# Checksum UDP ? read value from HVAC controller
**URL:** https://discourse.nodered.org/t/checksum-udp-read-value-from-hvac-controller/35040
**Category:** General
**Created:** [29 October 2020 20:41 UTC](https://discourse.nodered.org/t/checksum-udp-read-value-from-hvac-controller/35040 "2020-10-29T20:41:44Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@rogier](https://discourse.nodered.org/u/rogier)
#### Post date: [29 October 2020 20:41 UTC](https://discourse.nodered.org/t/checksum-udp-read-value-from-hvac-controller/35040/1 "2020-10-29T20:41:44Z")
Hello,
i have a HVAC controller. want to read values via a UDP port. In the program software of the controller i can monitor a input value and grap with Wireshark the HEX send to the controller. to do this is a lot of work when you want to read a lot of values.. i like to auto create a working string.
this is the string a grap:
0251fc209a000002b5095d55
what i know is the first 2 is the adres of the controller. the point i want to read is 2b509
here is another HEX grap of the same value:
0251fc289a000002b509cef8
is it possible to create an HEX string if you know the adres (02) and the point (2b509)??
---
### Author:  [@Steve-Mcl](https://discourse.nodered.org/u/Steve-Mcl)
#### Post date: [30 October 2020 23:32 UTC](https://discourse.nodered.org/t/checksum-udp-read-value-from-hvac-controller/35040/2 "2020-10-30T23:32:15Z")
hi, I doubt `2b509` is the full value as it is an odd number of bytes, starting at offset of 7.5th byte in the data `0251fc289a000002b509cef8`
It is more likely to be the 7th + 8th bytes (16 bit) e.g. 02b5 (as is circled in your picture)
Anyhow, that's the finer details - for starters, I will show you how you can pick & select any value you want using the [buffer parser](https://flows.nodered.org/node/node-red-contrib-buffer-parser) node...
Now i realise that ^ value is potentially not what you expect, but without a specification / manual for your HVAC (e.g. byte swapping?, int?, uint? etc) - I randomly chose 16 bit UINT (big endian)
**example flow...**
```auto
[{"id":"58bdc980.c562e8","type":"inject","z":"c9ca6ac7.25b568","name":"0251fc209a000002b5095d55","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"0251fc289a000002b509cef8","payloadType":"str","x":1220,"y":100,"wires":[["7544a660.fcb0e8"]]},{"id":"816105c6.6e6208","type":"inject","z":"c9ca6ac7.25b568","name":"0251fc289a000002b509cef8","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"0251fc289a000002b509cef8","payloadType":"str","x":1220,"y":140,"wires":[["7544a660.fcb0e8"]]},{"id":"7544a660.fcb0e8","type":"buffer-parser","z":"c9ca6ac7.25b568","name":"","data":"payload","dataType":"msg","specification":"spec","specificationType":"ui","items":[{"type":"int8","name":"address","offset":0,"length":1,"offsetbit":0,"scale":1,"mask":""},{"type":"uint16be","name":"data","offset":7,"length":1,"offsetbit":0,"scale":1,"mask":""}],"swap1":"","swap2":"","swap3":"","swap1Type":"swap","swap2Type":"swap","swap3Type":"swap","msgProperty":"payload","msgPropertyType":"str","resultType":"keyvalue","resultTypeType":"output","multipleResult":false,"setTopic":true,"x":1450,"y":120,"wires":[["309f9087.49213"]]},{"id":"309f9087.49213","type":"debug","z":"c9ca6ac7.25b568","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":1470,"y":160,"wires":[]}]
```
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### Author:  [@system](https://discourse.nodered.org/u/system)
#### Post date: [29 December 2020 23:32 UTC](https://discourse.nodered.org/t/checksum-udp-read-value-from-hvac-controller/35040/3 "2020-12-29T23:32:16Z")
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