Transmission Mode (Rtu); Device Address; Data Format; Query - Siemens RS-485 User Manual

Transmission mode (RTU)

Data format

Device address

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The transmission mode used is the RTU (remote terminal unit) mode. Data are trans-
mitted in binary format (hexadecimal) with 8 bits.
The LSB (least significant bit) is transmitted first.
ASCII mode is not supported.
The data format describes the structure of the transmitted byte.
Data word
8 bit
The slaves' device addresses can be selected between 1 and 99.
Device address 0 is reserved.
+
A maximum of 31 slaves can be addressed via the RS-422 / RS-485 interface.
There are 2 choices of data exchange:

Query

Data query / instructions delivered by the master to a slave via the re-
spective device address.
The addressed slave responds.
Broadcast Master instructions to all slaves via device address 0.
The addressed slaves do not respond.
A data query with device address 0 does not make sense.
A certain setpoint can be transmitted to all slaves, for example.
In that case, correct adoption of the value by the slaves should be
checked by subsequent setpoint readout.
Parity bit Stop bit ½ bit
Not
Number of bits
1 9
CC1A7865.1en
31.10.2002