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-------------------------Monitor------------------------
The Newsletter for PC-Based Data Acquisition and Control
Issue 69 www.windmill.co.uk April 2004
--------------------ISSN 1472-0221----------------------
Welcome to April's Monitor. We hope you find it useful,
but should you wish to cancel your subscription please
do so at
http://www.windmill.co.uk/newsletter.html
CONTENTS
========
* Windmill News: Windmill Detects PLC Communications
* Monitoring Current Signals
* Windmill Notes: Parsing Data with the Free Windmill
* Excel Corner: Scrolling through a Chart
________________________________________________________
________________________________________________________
Windmill News:
Windmill Detects Siemens PLC Communications
________________________________________________________
The Siemens S5 programmable logic controllers have been
widely used for automation purposes around the world.
Although now superseded by the S7, there are many still
in use. One of our readers was studying the properties
of the S5's serial connection, with a view to writing
his own software to communicate with this plc. He
downloaded Windmill comDebug and found that..."it was
the most simple way to detect the right data used for
communication". Windmill speeded up his programming
task and helped him identify and eliminate data transfer
errors.
You can download Windmill comDebug from
http://www.windmill.co.uk/serial.html
________________________________________________________
________________________________________________________
Monitoring Current Signals
________________________________________________________
Current is often used to transmit signals in noisy
environments, especially where there is a long distance
between the measurement point and the data acquisition
system. This is because current is much less affected
by environmental noise pick-up.
The full scale range of the current signal is normally
either 4-20 mA or 0-20 mA. A 4-20 mA signal has the
advantage that even at minimum signal value there
should be a detectable current flowing. The absence
of this indicates a wiring problem.
Before the data acquisition equipment can digitise the
current signal and transfer it to the computer, a
current-sensing resistor is usually used to convert
the current to a voltage. The resistor should be of
high precision (consider how much resolution the
analogue-to-digital converter will give you). It
should also match the signal to an input range of the
analogue input hardware. For 4-20 mA signals a 50 ohm
resistor will give a voltage of 1 V for a 20 mA signal
(V=IR). Choose 0.03% or 0.01% resistor accuracy. The
accuracy is a measure of the long-term stability of
the resistor and its tolerance to temperature changes.
________________________________________________________
________________________________________________________
Windmill Notes: Parsing Data with the Free Windmill
________________________________________________________
If you have an instrument or device which you can plug
into the PC's COM port, and it communicates using ASCII
messages, chances are you can use the free Windmill
software to collect its data. This is because
Windmill's free LabIML driver was designed to be as
flexible as possible and accommodate almost any
instrument with an RS232 port. However, this
flexibility has meant that the initial set up can be a
case of trial and error. These notes should reduce the
error and help you decide the best way to interpret, or
parse, the messages from your instrument. (Our latest
serial driver is more transparent, see
http://www.windmillsoft.com/comiml.html for details.)
When an instrument sends a message it rarely just
contains data. All sorts of extra characters and codes
are in there. Here are some examples of messages.
A weighing scale might send:
+ 125.32 g
A GPS receiver might send:
$GPGLL,5330.12,N,00215.31,W,134531,A
A Parallax BASIC Stamp might send:
+66.4
A spectrophotometer might send:
DR/4000U S/N: 9807U0000905 2.303-APR-04
10:05:56 MULTI-WAVELENGTHGroup 0000 Sample
0002 -0.000 ABSA=K1A1+K2A2
665.0 nm -0.004 ABS K1: 1.0000 750.0 nm -
0.004 ABS K2: -1.0000
These examples show why Windmill needs to be flexible in
extracting the relevant data from the message.
It's not only message formats that vary. Some
instruments need a command prompt from Windmill before
they will send data, others initiate data transfer
themselves - often regularly sending data. In this case
the message can be considered as a continuous stream of
data that has a recurring pattern.
LabIML calls an instrument that sends one message after
a command a Request/Response instrument. Devices which
constantly send data are called Continuous Flow
instruments.
To set up parsing instructions use the Windmill ConfIML
software and select the LabIML driver. Go to the
Channels settings.
The first thing to do is to locate the data. You can do
this by:
1. Searching for specific characters in the message
string.
2. Ignoring characters until one of the ones in which
you are interested appears.
3. Ignoring a number of characters in the message
string.
At first glance methods 1 and 2 seem to do the same job.
However, when you search for characters the next action
occurs AFTER the searched for characters. When you
ignore characters the next action occurs ON the
specified character. So if you searched for a + sign you
would not be able to extract it, but if you ignored
all characters until the + sign you would. Also, a search
will look for the entire string specified (eg "abc"),
whilst ignoring characters will stop at any of the
characters specified (eg "a" or "b" or "c").
Be aware that with instruments that constantly send data
LabIML will not know where the beginning of the message
is. So, using the GPS example above, it's no use
ignoring 8 characters and expecting the next value to
be a latitude reading. Instead search for a unique
string such as $GPGLL.
Once you've located the data you need to extract it.
There are two ways to do this:
1. Extract until a specified character.
2. Extract the next so many characters.
Using the GPS receiver as an example again
$GPGLL,5330.12,N,00215.31,W,134531,A
The data we might want to record from this string is the
latitude and the longitude. These are the 5330.12 and
00215.31 figures respectively. We should have already
created 2 channels: one to hold the latitude readings
and one to hold the longitude. For the latitude channel
we might:
Search for $GPGLL, and extract until ,
For the longitude we might:
Search for N, and extract until ,
As another example, the BASIC Stamp might continually
send a reading comprising a leading sign character, a trailing
carriage return to mark the end of data and a maximum length of
six characters. For example -55.27 or +66.4 (where
is the carriage return. In this case we could
ignore all characters until we reach the + or - sign,
and then extract all data up to the carriage return.
