Windmill Software Ltd
Data Acquisition Intelligence

fluorescence mapping GIS
April 2002

The Newsletter for PC-Based Data Acquisition and Control
Issue 45          April 2002
--------------------ISSN 1472-0221----------------------

Welcome to the April issue of Monitor
. This month a free 
upgrade for all COMIML serial driver users, plus, a 
discussion of triggering data acquisition.

We only send this newsletter to people who have 
subscribed - should you wish to cancel your free 
subscription please visit

* Windmill News: Free COMIML Serial Driver Upgrade
* Triggering Data Acquisition
  - What is a trigger?
  - Why use external triggers?
  - What do the external trigger specifications mean?
* Data Acquisition Exhibitions


Windmill News: Free Upgrade to the COMIML Serial Driver

We are pleased to offer all COMIML users a free upgrade 
to their serial driver.

COMIML lets you read and control almost any serial 
device communicating over RS232, RS422, RS485,  Modbus or TCP/IP. 
These include such diverse equipment as electronic 
balances used in a laboratory, GPS receivers used in the 
field and data loggers used in process plant. The COMIML 
driver reads data from your PC's COM port, and passes it 
to the Windmill logging, charting and DDE Panel 
programs. You can also use other Windows software, like 
Excel or Access, to display and analyse the data. 

The upgrade offers:
- Easier entry of "non-printing" characters, such as 
  carriage returns and line feeds.
- A "reverse float" data format option. As well as a 
  four-byte floating point representation, there is now 
  the option for a the bytes to be in reverse order: 
  needed for some instruments.
- Interactive parsing - changes to the results of parsing 
  the data from your instrument are now automatically 
  shown during editing.

If you purchased COMIML before the 21st of March,
download your free upgrade from

You will need to enter:
User Name = MonitorSubscriber
Password = Ucpogmriamdle

Use PKZip, WinZip or similar to unpack the upgrade 
files, then run setup.exe. Install the upgrade in your 
Windmill folder.


Triggering Data Acquisition

What is a trigger?
A trigger is something that causes a data acquisition 
system to start collecting data. It may be as simple as 
pressing a software button. More powerfully, programs 
like Windmill Test-Seq let you set conditions 
which when met trigger data capture.

You application might demand, though, that data 
collection starts independently of software on an 
externally generated, hardware signal. In this case your 
data acquisition equipment will need one or two inputs 
to which you connect a trigger signal from your 

Why use External Triggers?
External triggers let you synchronise data acquisition 
with external events. This has several advantages over 
a simple software button.
1. You reduce the amount of data collected.
2. Some instruments, spectrometers for example, have 
   their own timing control and can use triggers to tell 
   the acquisition interface to start collecting data.
3. Acquisition can automatically start when, for 
   example, you switch on a machine to be monitored.
4. You need only to collect data under specific 
   conditions. For example, you might wish to record 
   what happens to a test rig when an input signal goes 
   outside pre-defined limits.

What do the External Trigger Specifications Mean?
Product literature from data acquisition hardware 
manufacturers will tell you whether their equipment 
will accept external triggers, and what form these 
triggers should take. 

Positive Edge or Negative Edge Triggering
  The system may be triggered when the external signal 
  passes from a low to a high state: positive edge 
  triggering. Alternatively, it might wait for the 
  signal to change from a high to a low state: negative 
  edge triggering.

Digital Triggers
  A trigger can be an analogue or a digital signal. A 
  digital trigger input may be listed as TTL or  CMOS 
  compatible. TTL (transistor-transistor logic) is the 
  most commonly used. When the trigger input signal is 
  between 0 and 0.8 V it is considered low, when between 
  2.0 and 5.5 V high. 

Analogue Triggers
  Analogue triggers let you use, for example, changes 
  in temperature or strain to initiate sampling. You 
  can program the hardware to be triggered (or to cause 
  a trigger) when an analogue input crosses your chosen 
  threshold. With a flexible system you might set the 
  trigger level as a percentage of the analogue input 
  voltage. If the range was 0-10 V, setting 50 percent 
  would cause a trigger when the signal crossed the 
  5 V threshold. This is useful when you can choose the 
  hardware range.

Gated Triggering
  You can not only start sampling with an external 
  trigger, you can also stop it. When the trigger input 
  leaves the "stop" state, sampling begins. It 
  continues until the trigger signal returns to the 
  stop state. With digital triggers a low signal might 
  be the stop state and a high signal the start state. 
  For analogue triggers the hardware would need two 
  trigger inputs. One trigger might occur when a high 
  limit is passed with a positive slope (low to high) 
  and one when a low limit is passed with a negative 
  slope (high to low). This lets you record, for 
  example, what happens to a test-rig when an input 
  signal transgresses pre-defined limits.

Trigger Pulse Width
  This is the minimum time that the change in trigger 
  signal must be sustained before sampling starts, or 
  stop. Typically this might be 1 millisecond.

Trigger Delay
  Some hardware lets you specify a pause between 
  receiving the trigger and sampling starting. This is 
  useful if the software needs to prepare itself to 
  receive data. The delay might be around 20 millisecond.

Pre-Trigger Data
  Some hardware can collect data before the trigger 
  arrives. You specify how much of this should be 
  retained. This way you can record events leading 
  up to the trigger.

More information on hardware triggering is given in the 
Microlink Measurement and Control Systems Catalogue, 
which is available free from Biodata Ltd.

Streamer software handles external triggers - to 
download a free demo visit

The Microlink 770 transient capture unit, available 
from our on-line catalogue complete with Streamer 
software, accepts and can initiate digital external 

Other hardware options are available, contact 
[email protected] for details.


Data Acquisition Exhibitions and Conferences

Instrumentation North
1-2 May
Leeds UK

Instrumentation and Control Expo
9-12 May
Bombay India
2nd International exhibition and conference for 
instrumentation, control and automation.

Sensors Expo
20-23 May
San Jose California USA
Brings together the technologies that are shaping 
sensing, data acquisition, control and communications.

Drives & Controls
28-30 May
NEC Birmingham UK
Conference for drives, automation and control.

Control & Instrumentation Asia
9-12 July
Part of PSA 2002, the exhibition runs alongside Watertec, 
Envirotec, Chemtec and Motion, Drive & Automation.

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

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or publication, providing the above notice is 
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and an index of articles 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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