Siemens RS-485 - Manual (Page 2)

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Siemens Building Technologies CC1A7865.1en
HVAC Products 31.10.2002
Transmission mode (RTU)
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 Parity bit Stop bit ½ bit Number of bits
8 bit Not 1 9
Device address
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.
Data format