Handlers, Erlang, and hex strings...

[co...@perceptivethings.com]

I've been having a difficult time getting my Multitech mDot's data to parse in a handler.  I get consistent parse_failed errors and I don't understand why.  Specifically, the payload is 18 bytes, but the values are really 9 bytes, with each ASCII character being represented with two bytes as a hexadecimal value (i.e. 0x31 = 1, 0x32 = 2, etc).  I think this is a "hex string" in Erlang?  When converted to ASCII characters, first byte is a boolean ("1" or "0"), the next four bytes are a float temperature reading, and the last four are a float battery voltage reading.  As I mentioned, the values are sent as hexadecimal ASCII characters represented.  For example, a payload of "3132352E31332E3334" is...

3132352E31332E3334 --> 31 34 35 2E 31 33 2E 33 34 --> 125.13.34 --> 1, 25.1, 3.34

so my resulting object would look like:

{sensor: 1, temp: 25.1,  batt: 3.34}

I've tried all sorts of ways to parse the data, and the most promising in my mind is this function:

fun(Fields, <<Data/binary>>) ->
Fields#{sensor => binary_part(Data, {0,1}), temp => binary_part(Data, {2,4}), batt => binary_part(Data, {8,4})}
end.

This should just pull the ASCII characters out of the binary based on position...like a substring.  Unfortunately, I only get "badarg" errors from this function, but I'm at a loss as to why. This may not be the ideal way of doing it, but my other attempt based on the documentation always fails with a "function_clause" parse error:

fun(Fields, <<SENSOR, Temp:4/binary, Voltage:4/binary>>) ->
Fields#{sensor => SENSOR, temp => Temp, batt => Voltage}
end.

Any help would be appreciated!

[Petr Gotthard]

First of all, are you using the release 0.4.x or 0.5? The documentation under ‘master’ branch is for 0.5; if you have 0.4.x you need to switch to ‘stable’.

 

If your data is as you say, then <<Sensor:1/binary, Temp:4/binary, Voltage:4/binary>> shall work. The difference to your last function is that “Sensor” is a binary too.

 

 

Petr

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[co...@perceptivethings.com]

We're testing 0.5, but your solution worked!  Changing the first field to "Sensor:1/binary" seems to have done the trick. Thanks for your help and amazing work on this project.