Buses, Protocols and Systems for Home and Building Automation

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(synchronization can be done every bit) unidirectional serial communication with a
transmission rate of 1.0 kHz and data frame (Fig. 3.2.5.6) consisting of 34 bits (a startbit,
32 data bits, a stop-bit). The data bits consist of:
- 1 parity-bit (even)
- 3 message type bits = used for an identification of the content and meaning of
the frame (read-data, write-data, data-invalid…)
- 4 spare bits
- 8 data-ID bits, which identify uniquely the transmitted values. These bits are
called OpenTherm-ID. There are 256 OT-ID available: 128 of them are reserved
for OEM use, and the other 90 are functionally specified.
- 16 data-Value bits
In normal operation, a master sends a frame every second. Answer from a slave is
expected in 20ms – 800 ms. After a slave answers, there is a 100 ms pause.
Fig. 3.2.5.6: OpenTherm frame structure (51)
There exist only few standard interfaces for interconnection of the OpenTherm system with
BACnet IP, LON, Modbus, WWW (Fig. 3.2.5.7) or KNX (Fig. 3.2.5.8).
Fig. 3.2.5.7: A gateway to other systems (53)
Ondřej Nývlt - Buses, Protocols and Systems for Home and Building Automation 37