exchange. TIA, again with the EIA, officially calls it EIA/TIA 232 E, “Interface between data terminal equipment and data circuit terminating equipment using binary data exchange.” The suffix “E” indicates a higher version than the ordinary “C” version. This standard differs from RS422 and RS485 in that it defines both a mechanical interface and an electrical interface.
RS232 is suitable for signaling 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 circuit (data sent, data received), control circuit (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 universal parallel interface called the General Purpose Interface Bus (GPIB) or IEEE-488. Can interconnect up to 15 devices, typically personal computers and scientific equipment. It provides high data signaling rates up to 1 Mbps, but is limited in length. The allowed total 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 connector is a 24-pin connector; the devices on the bus use female sockets, while the 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 tank performs sample conditioning (pressure, flow and temperature control) and chemical analysis (pH, dissolved oxygen and conductivity) of water samples. The tank houses the pressure sensor, resistance temperature detector (RTD), thermocouple and reference junction. A 30-point scanner is used to multiplex data from all sensors. The scanner connects to a desktop or laptop computer using a GPIB interface. Under IEEE- 488, data can be acquired, stored, displayed and reduced efficiently and reliably using applications on your PC.
The medium used to implement the physical layer is usually a set of copper wires. Unshielded twisted pair (UTP) cable is the most affordable. It is lightweight, easy to pull, easy to terminate, and takes up less cable tray space than shielded twisted pair (STP). However, it is more susceptible to electromagnetic interference (EMI).
STP is heavier and more difficult to manufacture, but it can greatly increase signaling rates in a given transmission scheme. The twist cancels the magnetic field and current flow on a pair of conductors. Magnetic fields are generated 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. Class 5 copper cables suitable for 100 Mbps applications have more twist per inch than lower class copper cables. More twists per inch means more linear feet of copper wire used to make up a cable run, and more copper means more money.
Shielding provides a way to reflect or absorb the electric fields surrounding the cable. Shielding comes in many forms, from copper braid or mesh to aluminized Mylar tape wrapped around each conductor to twisted pairs.
As user applications require ever higher bandwidths, fiber optics 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 capacity or amount of data that can be sent over a given circuit.
The standard bandwidth using fiber optic cable is 100 Mbps. When it was first introduced, fiber optics was considered only for special applications because it was expensive and difficult to use. In recent years, the pursuit of greater bandwidth combined with easier-to-use fiber optics has made it more common. Tools and training are provided for installing and troubleshooting fiber optics.
There are three basic types of fiber optic cable: multimode step-indexed, multimode graduated-indexed, and singlemode. Multimode fiber is typically driven by LEDs at both ends of the cable, while singlemode fiber is typically driven by a laser. Single-mode fiber can achieve higher bandwidths than multi-mode fiber, but is thinner (10 microns) and physically weaker than multi-mode fiber. The equipment to transmit and receive single-mode fiber optic signals costs much more (at least four times) than multi -mode signals.
One clear advantage of fiber optic cables is noise immunity. Although fire ratings should be observed, fiber optic cables can be routed through high-noise areas with impunity. Cables that pass through multiple spaces in a factory should meet the National Fire Protection Association (NFPA ) fire/ventilation/air conditioning (HVAC) ventilation system ratings.
Keywords: Industrial Ethernet data transmission terminal