Category: General

Developed by the Modicon company (the original manufacturer of the Programmable Logic Controller, or PLC) in 1979 for use in its industrial control products, Modbus is a protocol designed specifically for exchanging process data between industrial control devices. The Modbus standard does not specify any details of physical networking, and thus may be deployed on many different types of physical…

A technological advance introduced in the late 1980’s was HART, an acronym standing for Highway Addressable Remote Transmitter. The purpose of the HART standard was to create a way for instruments to digitally communicate with one another over the same two wires used to convey a 4-20 mA analog instrument signal. In other words, HART is a hybrid communication standard, with one…

At the next OSI Reference Model layer (layer 4) is a set of protocols specifying how reliable communication “connections” should be established between devices on a digital network. Rather than specifying addresses for routing data packets on a large network (OSI layer 3), layer 4 works on the level of virtual data “ports” at the…

I remember first learning about the world-wide Internet, and wondering what it actually looked like. The first vision entering my mind when people told me about a computer network spanning nearly all of the United States and many other parts of the world was that of a thick cable strung along telephone poles and buried underground,…

An engineer named Bob Metcalfe conceived the idea of Ethernet in 1973, while working for the Xerox research center in Palo Alto, California. His fundamental invention was the CSMA/CD method of channel arbitration, allowing multiple devices to share a common channel of communication while recovering gracefully from inevitable “collisions.” In Metcalfe’s vision, all of the…

Some of the simplest types of digital communication networks found in industry are defined by the EIA (Electronic Industry Alliance) and TIA (Telecommunications Industry Alliance) groups, under the numerical labels 232, 422, and 485. This section discusses these three network types. 15.6.1 EIA/TIA-232 The EIA/TIA-232C standard, formerly40 known as RS-232, is a standard defining details found at layer…

One of the great benefits of digital technology is the ability to communicate vast amounts of information over networks. This very textbook you are reading was transmitted in digital form over the electronic network we call the Internet: a feat nearly impossible with any sort of analog electronic technology. The main benefit of digital data communication in industrial…

Modern analog-to-digital converters are phenomenally accurate, dependable, repeatable, and surprisingly inexpensive as integrated circuits considering their capabilities. However, even the best ADC is useless in a real application unless the analog voltage signal input to it is properly conditioned, or “made ready” for the ADC to receive. We have already explored one form of signal conditioning…

In order to provide an interface between the internal (digital) world of a computer and the external (analog) world of process measurement and control, there must be some form of conversion taking place between these two types of data. Devices that generate digital representations of analog measurements are called analog-to-digital converters, or ADCs. You will find ADC circuits in…

Binary patterns are not just able to represent numerical values. Given a standardized code, they may represent other types of data as well, such as alphabetical characters. The ability to encode non-numerical data in digital form is what allows computers to manipulate and communicate text. In this subsection, we will explore some of the ways language…

Process measurements are often of an analog nature: the temperature of a furnace, the rate of fluid flow through a pipe, the pressure of a fluid, etc. These data are all analog quantities: infinitely variable, not discrete. Discrete process measurements such as the number of units passed by on a conveyor belt are relatively easy to sense…