knx, docker: negative temperatures are not sent correctly to knx bus

[mivola]

Description

• the value of a temparature (e.g. OutsideTemp or broadcast/datetime/outsidetemp) on the knx bus differs from the value sent via MQTT or fetched via ebusctl if the value is negative

• see:

  ebusctl read -c ehp OutsideTemp
  -1.62;ok

  

• but on the knx bus the value is different (by factor 10?) -- see on the wiregate log: 2023-11-29 09:42:58.702,A_GroupValue_Write,15.15.22,9/2/10,80 D4,-18.36,,9,0,low,5,T_DATA_XXX_REQ,0 -- also in NodeRed:

• the incorrect value is also nicely visibly in a graph when compared to other (not ebusd based) sensors:

• the ebusd is started in docker with these env vars:

  
  -e EBUSD_SCANCONFIG= \
  -e EBUSD_DEVICE=/dev/ttyUSB0 \
  -e EBUSD_VERSION=23.2 \
  -e EBUSD_MQTTPORT=1883 \
  -e EBUSD_MQTTHOST=mediaserver \
  -e EBUSD_MQTTVAR="filter-direction=r|u|^w" \
  -e EBUSD_MQTTINT=/etc/ebusd/mqtt-hassio.cfg \
  -e EBUSD_MQTTJSON= \
  -e EBUSD_KNXURL= \
  -e EBUSD_KNXINT=/etc/ebusd/knx.cfg \
  -e EBUSD_LOG=other:debug \

### Actual behavior

- incorrect temparature values when the temperature is below zero

### Expected behavior

- the value should be correct even if negative

### ebusd version

23.2

### ebusd arguments

see description

### Operating system

other

### CPU architecture

x64

### Dockerized

latest

### Hardware interface

other

### Related integration

TCP (cmdline client like ebusctl or netcat), MQTT generic, KNX

### Logs

I couldnt find any log that shows the incorrect value that is sent out to the KNX bus

[c-huetter]

What a relief, I have exactly the same issue :-)

---

Description

Negative temperature values are sent incorrectly to KNX. Positive temperature values are sent correctly.

Actual behavior

ebusd sends telegrams to knxd with incorrect values for negative temperatures:

time: 2023-12-04T09:02:20Z
value: -19.78
description: OutsideAirTemperature
dpt: 9.001
dptdesc: Temperature
event: GroupValue_Write

Expected behavior

The actual air temperature was -0.7 °C

# ebusctl read -V outsideairtemperature                        
kwl OutsideAirTemperature data=-0.7 °C [OutsideTemperature]

ebusd version

23.2

ebusd arguments

-c /etc/ebusd -d ens:/dev/ttyUSB0 --knxurl=ip:localhost --knxint=/etc/ebusd/knx.cfg

Operating system

Synology DSM Linux 4.4.302+

CPU architecture

arm64

Dockerized

same as ebusd version

Hardware interface

adapter 3.0 USB

Related integration

KNX

Logs

2023-12-04 09:02:14.139 [update notice] sent poll-read kwl OutsideAirTemperature QQ=31: -0.7

[c-huetter]

Hi everyone,

I have analyzed the issue reported by @mivola that negative temperatures are not sent correctly to KNX. The functions floatToInt16 and int16ToFloat that translate 2-byte floats into KNX data type 9 (and vice versa) seem to be buggy.

I have fixed the bugs in the translation functions and added a unit test for knxhandler in my fork ebusd-with-knxd. Please verify the bugfix and consider merging my pull request #1051.

Best, Christian

[john30]

I can't reproduce this. the values are converted correctly, only in edge cases there might be a rounding issue, but this is not related to this. please post the hex value, the field definition and what was sent to knx for a wrong example

[c-huetter]

I've used the following test values from the repository knx-datapoints:

Hex	Float
0x0000	0.0
0x07FF	20.47
0x6464	46039.04
0x7FFF	670760.96
0x87FF	-0.01
0x8000	-20.48
0x8A24	-30.0
0xAC00	-327.68
0xC8C8	-9461.76
0xF800	-671088.64

Please check out the test cases in test_knxhandler.cpp to reproduce the bug. It would be easier to test the functions floatToInt16 and int16ToFloat if they were static.

[john30]

what are the values that fail from this list with the current implementation?

[c-huetter]

The test case test_knxhandler.cpp returns the following values when using the original (probably buggy) functions floatToInt16 and int16ToFloat:

Correct: 0x0 == 0.00
Correct: 0x7ff == 20.47
Correct: 0x6464 == 46039.04
Incorrect: 0x7fff != 4294967296.00, should be 670760.94
Incorrect: 0x87ff != -20.47, should be -0.01
Incorrect: 0x8000 != -0.00, should be -20.48
Incorrect: 0x8a24 != -10.96, should be -30.00
Correct: 0xac00 == -327.68
Incorrect: 0xc8c8 != -1024.00, should be -9461.76
Incorrect: 0xf800 != -0.00, should be -671088.62
Correct: 0.00 == 0x0
Incorrect: 20.47 != 0x7fe, should be 0x7ff
Correct: 46039.04 == 0x6464
Incorrect: 670760.94 != 0x7ffe, should be 0x7fff
Incorrect: -0.01 != 0x8001, should be 0x87ff
Incorrect: -20.48 != 0x8c00, should be 0x8000
Incorrect: -30.00 != 0x8ddc, should be 0x8a24
Incorrect: -9461.76 != 0xcf38, should be 0xc8c8
Incorrect: -671088.62 != 0x7fff, should be 0xf800