Computing stuff tied to the physical world


Reducing the payload size

In Software on Nov 21, 2012 at 00:01

The OpenTherm relay sketch presented yesterday sends out 9-byte packets containing the raw ASCII text received from the gateway PIC. That’s a bit wasteful of bandwidth, so let’s reduce that to a 3-byte payload instead. Here is some extra code which does just that:

Screen Shot 2012 11 11 at 12 37 33

I’m using a very hacky way to convert hex to binary, and it doesn’t even check for anything. This should be ok, because the packet has already been verified to be of a certain kind:

  • marked as coming from either the thermostat or the heater
  • the packet type is either Write-Data or Read-Ack
  • every other type of incoming packet will be ignored

Note the shouldSend() implementation “stub”, to be filled in later to send fewer packets.

Here are the results of this change, as received by the central node: 

L 11:29:26.651 usb-A40117UK OK 14 16 18 0
L 11:29:27.547 usb-A40117UK OK 14 20 236 30
L 11:29:28.635 usb-A40117UK OK 14 28 45 0
L 11:29:29.642 usb-A40117UK OK 14 0 2 0
L 11:29:30.650 usb-A40117UK OK 14 25 46 0
L 11:29:31.562 usb-A40117UK OK 14 1 10 0
L 11:29:31.658 usb-A40117UK OK 14 1 10 0
L 11:29:34.649 usb-A40117UK OK 14 25 46 0

Much better, although still a lot of duplication and far too many packets to send.

I’ll fix this tomorrow, in the final version of this otRelay.ino sketch.