Kipp&Zonen COMBILOG Data Logger - Manual (Page 129)

11.8 MODBUS protocol
11.8.1 Telegram Format for the MODBUS-RTU Protocol
The request and response telegrams in the RTU-mode used by
the sensor modules start with an idle-interval of at least 3.5
character lengths. The simplest way of attaining this is by wait-
ing for at least 4 character-times after receiving the last charac-
ter of a telegram. The telegrams have no Start-Delimiter and no
End-Delimiter either. The first field after that idle-interval is the
ISM-Address (ADR) followed by the function number (FNR)
and the function parameters or data respectively. At the end
the telegrams contain a check sum (CRC) with a length of 16
bits. The check sum is calculated from the whole telegram
without the CRC itself. The CRC-polynomial is: u
15 + u 13 + 1.
The start value is hex FFFF.
request telegram
idle interval ADR FNR function parameters / data CRC
> 3,5 CT 1 byte 1 byte n byte 2 byte
response telegram
idle interval ADR FNR function parameters / data CRC
> 3,5 CT 1 byte 1 byte n byte 2 byte
Hardware Manual COMBILOG 1022
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