-----------------------------Monitor------------------------------ The Newsletter for PC-Based Data Acquisition and Control Issue 28 www.windmill.co.uk November 2000 -------------------------ISSN 1472-0221--------------------------- Welcome to another edition of Monitor. This month we are pleased to announce the launch of Windmill Remote software, for condition monitoring over the internet. For those more interested in local data acquisition, we also discuss the GPIB. 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 ======== * Launch of Windmill Remote for Data Acquisition over the Internet * Local Monitoring: The GPIB - Connect up to 14 instruments to 1 PC - High speed - Large numbers of measurement and control channels - Technical details - Summary - Further Reading _________________________________________________________________ Windmill News: Windmill Software Launch Windmill Remote Windmill Software is proud to use this issue to announce the global launch of our new Windmill Remote software. Windmill Remote lets you monitor conditions at a distance, and view live data over the internet. It is especially useful for hazardous environments. Designed to incorporate GPS measurements, it is also ideal for mobile systems where location needs to be logged. Windmill Remote lets you monitor as many remote stations as necessary. With proper permission, Administrators may also control the monitoring station: setting different parameters to be logged (temperature, speed, etc), new update intervals, fresh alarm settings, different telephone numbers to be dialled to alert people to alarms, and so on. For more details of Windmill Remote contact [email protected] or visit http://www.windmill.co.uk/wmremote.html _________________________________________________________________ Local Monitoring - the GPIB? There are many things to consider when choosing data acquisition equipment, including how you will connect it to the PC. Many cabling options exist: USB, RS-232, Ethernet, Modbus, GPIB, etc. Each method has its advantages and disadvantages, many of which we summarised in Issue 2 of Monitor (http://www.windmill.co.uk/freewm.html). This month - why use the General Purpose Interface Bus? The original name for the GPIB was HPIB, or Hewlett-Packard Interface Bus. As the name implies, the bus was designed by Hewlett-Packard. It was subsequently accepted as the IEEE Standard 488. Originally peripherals like printers and plotters were commonly connected by the GPIB. Although this is no longer true, the GPIB continues to be a popular for data acquisition systems - especially in high speed applications. * Connect up to 14 Instruments to one PC ====================================== With the GPIB you can connect up to 14 instruments to one PC. Each instrument should be less than 2 metres from the next. You need a GPIB adaptor card in your computer and a GPIB lead. The GPIB adaptor will come with driver softwar. You might also receive functions that let you write your own programs to access the GPIB. Alternatively you could use off-the-shelf applications like Windmill, Streamer or WaveCap software. * High Speed ========== According to the specification, the GPIB allows speeds of 1 Mbyte per second. However, in practice, the maximum speeds tend to be around 250 Kbytes per second. * Large Numbers of Measurement and Control Channels ================================================= Each device on the GPIB must have a unique, primary, address which is a number from 0 to 30. Secondary addresses are also available for modular instruments. For example, a frame or case could contain a number of plug-in boards that perform different functions: timing, analogue input, digital input and output, etc. Using the GPIB the computer can address each module individually. As 961 secondary addresses are available, and each module might provide 16 or 32 inputs or outputs, there is potential for vast numbers of measurement and control channels. * Technical Details ================= At any time there must be one device on the bus that is a controller. This device issues commands to the other devices, and in our example is the PC. The other devices may be Talkers - putting data onto the bus, Listeners - reading from the bus, or inactive - neither talking nor listening. Only one device may talk at once, but more than one may listen to the Talker. The GPIB uses a 16 line parallel connection which has strictly defined mechanical and electrical properties. The 16 lines are divided into 8 data lines, 3 handshake lines to synchronise the transfer and 5 management lines to control use of the bus. The management lines are ATN - Attention - Indicates current use of data lines: commands (true) or data (false) IFC - Interface Clear - Used by the controller to reset all devices on the bus to inactive EOI - End or Identify - Depending whether ATN is in command or data mode, requests device status or indicates the end of data transfer SRQ - Service Request - A device which requires attention may use SRQ to signal the controller REN - Remote Enable - Used to switch a device between local control and remote control. Not used on many devices. The three handshake lines guarantee transmission integrity. NFRD - Not Ready for Data - indicates when a device is ready for a message NDAC - Not Data accepted - Indicates when a device has accepted a message DAV - Data Valid - Indicates when signals on the data lines are stable and valid and can thus be accepted The GPIB uses TTL levels with negative logic. Lines are true when they are at the TTL low level: 0.8 V or less. Lines are false when they are at the TTL high level: 2.0 V or more. * Summary of the GPIB =================== - Can connect 14 devices, from different manufacturers, to one PC - High speed: the GPIB transmits data bits in parallel rather than in series - Individual modules can be addressed, allowing an enormous number of measurement and control channels - Reliable and efficient method of communication because each byte is handshaked - For systems with instruments close to the computer - The GPIB specification gives precise talking and forgiving listening * Further Reading =============== ANSI/IEEE standard 488.1-1987 Standard Digital Interface for Programmable Instrumentation. ANSI/IEEE standard 488.2-1992 Codes, Formats, Protocols and Commands for Programmable Instruments Microlink Technical Notes: GPIB http://www.microlink.co.uk/gpib.html Agilent: Fundamentals of HP-IB http://www.agilent.com/ Monitor Issue 2, Summary of Communication Links http://www.windmill.co.uk/freewm.html Issue 15, RS-232 Communications http://www.windmill.co.uk/rs-232.html Issue 24, Universal Serial Bus http://www.windmill.co.uk/monitor24.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 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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