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Showing content with the highest reputation on 07/03/2012 in all areas

  1. 1 point
    MI's are faster and more robust. Datatable functions are not immediate or even guaranteed under all circumstances. Try putting 4 or more data table functions on one ladder and enable them with a positive transition. I have not used version 9 so this may have been addressed. However, The DT is the best way to list several items and then makes it easier to send them all to the PC. I would use one DataTable copy function and make it a FIFO. Then when the table is full, send it all to the PC and clear the table. 2 lines of ladder. If your data is in an array of MIs it is easy to copy the data into the first row of the datatable and then clear the data in the array. Shift the data in the datatable down one row and continue. If there is data in the last row, send the whole table to the PC and clear the datatable.
  2. 1 point
    can is a one way open loop broadcast modbus is a 2 way communication that uses a handshake to confirm that the information was sent adn received and both sides are aware of the result. Can is faster, but you dont know if the message was received, unless you trigger some sort of return message, which would slow it down. Can does have a heartbeat function that confirms the connection is live, if you use it. It is possible to configure a canopen network with only one node and transmit data and receive no error or indication that no other node received the data. This is not possible with modbus. The heartbeat function is the "old style" can, and will slow down the can network. You choice is fast or robust. And you also might want to consider that Unitronics is a bit tricky with canopen communications. The last time I used it (it could be better now) I found that the canopen function is asynchronous to the PLC ladder. The canopen function also contains pointers to data , the actual data is NOT loaded into a com buffer when the function is called. So if the data changes during the scan that you call the canopen function, the data will be corrupted. This led me to stop updating the data during the scan that a canopen function is activated. This is also a time cost. I needed fast and using two can nodes to talk back and forth was remarkably slow after all the limitations were dealt with. The canopen hardware runs independent of the ladder, the data may be sent at any time during the scan.
  3. 1 point
    Just open datatable and place mouse pointer over cell you want to get memory allocation. It shows type of register, length and allocation (FLASH: ## or RAM:##).
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