Node-RED decoding function for Catena 4450 data record port 1, type 0x14

decode-0x14.js

// this Node-RED decoding function decodes the record sent by the Catena 4450 M101 power monitor
// written in a big hurry, so no points for style

var b = msg.payload;  // pick up data for convenience; just saves typing.

// an empty table to which we'll add result fields:
//
// result.vBat: the battery voltage (if present)
// result.vBus: the USB charger voltage (if provided)
// result.boot: the system boot counter, modulo 256
// result.t: temperature in degrees C
// result.p: station pressure in hPa (millibars). Note that this is not
//   adjusted for the height above sealevel so can't be directly compared
//   to weather.gov "barometric pressure"
// result.rh: relative humidity (in %)
// result.lux: light level, in lux
// result.powerUsed: pulses from the WH pulse meter (consumption)
// result.powerRev: pulese from the WH pulse meter (sourced to the grid)
// result.powerUsedDeriv: derivative of total power used over the last 
//    sample period, normalised to kWh / hour (in other words, kW).
// result.powerRevDeriv: derivative of total power sourced over the 
//    last sample period, normalized to kWh/hour.
var result = { };

// check the message type byte
if (b[0] != 0x14)
    {
    // not one of ours: report an error, return without a value,
    // so that Node-RED doesn't propagate the message any further.
    node.error("not ours! " + b[0].toString());
    return;
    }

// i is used as the index into the message. Start with the flag byte.
var i = 1;
// fetch the bitmap.
var flags = b[i++];

if (flags & 0x1)
    {
    // set vRaw to a uint16, and increment pointer
    var vRaw = (b[i] << 8) + b[i+1];
    i += 2;
    // interpret uint16 as an int16 instead.
    if (vRaw & 0x8000)
        vRaw += -0x10000;
    // scale and save in result.
    result.vBat = vRaw / 4096.0;
    }

if (flags & 0x2)
    {
    var vRaw = (b[i] << 8) + b[i+1];
    i += 2;
    if (vRaw & 0x8000)
        vRaw += -0x10000;
    result.vBus = vRaw / 4096.0;
    }

if (flags & 0x4)
    {
    var iBoot = b[i];
    i += 1;
    result.boot = iBoot;
    }

if (flags & 0x8)
    {
    // we have temp, pressure, RH
    var tRaw = (b[i] << 8) + b[i+1];
    if (tRaw & 0x8000)
        tRaw = -0x10000 + tRaw;
    i += 2;
    var pRaw = (b[i] << 8) + b[i+1];
    i += 2;
    var hRaw = b[i++];
    
    result.t = tRaw / 256;
    result.p = pRaw * 4 / 100.0;
    result.rh = hRaw / 256 * 100;
    }

if (flags & 0x10)
    {
    // we have lux
    var luxRaw = (b[i] << 8) + b[i+1];
    i += 2;
    result.lux = luxRaw;
    }


if (flags & 0x20)   // watthour
    {
    var powerIn = (b[i] << 8) + b[i+1];
    i += 2;
    var powerOut = (b[i] << 8) + b[i+1];
    i += 2;
    result.powerUsed = powerIn;
    result.powerRev = powerOut;
    }
    
if (flags & 0x40)  // normalize floating pulses per hour
    {
    var floatIn = (b[i] << 8) + b[i+1];
    i += 2;
    var floatOut = (b[i] << 8) + b[i+1];
    i += 2;
    
    var exp1 = floatIn >> 12;
    var exp2 = floatOut >> 12;
    var mant1 = (floatIn & 0xFFF) / 4096.0;
    var mant2 = (floatOut & 0xFFF) / 4096.0;
    var powerPerHourIn = mant1 * Math.pow(2, exp1 - 15) * 60 * 60 * 4;
    var powerPerHourOut = mant2 * Math.pow(2, exp2 - 15) * 60 * 60 * 4;
    result.powerUsedDeriv = powerPerHourIn;
    result.powerRevDeriv = powerPerHourOut;
    }

msg.payload = result;
msg.local = 
    {
    nodeType: "Catena 4450-M101",
    platformType: "Feather M0 LoRa",
    radioType: "RF95",
    applicationName: "AC Power Monitoring"
    };

return msg;

format.md

Understanding Catena 4450 format 0x14

Catena 4450 format 0x14 messages are always sent on LoRaWAN port 1, and have the following layout.

byte	description
0	Format code (always 0x14, decimal 20).
1	bitmap encoding the fields that follow
2..n	data bytes; use bitmap to decode.

Bitmap fields and associated fields

The bitmap byte has the following interpretation. int16, uint16, etc. are defined after the table.

bit	length of field	encoding	description
0	2	int16	battery voltage; divide by 4096 to convert from counts to volts.
1	2	int16	bus voltage; divide by 4096 to convert from counts to volts. Note: this field is currently never transmitted.
2	1	uint8	counter of number of recorded system reboots, modulo 256.
3	5	int16, uint16, uint8	Temperature (divide by 256 to get degrees C); pressure (divide by 25 to get millibars); relative humidity (divide by 2.56 to get percent).
4	2	uint16	Lux, from 0 to 65536.
5	4	uint16, uint16	Watt-hour pulses since reset, modulo 65,536. First value is for power consumed from the grid, second for power sourced to the grid.
6	4	uflt16, uflt16	Instantaneous power over this measurement period. First value is for power consumed, second is for power produced.
7	n/a	n/a	reserved, must always be zero.

Data Formats

All multi-byte data is transmitted with the most significant byte first (big-endian format). Comments on the individual formats follow.

uint16

an integer from 0 to 65536.

int16

a signed integer from -32,768 to 32,767, in two's complement form. (Thus 0..0x7FFF represent 0 to 32,767; 0x8000 to 0xFFFF represent -32,768 to -1).

uint8

an integer from 0 to 255.

uflt16

A unsigned floating point number, transmitted as a 16-bit number, with the following interpretation:

bits	description
15..12	binary exponent b
11..0	fraction f

The floating point number is computed by computing f/4096 * 2^(b-15). Note that this format is deliberately not IEEE-compliant; it's intended to be easy to decode by hand and not overwhelmingly sophisticated.

For example, if the transmitted message contains 0x1A, 0xAB, the equivalent floating point number is found as follows.

1. The full 16-bit number is 0x1AAB.
2. b is therefore 0x1, and b-15 is -14. 2^-14 is 1/32768
3. f is 0xAAB. 0xAAB/4096 is 0.667
4. f * 2^(b-15) is therefore 0.6667/32768 or 0.0000204

Floating point mavens will immediately recognize:

• There is no sign bit; all numbers are positive.
• Numbers do not need to be normalized (although in practice they always are).
• The format is somewhat wasteful (it is common to assume all numbers are normalized, and then suppress the MSB of f -- in other words, insist that the actual value of f be in the range 2048.. 4095, and then transmit only f - 2048, saving a bit. This bit is then reused as the sign bit.
• Gradual underflow at the bottom of the range is, on the other hand, automatic with this encoding; the more sophisticated schemes need extra logic (and extra testing) in order to provide the same feature.