# How to work out what CRC calculation to use?
**URL:** https://discourse.nodered.org/t/how-to-work-out-what-crc-calculation-to-use/50913
**Category:** General
**Created:** [12 September 2021 07:44 UTC](https://discourse.nodered.org/t/how-to-work-out-what-crc-calculation-to-use/50913 "2021-09-12T07:44:49Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@craigcurtin](https://discourse.nodered.org/u/craigcurtin)
#### Post date: [12 September 2021 07:44 UTC](https://discourse.nodered.org/t/how-to-work-out-what-crc-calculation-to-use/50913/1 "2021-09-12T07:44:49Z")
Guys,
I have an undocumented energy meter that i am reading from - i have snooped on the protocol and found information online about similar units so can make some assumptions about the data involved.
However i am wondering how - given the current data i work out what CRC algorithm they are using for me to inject my own data ?
For instance this is the request my Inverter makes to the energy meter every 200ms
03 03 00 61 00 17 55 F8
03 - ID of slave device
03 - Function code
00 61 - Start register
00 17 - number of registers (23)
55 F8 - CRC
How do i work out what Algo it is using to calculate the CRC ?
regards
Craig
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### Author:  [@craigcurtin](https://discourse.nodered.org/u/craigcurtin)
#### Post date: [12 September 2021 07:51 UTC](https://discourse.nodered.org/t/how-to-work-out-what-crc-calculation-to-use/50913/2 "2021-09-12T07:51:08Z")
I found this forum thread and it appears to work with each of my requests so i assume now this is using standard Modbus CRC16
> [@How to calculate CRC-16 (Modbus)?](https://discourse.nodered.org/t/how-to-calculate-crc-16-modbus/48955):
>
> Hi everyone, I am trying to calculate CRC with a function I got from Node-RED forum, but when there is a 0 at the end or at the beginning of the CRC, it does not identify it, and I just get three bytes. The fact is that I do not want to download any node, only a function for the CRC-16 (Modbus) like a frame like this: 010600000001 + CRC Could anyone help me please? Thank you very much, Q.
Craig
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### Author:  [@system](https://discourse.nodered.org/u/system)
#### Post date: [26 September 2021 07:51 UTC](https://discourse.nodered.org/t/how-to-work-out-what-crc-calculation-to-use/50913/3 "2021-09-26T07:51:55Z")
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