Issue 2: PC Connections and Free Software Offer
You can now download a fully working version of the Windmill data acquisition software suite free from our web site! The software includes a generic driver for instruments that have an RS232 port and communicate with ASCII messages. (We've successfully interfaced devices from Bruël & Kjær, Datel, Mettler, Sartorius, Siemens, West Instruments and many other manufacturers.)
The software runs under Microsoft Windows and has comprehensive on-line Help. You can
- log data
- view trend charts
- control outputs
- share data in real-time with, say, an Excel spreadsheet
And all for free! Subscribe to the Monitor Newsletter and find out how to download your copy.
To make measurements with a personal computer you need a data acquisition interface. You connect your signals to the interface hardware and it converts them to a form the computer can understand.
The interface hardware comes in many different physical formats, a common one being the plug-in card. This fits into a free expansion slot in your PC. The card generally connects to the main (or a subsidiary) data and control bus of the computer. There are a number of PC bus structures for which cards are available: ISA, PCI and PC(MCIA).
The main advantages of a plug-in card are its low cost, its high speed and the ease with which it is installed. The main drawbacks are the limited potential for system expansion and the fact that the computer has be to located close to the signal source. To address these limitations choose a hardware device external to the PC.
There are many different methods of connecting external data acquisition device to the computer - summarised below.
These exchange messages between the PC and the data acquisition and control system.
RS232 Serial Link
The Electronic Industries Association gave birth to the RS232 standard in 1962. RS stands for recommended standard, but may now be replaced by EIA (EIA232). It is the simplest way to connect external hardware to your PC.
An RS232 link will run at up 38400 baud (bits per second) over short distances, and at lower speeds as the distance increases owing to capacitance effects. You can plug the RS232 lead directly into the computer's serial (COM) port. You may only connect 1 device to an RS232 lead, but multi-port serial cards are available for PCs which let you connect several devices at once. Many instruments have RS232 ports, such as balances, data loggers, chromatographs, oscilloscopes and spectrometers.
Our Serial Port and RS232 Connections page gives more information.
These have two major advantages over a standard RS232 link: they use differential line drivers allowing signals to be sent much further, and you can connect several devices to a single cable. The cable consists of a screened twisted pair wire and you need and an adapter card in your computer.
Our free ComDebug software will let you read data from RS485 and RS422 devices connected to serial Com port.
More about RS485 and RS422 is given in Issue 32 of Monitor.
The GPIB standard was designed to connect several instruments to computers for data acquisition and control. It is much faster than serial communications, and you can connect 14 devices to one PC. However, your hardware needs to be within a few metres of the computer. The GPIB standard is defined completely in the IEEE standard 488.1-1987 (R1994) IEEE Standard Digital Interface for Programmable Instrumentation. A general overview of the GPIB is on Biodata's site.
The Modbus protocol was developed by Modicon (part of Schneider Automation). It is a local area network system for industrial control applications.
Our free ComDebug software will let you read data from Modbus devices.
You can't open a control magazine without some mention of fieldbus - there is seemingly unending discussion on standards and implementation. So here we content ourselves to the briefest of descriptions and pointers to more comprehensive resources.
There are dozens of fieldbus protocols available, including LonWorks, WorldFIP and Profibus. Many devices can be connected, great distances can be covered (ten to hundreds of kilometres) and high speeds can be achieved.
The universal serial bus lets you daisy-chain as many as 127
devices. Personal computers equipped with USB (Pentium IIs for
example) allow devices to be automatically configured as soon as
they are physically attached to the bus. There's no need to
install cards into computer slots and reconfigure the system.
More about USB is given in Issue 34 of Monitor.
Ethernet offers a good, low-cost, option for distributed measurement and control. You may add data acquisition devices to existing Ethernet systems: saving installation costs and time. Speed-wise it is faster than serial communications. As with all distributed systems, you can place data acquisition hardware close to sensors reducing cabling costs and errors produced by noise.
Lon networks are ideal for widely distributed systems with a small number of sensors per node. (Any device connected to the network comprises a node.) LonWorks is a type of fieldbus developed by Echelon.
A WAN connects remote locations into a common infrastructure. It spans a large, or global, region. Examples include telephone wires and ISDN.
CHOOSING A CONNECTION METHOD
For the sorts of application where Windmill software programs are suitable, consider the following list.
- Maximum speed - how many readings or outputs controlled per second?
- Number of measurements?
- Types of transducers?
- Distribution - are the measurement and control functions grouped at different locations and if so, how are they distributed?
- Distance from the PC?
THere, and in the next few issues, we give short explanations of data acquisition and control terms. For a more comprehensive glossary see . Please let us know if you have any suggestions for terms we should include - email firstname.lastname@example.org.
- Trade name for an alloy of nickel with up to 5% aluminium, manganese and silicon, used with chromel in K-type thermocouples.
- Ampere (A)
- SI unit of electric current.
- A circuit that produces a larger voltage or current than was applied at its input.
- The size or magnitude of a signal.
- Analogue-to-Digital (A-D) Converter
- Converts an analogue signal (such as a voltage from a temperature sensor) into a digital signal suitable for input to a computer.
- Anti-Alias Filter
- An anti-alias (or anti-aliasing) filter allows through the lower frequency components of a signal but stops higher frequencies, in either the signal or noise, from introducing distortion. Anti-alias filters are specified according to the sampling rate of the system and there must be one filter per input signal.
- The major multi-channel link in a network, from which smaller links branch off.
- Background Noise
- Extraneous signals that might be confused with the required measurement.
- Batch Process
- Any process on which operations are carried out on a limited number of articles, as opposed to continuous process.
- The rate of data transmission in serial data communications, approximately equal to one bit per second.
- A passage for transmission of data, address and control signals.
- An alloy of nickel with about 10% chromium, used with Alumel in K-type thermocouples.
- COM Port
- A connection on a computer into which you can plug a serial device.
- Common Mode Signal
- A signal applied simultaneously to both inputs of a differential amplifier.
- Common Mode Rejection Ration
- The ability of the differential amplifier to obtain the difference between the + and - inputs whilst rejecting the signal common to both.
- Contact emf
- Electromotive force which arises at the contact of dissimilar metals at the same temperature, or the same metal at different temperatures.
- An alloy of 40% nickel and 60% copper. Used with copper in T-type thermocouples.
- Continuous Process
- Method of continuously producing an article.
- Current is often used to transmit signals in noisy environments because it is much less affected by environmental noise pick-up. Before A-D conversion the current signals are usually turned into voltage signals by a current-sensing resistor.
- Differential Amplifier
- One whose output is proportional to the difference between two inputs. Also known as an instrumentation amplifier.
- Differential Inputs
- Using differential inputs can reduce noise picked up by the signal leads. For each input signal there are two signal wires. A third connector allows the signals to be referenced to ground. The measurement is the difference in voltage between the two wires: any voltage common to both wires, including noise, is removed.
- Digital-to-Analogue (D-A) Converter
- Used to produce analogue output signals. These may be control signals or synthesised waveforms.
- Direct Current (dc)
- Current which flows in one direction. See also alternating current.
- Slow variation of a performance characteristic such as gain, frequency or power output. May be due to, for instance, temperature or ageing. Usually only significant when measuring low-level signals (a few millivolts) over long periods of time, or in difficult environmental conditions.
- Dynamic Data Exchange (DDE)
- A standard Microsoft Windows protocol that defines a way for Windows applications to share information with one another.
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