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-----------------------------Monitor-----------------------------
     The Newsletter for PC-Based Data Acquisition and Control    
Issue 26               www.windmill.co.uk          September 2000
-------------------------ISSN 1472-0221--------------------------

CONTENTS
========
* Windmill News: Windmill Help Files Available from Web
* Multiplexing Multiple Measurements

_________________________________________________________________

                         Windmill News:                          
            Windmill Help Files Available from Web             

The Windmill data acquisition software comes with 
comprehensive on-line Help. We hope you'll be able to find 
answers there to any questions you have. However, we operate a 
policy of continually upgrading the Help: adding more 
information depending on the questions you ask us. All updated 
versions are available from our web site at 
http://www.windmill.co.uk/help.html
Download the zip files and copy *.hlp and *.cnt to your 
Windmill folder.

Previously we only listed updated Help files for Version 4.3 
of Windmill, but now we've added those for Windmill 6. 

To keep improving the Help we need your comments, especially 
your criticisms. Please fill in the form on the Help page.
http://www.windmill.co.uk/help.html
_________________________________________________________________

Multiple Measurements: Multiplexing or Simultaneous Sampling?

When data acquisition equipment receives an analogue signal, 
it digitises it - converting it to number that the PC can 
understand. This is done by an analogue-to-digital converter. 
However, what happens when monitoring multiple signals? 
Clearly, one A-D converter cannot digitise all the signals at 
once. There are two answers to this: provide an A-D converter 
for each analogue input, or switch (multiplex) the signals in 
turn to one A-D converter.

Simultaneous Sampling
=====================
In true simultaneous sampling the data acquisition hardware 
provides an A-D converter for each analogue input signal. 
Providing sampling is initiated from a single clock source, 
this both permits simultaneous sampling and ensures that there 
is no reduction in sampling rate as the number of inputs 
increases. However, this method is very expensive.

Multiplexing
============
The second, and more common, approach is to use a single 
analogue-to-digital converter and switch each signal in turn 
to the converter. This is a much cheaper solution. However it 
does not provide simultaneous sampling across the inputs and 
as more inputs are added the maximum sampling rate is reduced 
per input. For example, if your system can read 1 input 
channel 100 times a second, when reading 10 input channels it 
is limited to 10 readings per second per channel.

Settling Time and Cross Talk
============================
As the multiplexer switches one input after another to the A-D 
converter, you must allow time for each reading to settle to 
its new value. This settling time defines the minimum interval 
between reading each input: the inter-channel interval. If you 
try to go faster than this there will be increasing cross-talk 
between channels, leading to inaccurate results.

Mixing Small and Large Signals
==============================
Low-level signals, like thermocouple voltages, take longer 
than higher voltages to settle to their true value. It's 
always good practice to group the signal sizes. Some data 
acquisition systems let you set different inter-channel 
intervals for different groups of channels. This is especially 
valuable when streaming data directly to disk at high speeds. 
For slower applications, software like Windmill lets you set 
different settling times for different channels.

Another consideration when monitoring different sized signals, 
is being able to choose the range of each input channel 
individually. The range refers to the maximum and minimum 
voltage that will be digitised by the A-D converter. It's 
always best to choose the smallest range that encompasses the 
signal, as this optimises resolution. For low speed 
applications (Hz) some software will automatically select the 
best range for you, but this should be turned off for high 
speed applications (kHz). Independent ranges lets you to mix, 
say, thermocouples, 2 V instrumentation outputs and 4-20 mA 
process signals in one multiplexed scan. 

Burst Scanning
==============
When collecting data fairly slowly it is possible for the scan 
rate (the total time to read all the channels once) to be much 
slower than the minimum inter-channel interval suggests. In 
this case it may make sense to sample the channels fairly 
quickly once a scan starts, giving readings that are taken 
closer together in time and so more easily compared. This 
relatively long break between scans is known as burst 
scanning.

Mixing Fast and Slow Changing Signals: Split-Rate Scanning
==========================================================
Suppose you are monitoring an engine. You are recording 
pressure signals from the cylinders as well as coolant 
temperatures. It is sensible to record the pressure signal 
much more often than the coolant temperature. This leads to 
the idea of split-rate scanning where a subset of channels is 
recorded every scan, while all channels are recorded every nth 
scan.

Channel Sub-Sets
================
For test applications, you may be monitoring some inputs in 
one test run and a completely different set of inputs in 
another. The ability to select a sub-set to scan is important 
to avoid having to rewire your connections each time you want 
to record some waveform data.

Software
========
Windmill software supports low-speed multiplexed systems with 
independent input ranges and channel sub-sets. Streamer software 
supports higher-speed multiplexed systems with independent input 
ranges, channel sub-sets, inter-channel intervals, burst scanning 
and split-rate scanning. (Assuming in both cases that the 
hardware also supports these features.) For more details of 
Windmill or Streamer send for a free leaflet at our web site.
http://www.windmill.co.uk/free.html

Further Reading
===============
Monitor Issue 3 - A-D Converter Specifications 
Monitor Issue 4 - Analogue Signals and Types of A-D Converter 
_________________________________________________________________

* Copyright Windmill Software Ltd
* Reprinting permitted with this notice included
* For more articles see http://www.windmill.co.uk

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 is at 
http://www.windmill.co.uk/newsletter.html

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: monitor@windmill.co.uk
http://www.windmill.co.uk/
http://www.windmillsoft.com/

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