Introduction to Communication-Based Acquisition Systems
Why should I use a communication-based, or front-end, system for my data acquisition and control? A communication-based data acquisition system offers many advantages when compared to the plug-in systems. Some of these include size and expandability. Simple products, such as the P1000 Series, will accept only one input, while others, such as the OMB-TEMPSCAN and Workhorse systems can be expanded to several hundred analog inputs. One large advantage of a front-end is that you can start small, and build the system as your requirements grow. Putting together a system in this way keeps your initial cost low, yet provides you with the ability to add channels at any time in the future. Lastly, as you build and expand your system, the cost on a per channel basis decreases. Since each system only requires one master, or CPU/power module, additional units are not needed for expansion; the only cost incurred is for the actual channels you desire.Unlike a plug-in card, a communication based system can be placed in almost any environment. To install a plug-in on the factory floor, for example, you have to dedicate a PC to that location; in fact, depending upon the exact location, you might incur the added expense of having to purchase an "industrialized" PC, that has been built for the more rigorous requirements for the factory. With a front-end, however, you can place the system in almost any location, including a 19" industrial rack. Industrial enclosures are also available, to provide additional protection for the system. And, to talk to the host computer, you don't even have to worry about a special communication cable; most front-ends can talk to the host over telephone lines, using a modem.
Perhaps one of the biggest reasons for selecting a communication-based data acquisition system is the type of computer you happen to be using. Plug-in cards are designed to work with only one computer type. Cards are available for either the IBM PC or Apple computers. If you don't have either of those computers or a true compatible, the plug-in won't work. On the other hand, a front-end can talk to any computer that has a communications port. Different systems can communicate with the host computer using the RS-232-C, RS-422, RS-485 or IEEE-488 standard.
If you'd ever like to move your data acquisition system from one location to another, it's an easy task for the communication-based system. With a plug-in, however, it can be a monumental task because the field wiring terminates at the computer. If you have a computer at the new location, you have to open up the old one, disconnect the wiring, remove the card, and close up the old computer, re-run the input wires; then, do everything in reverse to re-install it.
Software and ease-of-use are also important considerations in selecting a type of data acquisition system. Communication based systems use simple commands, that are easy to learn, and make common sense. They are also available with a selection of third party software, that's been designed to work with the system, and will help you to take full advcantage of its capabilities. Lastly, if the system you're looking to install now is only the first of many such data acquisition systems, a front-end can be duplicated easily for installation in a second or third location, as can the control software. What are the differences between the various communication-based acquisition systems?
There are a number of factors to consider when selecting a front-end system. Begin by analyzing your requirements; do you need only analog inputs, or also analog outputs and digital I/O? How many inputs and outputs will you need? Pricing is also a very important factor, in that different systems have different initial costs, with various upgrade/enhancement prices. One major consideration in choosing the right system in the future: if you anticipate future needs and requirements, make sure that the system you select either has them now, or can be easily upgraded with them later.
Lastly, there are two types of communication-based systems: ones that can stand-alone for a period of time, and those that must be in constant communications with a host to operate. This difference is profound. Are you willing to dedicate a computer/terminal for communications with the system when it is operational, or do you want the system to perform the acquisition and control without the use of a host, and have the system "dump" the information to the host at a later time, when you can analyze it? A stand-alone system offers you enhanced programmability, and has storage capacity, for both the program and data, for it to perform its functions. The trade-off for these capabilities, however, is the price. More enhanced systems, those with added features, are more expensive.
What is networking, and how can it help with my data acquisition requirements?
A networked system is a system in more than one location. It can consist of one central master, or host, unit that communicates to the main computer, with remote "satellites" that communicate only through the master unit. Another type of networked system is one without a master. Each unit is like a link in a chain with one end of the chain connected to the host computer. For the computer to communicate with each link, it only has to specify which link in the chain it is addressing.Network systems offer many advantages over conventional communication-based systems. They can be placed in many locations in one building, or even in different buildings. They are also very easy to expand; just add a new link to the chain, or attach a new "slave" to the master. The network allows you to collect your data wihtout tying up a host computer in each location.
Introduction to Automatic Remote Dialers
Just what is an automatic remote dialer?
An automatic remote dialer is a device that monitors conditions at a remote site and, if an alarm condition develops, will alert people (or data processing equipment) at other remote locations. The local conditions may be monitored by a variety of sensors to keep track of the environment, process parameters, intruders, etc. Inputs may include one or more of the following: temperature sensors, sound pickup, AC power sensors, 4-20 mAdc process signals, voltage analog signals, or dry contact closures. These inputs may be sensed on an alert basis in which an alarm condition generates an action, or a non-alert basis in which the input is monitored without any alarm limits. If an alarm condition develops, remote sites will be dialed and a warning message in English wil be trasmitted when the remote phone is picked up. If the remote phone number is not answered, the automatic dialer will successively try additional numbers entered into the system. The number of phone lines that can be called will vary from one system to another. In most cases, the waiting time before a new number is tried can be programmed to suit preferences.In addition, some automatic dialing remote monitors will store data acquired by their inputs and transmit them when interrogated by phone or by computer interface. Some units will continuously transmit monitored data from their inputs.
How can I choose among the various auto dialers?
The cost and features vary widely among the units available. Some are easy to set up and have limited range of capabilities. Others require use of audible or visual, menu-driven, software. Some provide the ability of using a programing language, such as "C", to implement a truly specialized set of application conditions. Regardless of whether the auto dialer supports a flexible programming language or a very simple set of structured menu selections, most can be set up at the local site or remotedly by use of serial communication ports, modems, or data communication lines.
