-------------------------Monitor------------------------ The Newsletter for PC-Based Data Acquisition and Control Issue 159 www.windmill.co.uk October 2011 --------------------ISSN 1472-0221---------------------- Monitor brings articles on data acquisition techniques, application stories, special offers for our readers and the highlights of the measurement and control news of the month. I hope you find it useful but should you wish to remove yourself from our mailing please go to https://www.windmillsoft.com/daqshop/Monitor_Newsletter.html Today we have a story of how Windmill data logging software helped food scientists determine characteristics of food cooked by high pressure frying. We are always on the look out for more stories of data acquisition in action, so please share your application with us. Many measurement devices can be connected to a PC via RS422 and RS485 cables. We explain the difference between RS232, RS422 and RS485; their advantages and drawbacks; and how RS422 and RS485 function in practice. Don't forget that as a subscriber you can download our free ComDebug software to log data from RS422, RS485 and RS232 instruments. CONTENTS ======== * Windmill News: Using Windmill to Study Pressure Frying * Understanding RS422 and RS485 * DAQ News Round-up ________________________________________________________ ________________________________________________________ Food Scientists Use Windmill to Study Pressure Frying ________________________________________________________ In industrial cooking, high pressure frying produces more juicy and tender food than traditional atmospheric frying - the vapour naturally released from the food generating the pressure. Food scientists from the University of Mersin, Turkey and Lund University, Sweden, are studying how modifying factors that affect the boiling temperature of the oil in pressure frying may make the food more desirable. As part of their research they are using Windmill Software to continually collect data (https://www.windmillsoft.com/daqshop/rs232-modbus.html). The scientists needed to determine accurate temperature distribution: from this they could judge texture development, colour changes and the formation of carcinogenic substance. During their experiments they fryed potato slabs in a modified pressure cooker. They placed thermocouples in the potato slab and oil to measure temperatures. During frying they placed the pressure cooker on an analytical scale from Elastocon AB. They connected the scale to a PC via RS232. Windmill software, running on the PC, took weight readings throughout the frying process. Loss of water vapour through the pressure cooker's safety valve led to a decrease in the system mass, and this mass change allowed the calculation of the rate of change in mass in the potato sample over time. Every 2 seconds, Windmill recorded the change in mass of the frying system. From this, and the temperature data, the researchers could determine the heat transfer coefficient and thus temperature distribution. The Windmill RS232 data collection software is currently reduced from to £145 to £50 in our on-line catalogue: https://www.windmillsoft.com/daqshop/rs232-modbus.html Further Reading: Erdogdu, F. and P. Dejmek, "Determination of Heat Transfer Coefficient during High Pressure Frying of Potatoes", J. Food Eng., 96, 528-532 (2010) http://journals2.scholarsportal.info/details.xqy?uri=/02608774/unassigned/nfp_dohtcdhpfop ________________________________________________________ ________________________________________________________ Understanding RS422 and RS485 Wiring ________________________________________________________ Those familiar with RS232 will know that the standard defines how RS232 lines should be driven electrically: what they are called, what information they carry and even the connectors and pin numbers to use (see https://www.windmill.co.uk/rs232-communication.html if you are interested in details of the RS232 standard). RS422 and RS485, by contrast, define only the electrical characteristics of the driver. Advantages or RS485 and RS422 ============================= RS485 and RS422 drive lines in a differential fashion: two wires are required for each signal. If you wish to transmit four signals then eight wires are needed. The figure below shows a single RS485 / RS422 signal being transmitted. To transmit a logic 1, line B is high and line A is low. To transmit a logic 0, line B is low and line A is high. The advantage of this arrangement is that signals can be transmitted faster and over greater distances than is possible with a single wire.Diffences Between RS422 and RS485 ================================= In both RS422 and RS485, one driver can drive many receivers. In RS485 each driver can be switched off allowing multiple units to send data over a single pair of wires. This is not possible in RS422. This extra feature causes extra problems for RS485 units. Other differences are small so hereafter I will refer to units as RS485 but the remarks apply to RS422 also. Interface units often describe themselves as RS422/RS485 because a RS485 unit which is not using its extra capability is RS422 compatible. When instruments are described as having an RS485 interface this tells you nothing for sure about the signals being transmitted. Usually though only the Transmit Data (TX) and Receive Data (RX) of a normal serial port are converted to RS485 or RS422. The other signals of the serial port are not used. Three arrangements are commonplace: Write only, 4-wire (full duplex) and 2-wire (half duplex). Write Only System: Sending Data Only ==================================== In this arrangement the computer sends data out over a single RS485 pair of wires to many instruments. It never reads any data back. An example of this is the remote control of CCTV cameras. The operator can send out messages to move the camera. His feedback is the TV picture so no return data is needed. This requires only conversion of the RS232 TX signal to RS485. Full Duplex System: Sending and Receiving Data over 4 Wires =========================================================== Full Duplex means that data can pass simultaneously both to and from the instruments. This requires 4 wires - one pair to Transmit and one pair to Receive. The computer alone controls the TX transmit line. It sends out a message which includes an Address. The addressed instrument alone replies by driving the RX line. This does mean that all the connected instruments must use the same software protocol - otherwise there will be confusion as to which instrument is being addressed. A simple conversion of RS232 to RS485 will suffice for this arrangement and no special demands are made on general purpose software such as ComDebug.