This translates in LabIML's parse string to
\I"+-"\E"\C013"
When you are extracting several channels of data from a
string, note that if you have chosen a continuous flow
protocol, the parsing proceeds along the string for each
channel. For a request/response protocol though, the
parser goes back to the beginning of the string each
time for each channel.
--
Further Reading:
The LabIML Help file. You can download the latest
version of this from http://www.windmill.co.uk/help.html
Our web notes on interfacing -
GPS receivers
Sartorius balances
Mettler Toledo balances
Desoutter torque meters
Molytek loggers
Parallax BASIC Stamps
________________________________________________________
________________________________________________________
Excel Corner: Scrolling through a Chart
________________________________________________________
With a continuous data logging program like Windmill
Logger, you can very quickly collect a vast amount of
data. There are times when you might want to replay the
data as a moving chart. The best way to do this is to
use the Windmill Replay program. This will re-run the
data and lets you fast-forward, rewind and zoom in and
out. It also display a table of data that, when you
double-click a point on the chart, shows you the exact
value of that point. For more on Replay see
http://www.windmill.co.uk/replay.html
However, if you prefer to use Excel rather than Replay,
with a little ingenuity you can create a chart which you
can scroll through. The speed the chart moves depends on
how fast you drag the scroll bar control. Our
explanation of how to do this (below) will probably be
easier to understand if you download our example
spreadsheet from
http://www.windmill.co.uk/scrollchart.xls. (This is an
Excel 2002 spreadsheet. For earlier versions of Excel
you can download our example Logger file,
http://www.windmill.co.uk/scroll.wl, and follow the
instructions below.)
Our solution makes use of dynamic named ranges and the
fact that you can link a scroll bar to a cell, where
scrolling causes the cell's value to change.
Say you have imported a Windmill Logger file into Excel
which has time in the first column (A) and voltage
readings in the second column (B). You want to chart
the voltage signal against time.
Defining Dynamic Ranges with the Offset Function
================================================
First you need to define two dynamic ranges. To do this
we're going to use the Offset function, which returns a
cell reference according to your settings.
To create our ranges:
1. From the Insert menu choose Name then Define.
2. Type Time into the Names box and
=OFFSET(Sheet1!$A$6,Sheet1!$E$5,0,Sheet1!$E$6,1)
into the Refers to box.
Click Add.
3. Type Signal into the Names box and
=OFFSET(Sheet1!$A$6,Sheet1!$E$5,1,Sheet1!$E$6,1)
into the Refers to box.
Click Add.
4. Click OK.
This is the syntax of Offset:
OFFSET(reference, rows, cols, height, width)
Reference is the location from which you want to base
the offset. In our example this is the leftmost cell
immediately above the data: A6.
Rows and columns define how far away the offset is from
the reference cell. We want the row number to change as
we scroll the chart, so we don't use an absolute value
for rows. Instead, we'll put the rows value into cell
E5, and link this to the scroll bar.
Columns tells us whether we are referring to time
(column 0) or voltage (column 1) readings.
In our example, height is the number of rows of data to
be displayed at any one time on the chart. That is, the
number of data items to be shown. We could enter a
number for this, 30 say. However, if we enter the
height value into a cell and reference that, we can then
change this value and zoom in and out of the chart.
We'll use E6 to store the number of data points to be
displayed.
Finally the width value. This is the number of columns
of the returned reference, or, in our example, the
number of data series in the chart. For our chart this
is 1.
Entering the Row Number and Data Points to be Displayed
=======================================================
We now need to set our row number (E5) and number of
data points displayed (E6). Enter 1 into E5 and 30 into
E6. (Remember, dragging the scroll bar will change the
value in E5 and hence the row of data displayed.)
Creating the Chart
==================
We can now create the chart.
1. From the Insert menu choose Chart.
2. Select Line as the Chart type and press Next.
3. Click the Series Tab. Press Add. Type into the boxes
as follows:
Name: Chan_1
Values: =Sheet1!Signal
Category (X) axis labels: =Sheet1!Time
You should see a chart of the first 30 data values.
You now need to fix the y axis, so it doesn't expand or
contract when another set of data is shown.
1. Right-click the y axis.
2. Select Format axis and the Scale tab.
3. Clear all the auto boxes and make sure that the
maximum and minimum values span your data. In our
example these are +10 and -10 (Volts).
Inserting the Scrollbar
=======================
The next step is to insert a scrollbar control.
1. From the View menu select Toolbars and show the
Control Toolbox.
2. Click the scrollbar control. (Click off the chart to
do this.)
3. On the worksheet, drag the scrollbar to the size you
want.
4. Right-click the scrollbar and select properties.
5. Enter E5 as the Linked Cell.
6. Set the minimum value to be 1 and the maximum value
to be the number of rows of data you have.
7. Click the Set-Square and Pencil icon on the Control
Toolbox to exit Design mode.
Zooming Into the Chart
======================
To zoom into the chart simply change the figure in E6:
the number of data points displayed. The less data
points the greater the magnification and vice versa.
--
Further Reading:
Our method is a modified version of Andy Pope's
scrolling chart example -
http://www.andypope.info/charts/Scrolling.htm
Other Excel Charting Tips
http://www.windmill.co.uk/xlchart.html
More on Windmill Replay
http://www.windmill.co.uk/replay.html
________________________________________________________
________________________________________________________
* 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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