Fm210_01b_oi_e.docx / Nov-20
Page 35 / 40
Modbus CRC Calculation
The Modbus CRC Reset parameter is used to reset the CRC Calulator. If Modbus CRC Reset = 0,
the CRC Calculator is set to FFFF in the monitor field (Page 1; Variable <4). If Modbus CRC Reset =
1, the CRC value can be calculated with every Modbus byte.
The parameter Modbus CRC Byte is used to enter the bytes for the CRC calculation.
Into this memory address the Modbus bytes can be transferred one after the other and then the CRC
checksum can be read out in the monitor field (Page 1, variable <4).
Using the OS
For the Modbus command 0x07 0x11 the checksum shall be determined. First the Serial Page
parameter should be set to 1 to select Page 1.
- Modbus CRC Reset = 0, Modbus Byte = 7dec
(Press OS Transmit change)
-> Variable <4 = 0x0000 FFFF (reset)
- Modbus CRC Reset = 1, Modbus Byte = 7dec
(Press OS Transmit change)
-> Variable <4 = 0x0000 82FE
- Modbus CRC Reset = 1, Modbus Byte = 17dec
(Press OS Transmit change)
-> Variable <4 = 0x0000 8CC3
Now the command can be set together
-> 07 11 C3 8C
Note that with Modbus the CRC sends first the LByte and then the HByte, therefore byte swapping is
necessary.
Correction of the difference increments
The correction of the difference increments can be done by the parameters Correction Basis,
Correction Freq and Correction Value. If Correction Freq and Correction Value are set to zero, then
the extended correction is switched off.
A simple correction can be done by using the Correction Basis parameter. If all Correction parameters
are set to zero, the correction is deactivated and the output frequency is not corrected, so that a slight
deviation of the position in time is noticeable. Here, the differential counter slowly rises or falls to the
maximum value (approx. +/- 244 000) and then stops. An Out of Sync event is also triggered.
If you now set the Correction Base parameter at a certain frequency, you can see that with higher
parameter values, the differential counter runs around zero. With lower parameter values a differential
counter value is generated, which results in a constant position shift. Too high settings can make the
system unstable, resulting in an oscillation. If you now change the frequency, you can see that the
position shift increases or decreases.