# Learning UDP message

**URL:** <https://discourse.nodered.org/t/learning-udp-message/40055>\
**Category:** General\
**Created:** [30 January 2021 16:06 UTC](https://discourse.nodered.org/t/learning-udp-message/40055 "2021-01-30T16:06:35Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![rogier](https://avatars.discourse-cdn.com/v4/letter/r/ecc23a/32.png) [@rogier](https://discourse.nodered.org/u/rogier)\
**Post date:** [30 January 2021 16:06 UTC](https://discourse.nodered.org/t/learning-udp-message/40055/1 "2021-01-30T16:06:35Z")

</div>

Can anyone te me how this works / use it in Node Red

in the 2 picture below you see a snap of wire shark.  
in picture 1 i ask (with company software) 010101 from a DDC device (red understripe).  
in picture 2 i ask (with company software) 010102 from de DDC device (red understripe)

the only thing that change in the 2 requests is de last part of the data (green understripe)  
my question: how does the company software calculate the last part.. and the big question: can node red do the calculation?

 ![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/3/3/33dd281523848e421b8d631826825401ff077ce1.png) ![udp2](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/0/8/089c303da0f401bdba117fc82d5398fdde7cd4d1.png)

---

<div class="post-metadata">

**Author:** ![TotallyInformation](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/totallyinformation/32/31_2.png) [@TotallyInformation](https://discourse.nodered.org/u/TotallyInformation)\
**Post date:** [30 January 2021 16:21 UTC](https://discourse.nodered.org/t/learning-udp-message/40055/2 "2021-01-30T16:21:37Z")

</div>

There is absolutely no way we can help with this. We have no idea what is generating the data nor what it means.

---

<div class="post-metadata">

**Author:** ![rogier](https://avatars.discourse-cdn.com/v4/letter/r/ecc23a/32.png) [@rogier](https://discourse.nodered.org/u/rogier)\
**Post date:** [30 January 2021 16:42 UTC](https://discourse.nodered.org/t/learning-udp-message/40055/3 "2021-01-30T16:42:47Z")

</div>

thank you. I thought it was a reducible (calculated) value. it probably is but not standard in a UDP message ... unfortunate

---

<div class="post-metadata">

**Author:** ![TotallyInformation](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/totallyinformation/32/31_2.png) [@TotallyInformation](https://discourse.nodered.org/u/TotallyInformation)\
**Post date:** [30 January 2021 17:03 UTC](https://discourse.nodered.org/t/learning-udp-message/40055/4 "2021-01-30T17:03:32Z")

</div>

UDP is just a protocol. It does not define the data.

---

<div class="post-metadata">

**Author:** ![dceejay](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/dceejay/32/38_2.png) [@dceejay](https://discourse.nodered.org/u/dceejay)\
**Post date:** [30 January 2021 18:08 UTC](https://discourse.nodered.org/t/learning-udp-message/40055/5 "2021-01-30T18:08:09Z")

</div>

indeed - you can see the UDP checksum a few lines earlier in your dump - nothing to do with the data. But if you use something like [https://crccalc.com/](https://crccalc.com/) in CRC-16 mode and stick in the data part I'm sure you'll find it...

Hint - try CRC-16/XMODEM

---

<div class="post-metadata">

**Author:** ![rogier](https://avatars.discourse-cdn.com/v4/letter/r/ecc23a/32.png) [@rogier](https://discourse.nodered.org/u/rogier)\
**Post date:** [1 February 2021 16:41 UTC](https://discourse.nodered.org/t/learning-udp-message/40055/6 "2021-02-01T16:41:09Z")

</div>

thats it. the CRC-16/XMODEM . hit de string in [https://crccalc.com/](https://crccalc.com/) and the correct value came up! thank you.. now trying in node red...

---

<div class="post-metadata">

**Author:** ![rogier](https://avatars.discourse-cdn.com/v4/letter/r/ecc23a/32.png) [@rogier](https://discourse.nodered.org/u/rogier)\
**Post date:** [2 February 2021 19:15 UTC](https://discourse.nodered.org/t/learning-udp-message/40055/7 "2021-02-02T19:15:12Z")

</div>

ok, now getting it done in node red is my next issue...  
i found a funtion wat does the math. now i neet to paste the crc at the ens of the buffer (10 and 11).

