-------------------------Monitor------------------------ The Newsletter for PC-Based Data Acquisition and Control Issue 74 www.windmill.co.uk September 2004 --------------------ISSN 1472-0221---------------------- Welcome to September's edition of Monitor. We hope you enjoy this issue, but should you wish to cancel your subscription you can do so at https://www.windmill.co.uk/newsletter.html CONTENTS ======== * Windmill News: Live Instrumentation News Summaries * Interfacing Instruments with RS232 Ports: Handshaking Explained * Excel Corner: Adding a Time Stamp to Logged Data ________________________________________________________ ________________________________________________________ Windmill News: Live Instrumentation News Summaries ________________________________________________________ If you have put our live news headlines on data acquisition and control onto your web site, then thank you. We hope you are pleased with the results. We welcome any suggestions on the news feed: e-mail monitor@windmillsoft.com. You can now also add short summaries of the news to your web pages. To do this edit the javascript code given at https://www.windmill.co.uk/newsfeed.php, and change feed_compact=1 to feed_compact=0 The news feed regularly updates on your page - giving your visitors the latest news in one place. The news changes daily and is completely free. It is selected by human editors, not by a computer program, and so is guaranteed relevant to the data acquisition, control, measurement and instrumentation fields. For full details of incorporating live news into your web site see https://www.windmill.co.uk/newsfeed.php ________________________________________________________ ________________________________________________________ Interfacing Instruments with RS232 Ports: Handshaking Explained ________________________________________________________ Windmill software lets you connect RS232 instruments to your computer. As a subscriber to this newsletter you can download Version 4.3 of the software for free. Alternatively, if you would prefer the up-to-date release (called COMIML 6), you can buy this from https://www.windmillsoft.com/daqshop/rs232-modbus.html The RS232 protocol includes handshaking. Although this is often not necessary, it has two functions. 1. It allows the computer to stop your instrument from sending data when the PC is not ready for it. 2. It allows your instrument to prevent the PC from sending data when the instrument not ready for it. The fact that your instrument comes equipped with inputs and outputs that can be used for handshaking is no guarantee that handshaking is needed. Unless you know differently, when connecting the instrument to the PC it is best to start with the assumption that handshaking is not required. If you start with no handshaking what symptoms might indicate that it really is needed?. Well one possibility is that the computer misses part of a message because its input buffer overflows. With Windmill's COMIML software the buffers are 3000 bytes long so you are unlikely to be bothered by this problem. The other possibility is that the instrument misses part of a message sent by the computer. This will probably cause the instrument not to work properly. There are two types of handshaking: hardware and software. Software Handshaking ==================== Xon \ Xoff Handshaking is a software protocol that is often used to control data flow. Suppose that the computer were sending data to an instrument which could accept no more data for the time being - the instrument would send the single Xoff character to the computer which would stop sending data until it received an Xon character to restart transmission. The same arrangements would apply for the reverse direction of data flow. Xon is ASCII character 17 and Xoff ASCII character 19. Hardware Handshaking ==================== Handware handshaking is not often used, so you can tie any potential handshake lines to fixed voltages so that they do not affect operation. In fact many manufacturers add tying resistors to handshake lines so that if you do not want to use them you simply make no connection. If you do decide that handshaking must be used then you need to know which lines your communications software (such as COMIML) and your instrument use. COMIML, for example, uses the DTR (Data Terminal Ready) line to indicate when it is able to receive data. Other programs may use the RTS (request to Send) line. The instrument will probably use the CTS (Clear To Send) line to indicate when it is ready to receive data. Once you select hardware handshaking in COMIML, the state of the CTS input to the computer becomes important. When hardware handshake is not selected the CTS line state is ignored. COMIML keeps the DTR output permanently high. Further Reading =============== The COMIML serial driver https://www.windmillsoft.com/ Serial Ports and RS232 Communications https://www.windmill.co.uk/rs232-communication.html ________________________________________________________ Excel Corner: Adding a Time Stamp to Logged Data ________________________________________________________ When the Windmill Logger program saves data, it "stamps" each reading with the time and date that it was collected. You can also do this in Excel, by means of a macro and the Windmill DDE Panel. The macro below repeatedly samples data from all the instruments' channels and stores the data in Sheet1. It then inserts the timestamp at the end of the row. When the macro runs you are asked how many times to sample all the channels, and the interval between taking sets of samples. Make sure that Windmill DDE Panel is running and displaying data before running the macro. Declare Sub Sleep Lib "kernel32" (ByVal dwMilliseconds As Long) Sub SampleData() 'If NoOfRows = 1, the first data value 'will be placed in row 1. NoOfRows = 1 'Ask for number of sets of samples and sample interval. NoOfSamples = Val(InputBox("Please enter no of samples to collect", "No of Samples")) SamplePeriod = InputBox("Please enter sample interval in seconds", "Sample Interval") ' Convert to milliseconds - needed for sleep statement below. SamplePeriod = SamplePeriod * 1000 'Initiates conversation with DDE_Panel ddeChan = DDEInitiate("Windmill", "Data") 'Keeps conversation open until the required 'number of samples have been collected. While NoOfRows < NoOfSamples + 1 'Requests data from all channels and stores it in 'memory in an array called mydata. mydata = DDERequest(ddeChan, "AllChannels") 'Ignores any warning messages which would halt the macro On Error Resume Next 'Finds the lower & upper boundaries of array, 'to determine the number of columns needed to 'store the data. Lower = LBound(mydata, 1) Upper = UBound(mydata, 1) 'Inserts data from the array into a row of cells in Sheet 1. For Column = Lower To Upper Sheets("Sheet1").Cells(NoOfRows, Column).Value = mydata(Column) Next Column 'Inserts the time into the next column Column = Upper + 1 Sheets("Sheet1").Cells(NoOfRows, Column).Value = Now 'Waits for the specified sample interval Sleep SamplePeriod 'Increments number of rows, so next set of samples 'is inserted in the next row down. NoOfRows = NoOfRows + 1 'Stops loop when required sets of samples collected. Wend DDETerminate (ddeChan) End Sub -- Notes: This macro is a modified version of one given at https://www.windmill.co.uk/excel/. If you are already using this then just insert these lines of code at the relevant place. 'Inserts the time into the next column Column = Upper + 1 Cells(NoOfRows, Column).Value = Now Further Reading =============== More on using Excel for data acquisition... For more on data logging with Windmill Logger... ________________________________________________________ ________________________________________________________ * Copyright Windmill Software Ltd * Reprinting permitted with this notice included * For more articles see https://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 https://www.windmill.co.uk/newsletter.html and an index of articles at https://www.windmill.co.uk/newsletter.html Windmill Software Ltd, PO Box 58, North District Office, Manchester, M8 8QR, UK Telephone: +44 (0)161 834 6688 Facsimile: +44 (0)161 833 2190 E-mail: monitor@windmillsoft.com https://www.windmill.co.uk/ https://www.windmillsoft.com/
Windmill Software Ltd
Windows Engineering Software
September 2004
