Parsing LLAP packets using NodeRED on the Raspberry Pi

It’s taken a few weeks but here is the next instalment in my promised series of articles. Part of the scenario requires me to detect a particular scenario using a remote sensor and relay it on to a variety of other systems via my Raspberry Pi. The sensor itself is assembled using the rather brilliant Ciseco Wireless Inventor’s Kit which provides all you need for prototyping remote sensors and transmitting data over a short range radio link back to a waiting Pi.

It’s a pretty simple yet powerful set of kit. You build your sensor using the breadboard and XinoRF module supplied which communicates with a Slice of Radio transceiver module you attach to the Pi’s GPIO pins. Your applications then send and receive data to and from the remote sensor via the Pi’s serial port and you’re away. The protocol used for the serial communications is designed by Ciseco and is called Lightweight Logical Application Protocol (LLAP). It takes the form of a simple fixed-field text datagram and as such can be parsed by pretty well any programming language that can interrogate the serial port.

In my case I’m using NodeRED to parse the LLAP packets so I can invoke other services in response to an event triggered via a sensor. The event is detected by a switch (a pressure mat in fact) that sets the D03 pin on the XinoRF module to be “high”. The nice thing about how the XinoRF works is that it operates in an event-driven way — i.e. you don’t need to poll to check the state of the pin, it transmits when there is a change in the state from “low” to “high” or vice versa. The other XinoRF pins perform different functions in different ways, but I thought I’d show how to at least send and receive so others can tailor to their scenario.

I’m assuming here that folks are familiar with the NodeRED concepts, so will get straight into the meat of how it works.

Configuring A serial input node to Receive llap packets into a flow

The first thing to do is configure the serial input node in NodeRED. The settings for my node are shown below.

Screen Shot 2014-10-14 at 17.56.37

You can confirm the serial port name using the RasWIK documentation and tools. A particular point to note is the Split input settings which determines at which point the input node offers up data to the next node in the flow. This can be set to a delimiter, or set to a timeout, wherein if no data is received within a certain period, the transmission is considered complete. I discovered that the latter timeout approach worked with a setting of 250 milliseconds, though it does introduce the possibility of multiple LLAP packets being received in one go, which we now need to address within the next node in our flow.

Creating a function node to segment multiple llap packets into individual messages

For greatest efficiency want all subsequent nodes in the flow to be designed to ingest a single LLAP packet in a single message. To ensure this downstream behaviour is honoured we create a function node after our serial input node to check for multiple packets and segment them into individual messages if so. The way NodeRED is designed to work, we can return an array of messages and it will cycle through each message in turn, invoking the remainder of the flow as if triggered by a single message.

The first thing we do in our function node is test for the simple case of a single packet which we can test by the length of the payload, LLAP packets being 12 characters in length.

var instring = msg.payload;
if (instring.length<=12) {
   console.log("Single packet: "+instring);
   return msg;

If the length of the payload is longer, it means we have multiple packets so we need to iterate over the payload, extracting each packet into an array of new messages to return at the end of the function.

else {
   var packets = [];
   console.log("Multiple packets: "+instring);
   while (instring.length>=12) {
      var packet = instring.substr(0, 12);
      console.log("Unspliced packet: "+packet);
      packets[packets.length] = {payload:packet};
      instring = instring.substr(12);
      console.log("instring is now length "+instring.length);
   return [packets]; 
return null;

An important point to note (highlighted above) is that although we have created an array of payload objects, NodeRED requires that we in turn encase that array in another array when we return it for processing downstream.

Parsing the LLAP packet

In our subsequent function node, we can now parse the single LLAP packet to determine whether we have a “high” event for the D03 pin we’re interested in. The format of an LLAP packet in this case would be:


Our code simply needs to check the packet that is received, and parse it see if it corresponds to the above event. Note that in a simple scenario we could just do a direct string comparison between what we’ve received and a template packet such as that above, but if we have multiple stations (denoted by characters 1 and 2, “–” in the example) then we might have more complex flow control logic. My function node simply emits the word “HIGH” if we’ve got a D03 “high” event from the XinoRF.

var thePayload = msg.payload;
var pin = thePayload.substring(3,6);
if (pin == "D03") {
   // Now test if it was HIGH
   if (thePayload.substring(6,10) == "HIGH") {
      console.log("HIGH on D03");

Now in my scenario, the switch (pressure mat) could toggle on and off in quick succession which would technically send a number of LLAP packets back to the Pi. Within a certain time window, I’m only interested in a single “high” event, so have added some logic using NodeRED’s context to test when I last received a “high” event and only taking action if it was more than a minute ago.

      var timeNow = new Date();
      console.log("timeNow is "+timeNow);
      if (context.lastHigh) {
         console.log("context.lastHigh is "+context.lastHigh);
         var lastTimeMillis = context.lastHigh.getTime();
         var currentTimeMillis = timeNow.getTime();
         var delta = currentTimeMillis - lastTimeMillis;
         if (delta < 60000) { 
            console.log("Less than a minute since last HIGH: "+
               delta+" ms.");
            return null;
         console.log("Over a minute, actually "+delta+" ms.");
      } else {
      context.lastHigh = timeNow;
      return {payload: "HIGH"}
return null;

So, if we get a “high” event and we haven’t seen one for at least a minute, the subsequent nodes in the flow will receive a packet with a payload of “HIGH”.

And with that we’re free to continue our processing, which could be as simple as a debug node. Here’s a snapshot of my NodeRED workspace — in this case I’m using the “high” event to trigger a Tweet.

Screen Shot 2014-10-14 at 18.33.17




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