Radiometrix Ltd

KRX2

page 4

Activating outputs

Once a transmitter has been learned its transmissions will be acted upon, i.e. the function code

contained within the transmission is fed to the KRX2 outputs S0 – S3. Either a 10ms single-shot

output or a longer momentary output can be used according to the MODE selected. In the circuit shown

in Fig. 3, closing the switch will select “momentary” mode.

In 10ms “single-shot” mode the ouput(s) are activated for only 10ms (nominal) per message received.

This means that regardless of whether someone “holds down the button” at the sending device, the

KRX2 output will only be a single 10ms pulse. However, if the function code is added to by the sender

(e.g. another function code button is pressed in addition to the first) then both the original output and

the new output will provide a 10ms pulse. This is because a new function code = new message.

In Momentary mode the output is activated for a short period (approx. 500ms) for reception of a single

message. If the same message is received repeatedly the output status is refreshed during this period:

in effect, the output is “held-on” whilst a user keeps the relevant function line enabled at the

transmitter.

If the message was received from a transmitter being supplied less than approx. 3.8V the Low battery

output (LED “B” in the circuit shown in Fig. 3) will be activated.

Clearing the memory

When the memory is full, no new transmitters can be learned. To clear the memory of all stored

transmitters, the Learn button is pressed and held for more than approx. 5s. The Learn LED will go

out and upon releasing the button it will flash once to indicate that the memory has been cleared. It is

not possible to remove individual transmitters from the memory.

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 KRX2 to the transmitter(s) in use. Put

another way: KRX2 “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 KRX2 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 KRX2

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, KRX2 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, KRX2 will only

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