Half Duplex System: Sending and Receiving Data over 2 Wires =========================================================== Half Duplex means that data can only pass in one direction at a time. In this arrangement both TX and RX signals share a single pair of wires. This can save in installation costs. The computer drives the lines to send out its request message but must turn its driver off to allow the instrument to reply. This is the tricky bit. Many RS232-to-RS485 converters recruit the RTS line of the serial port to switch the RS485 driver On and Off. However Windows has never made more than a half hearted attempt to support such an arrangement. The multi-tasking nature of Windows means that it is always difficult to make actions occur at exact times - in this case there can be a significant delay between the request being sent out and the driver being switched off. If the instrument starts its reply before the switch-off then the start of the reply will be missed. Instrument manufacturers are forced to find their own solutions to this problem. This may include special software, special hardware or both. In recent years most Serial Ports have been implemented as USB-to-Serial arrangements. These units always include a microprocessor and this can perform the task of controlling the RS485 driver very well. If you need to implement a 2-wire RS485 system you should always obtain a USB-to-RS485 converter that advertises this special feature. An example type that we use is Moxa 1130. This unit automatically switches off the driver when there is no more data to send. Such a unit can work with general purpose software such as ComDebug or HyperTerminal. Beware though that it may not work with the special software supplied by instrument manufacturers which may assume a particular RS232- to-RS485 converter and may produce errors when used with any other unit.
Preventing Voltage Reflection ============================= When a signal is sent down a cable there is always a reflection voltage which returns up the cable. This reflection is bigger as signals' edges get faster and lines get longer. It can be minimised by terminating the ends of the line with the characteristic impedance of the system. This is 120 ohm for RS485. How to Arrange the RS485 Wires ============================== Ideally an RS485 system consists of a single linear cable (no branches) with 120 ohm resistors connected across the 2 wires at each end of the cable. RS485 can handle speeds of over 10 Mbits per second and line lengths of over 1 km. If you are operating anywhere near these values you must arrange your wiring close to the ideal. For many applications where baud rates are slow (say 9600 baud), and lines are only tens of meters long, this is not essential. The wiring requirements can then be relaxed to allow cables to run in star arrangement from the computer and the terminating resistors become optional. Line Pull Ups: Avoiding Serial Errors ===================================== In many RS485 multidrop systems there are times when no unit is driving the RS485 lines. They can then go to any voltage and may produce serial errors. To avoid this it is often necessary to fit resistors which pull one line high and the other line low putting the line into the passive state. The RS485 converters usually include such resistors which can be connected if needed. Signal Names: Caution Required ============================== You will find that your RS485 signal names for a line pair are either A,B or +,-. In an ideal world you would simply connect A on the computer to A on the instrument. However these names are not consistently applied by manufacturers and you may need to connect A on the computer to B on the instrument. Further Reading: ================ Monitor Issue 32--Serial Communications: RS422, RS485 and Modbus ________________________________________________________ ________________________________________________________ DAQ News Round-up ________________________________________________________ Welcome to our round-up of the data acquisition and control news. If you would like to receive more timely DAQ news updates then either grab our RSS newsfeed at https://www.windmillsoft.com/monitor.xml or follow us on Twitter at http://twitter.com/#!/DataAcquisition Discovery of Duqu malware sparks fears of Stuxnet 2.0 Security researchers have discovered a new form of malware that appears to have been created by the same people behind the Stuxnet worm uncovered last year. The malware, named Duqu, gathers intelligence from industrial control system manufacturers, possibly for use in mounting Stuxnet-like attacks against facilitates such as power stations that use the industrial control systems. Source: New Scientist http://www.newscientist.com/blogs/ Wind Energy Test and Monitoring Market Poised for Growth According to a report by Frost & Sullivan, new opportunities are unfolding for the wind test and measurement market as the need for certification and verification of wind turbine components increases. Both component manufacturers and wind power operators need to test, monitor, and inspect procedures during the product's lifecycle. Source: Frost & Sullivan http://www.frost.com/ Smart sensors stop flickering wind turbines An algorithm that determines when annoying shadows from wind turbines reach residential areas could end complaints of "shadow flicker". Source: New Scientist http://www.newscientist.com/article/ Wireless and Sustainability Report Reveals Big Cost Savings An emerging wireless-enabled environment has the potential to drive billions of dollars in energy savings and reshape how industry, agriculture and the public sector approach sustainability, according to a new report. Source: CTIA-The Wireless Association http://www.prnewswire.com/ EcoDriver project aims to help drivers improve fuel efficiency Motorists could soon be provided with real-time interactive feedback about their fuel-efficiency performance. The £13m pan-European EcoDriver initiative, led by Leeds University, will consider a suite of potential add-on devices over the next four years. Source: The Engineer https://www.theengineer.co.uk/ ________________________________________________________ ________________________________________________________ * 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/monitorindex.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/
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