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2 Protocol description
2.1 Master-slave principle
The communication between a PC (master) and an instrument (slave)
using MODbus /Jbus takes place according to the master-slave princi-
ple in the form of a data request/instruction - response.
The master controls the data exchange, the slaves only have response
function. They are identified by their instrument address.
2.2 Transfer mode (RTU)
The transfer mode used is the RTU (Remote Terminal Unit). Data are
transferred in binary format (hexadecimal) with 8 bits. The LSB (least
significant bit) is transferred first. The ASCII operating mode is not sup-
ported.
Data format
The data format describes the arrangement of a transferred byte. The
following options are available for the data format:
Master
Slave 1 Slave 2 Slave n
Data word Parity bit Stop bit
1/2 bits
No. of bits
8 bits — 1 9
8 bits even 1 10
8 bits odd 1 10
8 bits — 2 10
8 bits even 2 11
8 bits odd 2 11
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