SCADA & DATA ACQUISITION FEATURE
each of the network nodes, the system employs a 9-bit communication protocol which adds an ‘address’ bit onto the standard data byte. Using this methodology ensures that each node only has to interrogate data packets with the address bit set to 1, and then only responds if the address is that of itself. All data packets with the address bit set to 0 are deemed to be data bytes from other nodes and are ignored. Information about the protocol of the communication was not available and had to be determined by means of a storage scope hooked onto the RS- 485 lines, and protocol analysis software. At this stage a hardware solution was needed which was able to be placed onto the RS-485 network and collect the data. The main obstacle to overcome was the 9-bit protocol, which posed a
problem as all PC UART chips work on the basis of expecting 8-bits of data. SSDC created a workaround which allows a standard 8-bit UART to receive and process 9-bit data. This method makes use of the parity bit which is appended to the byte of data in normal 8-bit protocol - the transmitter appends this bit and sets it to 0 or 1 depending on the parity type in use. The Racal system does not actually use parity, but by setting the receiver to space parity means that for every 9-bit data packet with the address bit set to 1 there will be a parity error from the UART, hence each address data packet can be detected by seeing a parity error. The validity of this theory was tested by using a standard PC with a small
LabVIEW test code. A waveform was injected into the serial port and NI monitored the response using NI’s parity trick. The result proved the theory, but NI were missing far too many address packets to make it a viable solution. The reason for the losses was due to the PC not keeping up with 9,600 baud data stream. For the system to work, it required that each and every byte in the stream is brought through the UART and individually tested for parity - this was simply not possible on the Windows platform. The solution to the problem was to employ a NI 9075 CompactRIO (cRIO) system with a NI 9871 RS22/RS485 serial module and use the FPGA to process the serial data. FPGA is reconfigurable hardware which allows for inline processing at very high speeds. This speed ensured that NI were able to analyse each individual byte in
the serial stream and detect each address bit. The data was then simply converted to 16-bit numbers with one byte for data and one for address indication, this was then passed up to the real time computer on the cRIO for further processing. Having solved the 9-bit problem NI were left with processing the data stream so that data stream packages from each node are stored and ultimately sent to a database on a shore-based server. The cRIO real time processor was used to package the data and this was then sent on to a small embedded PC running a LabVIEW application. The NI software on the PC gives out a live data display of all parameters and stores the collected data on a TDMS file for the voyage. On completion of the voyage the TDMS file is sent to the shore based server via a WiFi link from the ship to the port of arrival. Another LabVIEW application on the server then processes the TDMS file so that scaled real values are determined from the raw data bytes and this data is sent to a database. The customer has a front end onto the database and is able to query data from each of the three ships so that data trending and predictive maintenance can be undertaken.
CONCLUSION The system is being successfully used by Condor Ferries to collect and log data thereby aiding in the preventive maintenance of its three fast ferries.
National Instruments
uk.ni.com T: 01635 523 545 /AUTOMATION Enter 209
paul@p4alt.com
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