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iJAF

Modbus Grundfos Pump CIM200 - Problems

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Hello, I have to integrate several grundfos pumps with modbus interface CIM200. Firstly i'm using a modbus scanner (modbus poll) to check the registers. But I cant communicate sucessfuly with the cim200 module. When observing the data in message exchanges(just reading reg 23 ) : 000002-Tx:4E 03 00 17 00 01 3A 31 000003-Rx:FF 4E 03 02 00 4E 6D TX - Is my master software RX - Is the slave I'm testing with add. nº78 (4E hex) The response from the slave has a FF byte before the response itself, in every message exchanged!! How is this possible ?! Thank you !

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Your response is missing the one of the two CRC bytes; it should be 4E 03 02 00 4E 6D BE but I don't think that's your problem when the low order half the CRC is correct. It appears that the master is looking for a response too soon, earlier than the response is actually arriving and has time to receive what it thinks is a start bit and the following line logic state that it interprets as all ones. I'll bet that you're probably running 8-N-1, with no parity so there's not even a parity check on the bogus data word. There's probably a setting (sometimes called turn around) that tells the master to wait x milliseconds before looking for a response. Try increasing that value to see what happens.

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Thank you! This modbus telegram was from an app called modbus poll, that I used to test the equipment. Yes, i'm using 1stopbit with none parity check and 8 databits (modbus rtu). With this software i'm just seeing that i can only change the response timeout, the delay between polls and the scan time to the slaves! Edited by iJAF

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Maybe try using even or odd parity, in hopes that the bogus leading byte will be rejected.

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The lack of hard biasing, pull -up or pull down resistors that drive the bus lines to a known state during the idle condition when no driver is active can cause a false data bit, which the UART can interpret as a start bit. I suspect that the absence of data following a start bit could show up as FF because I think a logic zero shows up as a one in serial voltage logic. It might be worth looking into biasing resistors. Pages 7 and 8, Failsafe Biasing, in the Analog Devices RS-485 Circuit Implementation manual gives a sample calculation. http://www.analog.com/static/imported-files/application_notes/AN-960.pdf

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