Windmill Software Ltd
Data Acquisition Intelligence

RS-232 Interface
28 July 2000

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     The Newsletter for PC-Based Data Acquisition and Control    
Issue 24                    July 2000
-------------------------ISSN 1472-0221--------------------------

Welcome to issue 24 of Monitor from Windmill Software. Today we 
are pleased to announce that Windmill is now available for 
USB data acquisition units! These plug into the computer's 
universal serial bus port. We discuss USB below, highlighting 
its benefits for measurement and control.

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* Windmill News: New USB Unit Uses Windmill
* What is the universal serial bus?
* Data acquisition abbreviations explained


            Windmill News: New USB Unit Uses Windmill      

The new USB unit from Biodata takes full advantage of the 
Windmill software suite. Called the Microlink 751, the unit 
lets you:
* measure voltage, current, temperature and strain
* switch digital outputs
* monitor digital inputs
* count up to 65535

You simply plug the compact Microlink 751 into the computer's 
USB port - no need to switch off your PC or even restart 
Windows. With the powerful yet easy-to-use Windmill applications 
you can be logging, charting and counting in no time. To discover
just how easy it is, visit

The Microlink 751 and Windmill software package costs just 595 
British pounds (around 895 US dollars or 945 euros). This 
includes free technical support for life and a 
money back guarantee if not absolutely satisfied. For secure 
ordering go to

                What is the Universal Serial Bus?                

The USB is a relatively recent way to connect instruments and 
devices to the PC. Most new computers have one or two USB ports, 
in addition to the familiar RS232 (COM) and parallel ports. You 
can use the USB ports to connect peripherals like digital 
cameras, printers and data acquisition and control units.

Why use the USB for Data Acquisition?
The USB is extremely convenient for data acquisition for several 
* The equipment can obtain power from the USB; it doesn't need to 
  be battery powered or plugged into the wall. This makes USB 
  ideal for portable data acquisition with a laptop.
* Using a USB hub you can connect many devices to one USB port - 
  letting you easily expand your system should requirements grow.
* USB ports are provided on most new PCs - no need to open the 
  computer and install adaptor cards.
* You can plug in and unplug your equipment without switching off 
  your computer or even restarting Windows.
* The USB cable can be up to 5 m long. However, using USB hubs 
  between cables you can reach 30m.
* Faster speeds than those allowed by RS232 connections are 
* You can use USB devices alongside existing data acquisition 
  equipment (such as cards that you've installed in your PC or 
  instruments that plug directly into the RS232 port).

What about older PCs?
If your PC does not have a USB port you can buy a PCI-based card 
which provides one. However, you need to be running Windows 98 or 
Windows 2000. (Early releases of Windows 95 do not support USB, 
later releases have some support but it's better to use Windows 
98 or 2000.) You can test whether your PC is USB compatible by 
visiting and downloading their evaluation utility 

If you don't wish to upgrade your machine there are many other 
ways to connect data acquisition equipment to computers, such as 
Ethernet, RS232, RS485, Modbus or GPIB. Issue 2 of Monitor - - discusses a range of 

Cable Details
The USB cable should have an "A" plug at one end (for the 
computer) and a "B" plug at the other: no sockets. Any other 
arrangement doesn't conform to the standard. The cable comprises 
four wires: two carry your signal data and two supply voltage. 
The bus can operate at two different speeds, depending on the 
attached device. The device itself tells the bus what its speed 
is through the voltage cables. The cable should not be longer 
than 5 metres for fast devices, or 3 metres for slow devices. 
However, you can use up to 5 USB hubs to connect cables, giving a 
maximum distance of 30 m.

The Future for USB
In April this year the USB 2.0 specification was released. This 
allows for faster data transfer of 480 megabits per second. 
However, the specification has been designed to be totally 
compatible with the existing USB standard. The new specification 
was developed by the USB Promoter Group, which includes Compaq, 
Intel, Microsoft and Philips Semiconductors. Future upgrades are 
promised to have equal compatibility, so you can be sure that as 
technology advances so will your data acquisition system.

Further Reading
USB Implementers Forum

USB Complete, Axelson J, 1999. ISBN: 0965081931, Lakeview Research
More details at or

USB Explained, McDowell and Seyer, 1998. ISBN: 013081153X, 
Prentice Hall
More details at or

            Data Acquisition and Control Abbreviations           

Continuing our acronym and other abbreviation reference guide. 
For A-M see Issue 22 and 23 of Monitor

Open database connectivity
Developed by Microsoft, a standard for accessing a database. By 
using ODBC statements in a program you can access files in a 
number of different databases, including Access, dBase and Text. 
In addition to the ODBC software, a separate module or driver is 
needed to access each database.

Original Equipment Manufacturer
A company that makes basic hardware for other manufacturers to 
build into their products. 

Object-Orientated Programming
Languages using OOP include Visual Basic, Java and C++.

Operating System
Examples include Windows, DOS and UNIX.

Personal Computer
Generally applied to computers conforming to the IBM designed 

Printed Circuit Board

Peripheral Component Interconnect
A local bus standard developed by in 1992. PCI cards plug into 
your computer and are configured through software. They do not 
have jumpers or switches.

Personal Computer Memory Card International Association
Industry group that developed the specification for credit card-
sized plug-in cards for laptop computers.

