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7.3 Check the cabling of the RS485
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The non-inverting and the inverting output wires should be cabled respectively
on non-inverting and inverting input respectively. No communication will take
place if non-inverting wires are mixed with inverting wires.
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Load termination resistors are important at both end of the network to avoid
distortion due to reflections in the communication line. Check that the 'Load
switch' is at the right position (ON or OFF) according to the network
configuration.
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Connection of a ground wire and presence of a shielding will improve reliability.
7.4 Check the Transmit – Receive indicators
This is a convenient way to check the presence of TX – RX signals:
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The Receive Led (yellow Led) indicates that a Modbus query is being received by
the RS485 Modbus adapter.
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The Transmit Led (green Led) indicates that a Modbus response is being
transmitted from the RS485 Modbus adapter.
If the Rx Led and the Tx Led never lit, the problem may come from:
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The wires of the RS485 cable are not properly fixed or cabled
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The query is not sent (see the application software)
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The controller and then the RS485 are not powered
If only the Tx Led never lit, the problem may come from:
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The controller settings are not correct (check communication parameters
settings)
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The controller entered the 'listen only' mode (reset the controller or send the
required command to disable the 'listen only' mode.)
·
The master sends Broadcast messages (address 0) which are to be acted
on by all connected devices and do not require a reply
7.5 Check the function called and the register addresses
To ensure that a read or write message to a Modbus address will produce a normal
response, check that the register address corresponds exactly to the desired data
requested.
In Modbus register addressing there are several categories of register, including holding
registers (numbered from 40001 upwards) and input registers (numbered from 30001
upwards). The category of register addressed is implicit in the Modbus function used,
e.g. function 03 addresses holding registers and function 04 addresses input registers.
Irrespective of the function used, the register address 30001 or 40001 would be
addressed in the transmitted message as register 0, 30002 or 40002 as register 1, 30010
or 40010 as register 9 and so on.
RS485 ç Troubleshooting - 17
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