------------------------Monitor------------------------- The Newsletter for PC-Based Data Acquisition and Control Issue 45 www.windmill.co.uk 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 http://www.windmill.co.uk/newsletter.html CONTENTS ======== * 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 https://www.windmillsoft.com/upgrade/ 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 instrument. 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 http://www.windmill.co.uk/strwindmill7.html The Microlink 770 transient capture unit, available from our on-line catalogue complete with Streamer software, accepts and can initiate digital external triggers. Other hardware options are available, contact [email protected] for details. ________________________________________________________ ________________________________________________________ Data Acquisition Exhibitions and Conferences ________________________________________________________ Instrumentation North 1-2 May Leeds UK http://www.instrumentation.co.uk/ Instrumentation and Control Expo 9-12 May Bombay India 2nd International exhibition and conference for instrumentation, control and automation. http://www.india-tech.com/menu/instrumentation.htm Sensors Expo 20-23 May San Jose California USA Brings together the technologies that are shaping sensing, data acquisition, control and communications. http://www.sensorsexpo.com/ Drives & Controls 28-30 May NEC Birmingham UK Conference for drives, automation and control. http://www.drivesexpo.com/ Control & Instrumentation Asia 9-12 July Singapore Part of PSA 2002, the exhibition runs alongside Watertec, Envirotec, Chemtec and Motion, Drive & Automation. http://www.psa2002.com/main.htm ________________________________________________________ ________________________________________________________ * 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 and an index of articles 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: [email protected] http://www.windmill.co.uk/ https://www.windmillsoft.com/
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