Proportional Derivative

potential of Hydrogen
A measure of acidity. At 25 C (77 F) a neutral solution such as 
pure water has a pH of 7; a pH of less than 7 indicates acidity, 
more than 7 alkalinity.

Proportional Integral

Piping and Instrumentation Diagram

Proportional Integral Derivative
Proportional gain, integral action time and derivative action 
time. A feedback method for control. PID software, for example, 
compares an analogue input value with a set point and if there is 
a discrepancy outputs an appropriate analogue or digital control 
value, according the PID calculations. 

Process and Instrumentation Design

Programmable Logic Controller
A microprocessor-based device used in industrial monitoring and 

Parts Per Billion

Parts Per Million

Parts Per Thousand

Process Fieldbus

Platinum Resistance Thermometer
A type of RTD (resistance temperature device) made of platinum. 
Has long term stability.

A PRT specified to have a resistance of 100 ohm at 0 oC.

Quality Assurance

Quality Control

Research and Development

Radio Frequency
Any frequency within the electromagnetic spectrum suitable for 
radio broadcasting and communications. Frequencies extend from 9 
kilohertz to thousands of gigahertz. The RF spectrum is divided 
into several bands, including VHF and UHF.

Radio Frequency Interference

Root mean square
The square root of the sum of the squares of a set of quantities 
divided by the total number of quantities. Used when monitoring 
ac (alternating current) signals. Many power supplies, for 
example, issue an ac signal. This needs to be converted to a dc 
(direct current) signal for the PC interface. The solution is a 
signal conditioning input that produces a dc signal proportional 
to the rms of the amplitude of the input signal. The rms 
operation means the reading will always be positive.

Resistance Temperature Device or Resistance Thermometer Detector
Rely on the principle that the resistance of a metal increases 
with temperature.

Recommended Standard 232
A protocol for serial data communications. An RS232 link will run 
at up 38400 baud (bits per second) over short distances, and at 
lower speeds as the distance increases. You can plug the RS232 
lead directly into the computer's serial (COM) port. 

Recommended Standard 485
Another protocol for serial communications. Allows several 
devices to be connected to a single cable, distributed over a 
wide area.

Supervisory Control and Data Acquisition

Small Computer Standard Interface
SCSI is a parallel interface standard used by Apple Macintosh 
computers, PCs and many UNIX systems for attaching peripheral 
devices to computers.

Sample and Hold 
A component of a type of analogue-to-digital converter. The 
analogue signal is frozen in a sample and hold circuit to prevent 
it changing during digitisation.

An analogue input that is measured with respect to a common 
earth. Single ended inputs are only suitable for signals that are 
of good size - 100 mV full scale or above.

Secure-Hypertext Transfer Protocol
An extension to the HTTP protocol to support sending data 
securely over the World Wide Web. 

Simple Mail Transfer Protocol. 
The Internet standard protocol for transferring electronic mail 
messages from one computer to another. Regulates the 
communication between mail servers.

International system of units. Abbreviation for Systeme 
International (d'Unites).

Set Point
Value of a controlled variable, departure from which causes a 
controller to operate to reduce the error and restore the 
intended steady state. 

Statistical Process Control
A method to separate random variations in data from non-random 
variations. Used in automatic product testing.

Structured Query Language
A query language for requesting information from a database. The 
original SQL was introduced in 1979 by Oracle Corporation. 
Supports distributed databases, spread over several computer 

Transmission Control Protocol/Internet Protocol
The communications language used between computers on the 
Internet. IP is the specification that determines where packets 
of information are routed to, based on their destination address. 
TCP makes sure that the packets arrive correctly at their 
destination address. If TCP determines that a packet was not 
received, it will try to resend the packet until it is received 

Transistor-Transistor Logic
Referring to logic circuits consisting of two or more directly 
interconnected transistors, to provide conditional switching 
capability. For digital input circuits, a logic 1 is obtained for 
inputs of 2.0 to 5.5 V which can source 40 microA, and a logic 0 
for inputs of 0 to 0.8 V which can sink 1.6 mA. For digital 
output signals, a logic 1 is represented by 2.4 to 5.5 V with a 
current source capability of at least 400 microA; and a logic 0 
by 0 to 0.6 V with a current sink capability of at least 16 mA. 

Ultra High Frequency
Radio waves between 300 MHz and 3000 MHz.

Uninterruptable Power Supply
Used to keep critical equipment, including computers, running in 
the event of a mains power failure. 

Universal Serial Bus
A serial communications protocol for connecting peripherals to 
computers - is gradually replacing RS232 and parallel ports.

Very High Frequency
Radio waves between 30 MHz and 300 MHz.

Wide area network. A network of circuits spanning a large region 
that is used to transmit data. 


Next month:
  It's Monitor's second birthday and we're including a complete 
  index of all subjects covered in our previous issues.


* Copyright Windmill Software Ltd
* Reprinting permitted with this notice included
* For more articles see

We are happy for you to copy and distribute this 
newsletter, and use extracts from it on your own web site 
or publication, providing the above notice is 
included and a link back to our website is in place.

An archive of previous issues - including articles on temperature 
and strain measurement - is at

Windmill Software Ltd, PO Box 58, North District Office,
Manchester, M8 8QR, UK
Telephone: +44 (0)161 833 2782
Facsimile: +44 (0)161 833 2190
E-mail: [email protected]

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