Hi Steve,
Further to the above, I took the KNX nodes entirely out of the picture to try and narrow the problem down. I simply created an HTTP in, connected to your function node, set two topics and then joined them.
In the example below, if I manually inject a payload, the flow works and I get a combined key/value object out of the join node. If I call the http in node absolutely nothing happens... there isn't even an output from the HTTP in node with the res/req where normally a HTTP in would output that with nothing connected. If I break the flow to the join node, then initiating the flow from the HTTP in works (up to the join node where it's broken, obviously).
Assuming there's nothing wrong with my flow, I am beginning to lean towards the HTTP in and the join node not playing nicely.... to the extent that the HTTP in node isn't doing anything if there's a join in the flow (admittedly this doesn't answer how we got some KNX readings before but goes some way to narrowing things down).
[{"id":"b51e5ba0.4e885","type":"tab","label":"Flow 1","disabled":false,"info":""},{"id":"10d85f99.17fa48","type":"http response","z":"b51e5ba0.4e885","name":"","statusCode":"","headers":{},"x":1130,"y":320,"wires":[]},{"id":"34799786.571688","type":"join","z":"b51e5ba0.4e885","name":"","mode":"custom","build":"object","property":"","propertyType":"full","key":"topic","joiner":"\\n","joinerType":"str","accumulate":false,"timeout":"","count":"2","reduceRight":false,"reduceExp":"","reduceInit":"","reduceInitType":"num","reduceFixup":"","x":930,"y":320,"wires":[["10d85f99.17fa48","584621b0.a4bc6"]]},{"id":"70be2264.90bddc","type":"change","z":"b51e5ba0.4e885","name":"","rules":[{"t":"set","p":"topic","pt":"msg","to":"http","tot":"str"}],"action":"","property":"","from":"","to":"","reg":false,"x":710,"y":360,"wires":[["fa698b34.2d46a8","34799786.571688"]]},{"id":"fa698b34.2d46a8","type":"debug","z":"b51e5ba0.4e885","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","x":850,"y":420,"wires":[]},{"id":"584621b0.a4bc6","type":"debug","z":"b51e5ba0.4e885","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","x":1050,"y":260,"wires":[]},{"id":"f45c8ae8.d3d5a8","type":"debug","z":"b51e5ba0.4e885","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","x":470,"y":240,"wires":[]},{"id":"b107986b.a54cb8","type":"inject","z":"b51e5ba0.4e885","name":"Mode check","topic":"","payload":"true","payloadType":"bool","repeat":"","crontab":"","once":false,"onceDelay":"55","x":330,"y":300,"wires":[["4d3c6427.5b2e24","f45c8ae8.d3d5a8"]]},{"id":"4d3c6427.5b2e24","type":"function","z":"b51e5ba0.4e885","name":"fan out","func":"msg.topic = \"http\" //dont forget to set the topic on the http msg (for the join node)\nreturn [{\"payload\":true},msg];//return 2 messages (http msg goes out of output 2)","outputs":2,"noerr":0,"x":540,"y":320,"wires":[["645dc2a5.e7ef9c"],["70be2264.90bddc"]]},{"id":"c62a9c51.edf588","type":"debug","z":"b51e5ba0.4e885","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","x":470,"y":420,"wires":[]},{"id":"645dc2a5.e7ef9c","type":"change","z":"b51e5ba0.4e885","name":"","rules":[{"t":"set","p":"topic","pt":"msg","to":"blue","tot":"str"}],"action":"","property":"","from":"","to":"","reg":false,"x":710,"y":280,"wires":[["c5cc6813.4fca4","34799786.571688"]]},{"id":"c5cc6813.4fca4","type":"debug","z":"b51e5ba0.4e885","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","x":870,"y":240,"wires":[]},{"id":"415aa5e2.1e25cc","type":"http in","z":"b51e5ba0.4e885","name":"","url":"test","method":"get","upload":false,"swaggerDoc":"","x":340,"y":340,"wires":[["4d3c6427.5b2e24","c62a9c51.edf588"]]}]
