Siemens RS-485 User Manual
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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.
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.
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.
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.
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