RFXCOM RFXtrx433 Especificações descarregar pdf (Página 20)

Chapter 2. Related Work/Fundamentals 13

connected until the memory is cleared by a manual reset. If the memory was full, connection will

fail.

Data is transmitted via 433.92 MHz using OOK modulation. Connections are unidirectional, which

means that participants can either send or receive. As a result, transmitters are not aware of each

other and the receiver cannot acknowledge received messages. This lowers costs, because radio

modules are simpler, but it also leads to increased unreliability. Therefore, products of the EU-

series send their data up to seven times, whereof at least one must be received. Data transmission

is not encrypted; just a few devices of the UK-series use rolling code to encrypt their data.

However, these encrypted messaged need special receivers and cannot be read by other devices.

Protocol As already mentioned, there are diﬀerent protocol types. Since they are all proprietary,

diﬀerent data streams were recorded with a logic analyser and decoded manually. Figure 2.5 shows

three diﬀerent protocols, which were sent by a HE853 USB dongle.

Figure 2.5.: Three diﬀerent HE protocol types. The short bit at the start is a sync pulse, which

slightly increases the sensitivity of receivers.

a) encrypted UK-protocol with 29 bit

b) UK-simple protocol with 25 bit

c) Snippet of the 58 bit long EU-protocol

Each Bit is represented by one transition with two diﬀerent timings for each level. Figure 2.6

shows the bit timings.

Every message is 58 bit long and contains a unique address as well as a state code. Figure 2.7

depicts a typical message from an on/oﬀ-switch. The address code consists of a ﬁxed part

and

This part stayed the same for all combinations of device and group codes. It is not certain that this part is really

ﬁxed, but the author could not prove the opposite.

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RFXCOM RFXtrx433 Especificações Página 20