 ![crc](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/9/7/975b88edd69503f7eccb7266ba20dc62f4f8cb3a.png)

```auto
[{"id":"95de803e.d9674","type":"tab","label":"Flow 3","disabled":false,"info":""},{"id":"272ea949.947526","type":"inject","z":"95de803e.d9674","name":"as hex string 745207020315d2","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"0251FA019A00000A6209","payload":"0251FA019A00000A6209","payloadType":"str","x":1190,"y":360,"wires":[["c493e18d.d3b86"]]},{"id":"70ac250.402efdc","type":"debug","z":"95de803e.d9674","name":"","active":true,"tosidebar":true,"console":true,"tostatus":true,"complete":"true","targetType":"full","statusVal":"payload","statusType":"auto","x":1730,"y":360,"wires":[]},{"id":"c493e18d.d3b86","type":"function","z":"95de803e.d9674","name":"calculate CRC","func":"msg.payload = Buffer.from(msg.payload,\"hex\")\n\nconst ccitt_16 = [\n 0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50A5, 0x60C6, 0x70E7,\n 0x8108, 0x9129, 0xA14A, 0xB16B, 0xC18C, 0xD1AD, 0xE1CE, 0xF1EF,\n 0x1231, 0x0210, 0x3273, 0x2252, 0x52B5, 0x4294, 0x72F7, 0x62D6,\n 0x9339, 0x8318, 0xB37B, 0xA35A, 0xD3BD, 0xC39C, 0xF3FF, 0xE3DE,\n 0x2462, 0x3443, 0x0420, 0x1401, 0x64E6, 0x74C7, 0x44A4, 0x5485,\n 0xA56A, 0xB54B, 0x8528, 0x9509, 0xE5EE, 0xF5CF, 0xC5AC, 0xD58D,\n 0x3653, 0x2672, 0x1611, 0x0630, 0x76D7, 0x66F6, 0x5695, 0x46B4,\n 0xB75B, 0xA77A, 0x9719, 0x8738, 0xF7DF, 0xE7FE, 0xD79D, 0xC7BC,\n 0x48C4, 0x58E5, 0x6886, 0x78A7, 0x0840, 0x1861, 0x2802, 0x3823,\n 0xC9CC, 0xD9ED, 0xE98E, 0xF9AF, 0x8948, 0x9969, 0xA90A, 0xB92B,\n 0x5AF5, 0x4AD4, 0x7AB7, 0x6A96, 0x1A71, 0x0A50, 0x3A33, 0x2A12,\n 0xDBFD, 0xCBDC, 0xFBBF, 0xEB9E, 0x9B79, 0x8B58, 0xBB3B, 0xAB1A,\n 0x6CA6, 0x7C87, 0x4CE4, 0x5CC5, 0x2C22, 0x3C03, 0x0C60, 0x1C41,\n 0xEDAE, 0xFD8F, 0xCDEC, 0xDDCD, 0xAD2A, 0xBD0B, 0x8D68, 0x9D49,\n 0x7E97, 0x6EB6, 0x5ED5, 0x4EF4, 0x3E13, 0x2E32, 0x1E51, 0x0E70,\n 0xFF9F, 0xEFBE, 0xDFDD, 0xCFFC, 0xBF1B, 0xAF3A, 0x9F59, 0x8F78,\n 0x9188, 0x81A9, 0xB1CA, 0xA1EB, 0xD10C, 0xC12D, 0xF14E, 0xE16F,\n 0x1080, 0x00A1, 0x30C2, 0x20E3, 0x5004, 0x4025, 0x7046, 0x6067,\n 0x83B9, 0x9398, 0xA3FB, 0xB3DA, 0xC33D, 0xD31C, 0xE37F, 0xF35E,\n 0x02B1, 0x1290, 0x22F3, 0x32D2, 0x4235, 0x5214, 0x6277, 0x7256,\n 0xB5EA, 0xA5CB, 0x95A8, 0x8589, 0xF56E, 0xE54F, 0xD52C, 0xC50D,\n 0x34E2, 0x24C3, 0x14A0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,\n 0xA7DB, 0xB7FA, 0x8799, 0x97B8, 0xE75F, 0xF77E, 0xC71D, 0xD73C,\n 0x26D3, 0x36F2, 0x0691, 0x16B0, 0x6657, 0x7676, 0x4615, 0x5634,\n 0xD94C, 0xC96D, 0xF90E, 0xE92F, 0x99C8, 0x89E9, 0xB98A, 0xA9AB,\n 0x5844, 0x4865, 0x7806, 0x6827, 0x18C0, 0x08E1, 0x3882, 0x28A3,\n 0xCB7D, 0xDB5C, 0xEB3F, 0xFB1E, 0x8BF9, 0x9BD8, 0xABBB, 0xBB9A,\n 0x4A75, 0x5A54, 0x6A37, 0x7A16, 0x0AF1, 0x1AD0, 0x2AB3, 0x3A92,\n 0xFD2E, 0xED0F, 0xDD6C, 0xCD4D, 0xBDAA, 0xAD8B, 0x9DE8, 0x8DC9,\n 0x7C26, 0x6C07, 0x5C64, 0x4C45, 0x3CA2, 0x2C83, 0x1CE0, 0x0CC1,\n 0xEF1F, 0xFF3E, 0xCF5D, 0xDF7C, 0xAF9B, 0xBFBA, 0x8FD9, 0x9FF8,\n 0x6E17, 0x7E36, 0x4E55, 0x5E74, 0x2E93, 0x3EB2, 0x0ED1, 0x1EF0\n];\nfunction Hi(word){\n return (word >> 8) \n}\nfunction Lo(word){\n return (word & 0x00ff) \n}\nfunction CalculateCharacterCRC16(crc, byte){\n let shifted8_int16 = ((crc << 8) & 0xffff);\n let b = (Hi(crc) ^ byte);\n let newCRC = (shifted8_int16 ^ ccitt_16[b]);\n return newCRC;\n}\n\nvar data = msg.payload;\nvar crc = 0;\nfor (let i=0; i < data.length; i++) {\n crc = CalculateCharacterCRC16(crc, data[i]);\n} \nmsg.crc = crc ;\n\nreturn msg;\n\n\n","outputs":1,"noerr":0,"initialize":"","finalize":"","x":1520,"y":360,"wires":[["70ac250.402efdc"]]}]

```

does anyone know how to do this in one function code??

---

<div class="post-metadata">

**Author:** ![system](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/1X/d073cd938eafa2e558d7c2cd59003b3ef4963033.png) [@system](https://discourse.nodered.org/u/system)\
**Post date:** [3 April 2021 19:15 UTC](https://discourse.nodered.org/t/learning-udp-message/40055/8 "2021-04-03T19:15:58Z")

</div>

This topic was automatically closed 60 days after the last reply. New replies are no longer allowed.