Radiometrix Ltd

KDEC

page 4

Function S0 =1

Message repeated....

75ms: end of message

120ms: function activated

Start of new message

S0 Output: Relay 1

RXD

Code-hopped messages – technical brief

Transmission format is 1250bps (nominal rate) Manchester encoding. Although the function-code is only 4

bits, total message length is 67 bits. This includes a 28-bit serial number and a 32-bit encrypted hop-code.

Note that the serial number and function code data is not encrypted. The correct status of the transmission is

used to validate the data. This is achieved through use of the ever changing hop-code (changes for every

new message) and by synchronising the KDEC to the transmitter(s) in use. Put another way: KDEC “Learns”

individual unique transmitters, as required by the user.

The hop-code is encrypted using the serial number of the transmitter plus the manufacturer code (64- bit),

which is kept secret and is not transmitted. The function-code (S0-S3) appears twice in the message and

forms a part of the hop-code encryption process.

If KDEC loses synchronisation with the transmitter (e.g. repeated transmitter activation whilst out of range)

then the transmitter must be activated twice with valid and sequential hop-codes before KDEC will allow the

control to be processed. The hop-code is only valid if it is within a forward window of +32768 activations. This

mechanism prevents “code-grabbers” from breaking the security of the system by recording and re-

broadcasting old codes. Additionally, KDEC allows up to 14 transmissions to occur whilst the transmitter is out

of range before two valid sequential transmissions are required to use the system. In practice a user may not

notice the latter, since human reaction to the unsuccessful attempt is simply to “press the button again”.

The level of redundancy and encryption in a message mean that, in simple terms, KDEC will only output a

function-code that has been received from a properly recognised and synchronised transmitter.

Figure 3: Timing diagram – showing action upon receiving a message with S0 = 1

The typical time delay between a “button-press” at the transmitter and relay activation by the KDEC board is

140ms.

Ordering Information

KDEC-xxx-000

Where “xxx” is frequency band in MHz – 433 is standard.

Please contact sales for other requirements.

Note

:

Whilst the KDEC will operate with both KFX and KTX transmitters regardless of Jumper settings, it is

recommended that Jumper T be placed in position A (or not fitted at all) when KDEC is used with 4

and 5 button Key-fobs. For these key-fobs, a combined function-code (S0, S2) is used as the code

for one of the buttons.