Hi, welcome to the forum.
I am unfamiliar with ACC64 but a quick search reveals it is a number format SunSpec uses - are you using SunSpec gear?
According to a Sunspec PDF, ACC64 is essentially a smaller uint64
with a range of 0 ... 9223372036854775807
9223372036854775807
is bigger than JavaScripts Number.MAX_SAFE_INTEGER so you likely need to use BigInt value (which is a true 64bit number)
Can you copy the output of the Modbus node and paste it in a reply. Also, tell us the value you expect the 4 UINT16 modbus values to be once converted.
e.g.
Modbus node debug capture:
{ payload: [0,0,16,55432], xxx: {], yyy: {}, zzz:{} }
Expected value:
203685477
How to grab data from a node
There’s a great page in the docs (Working with messages : Node-RED) that will explain how to use the debug panel to find the right path/value for any data item.
Pay particular attention to the part about the buttons that appear under your mouse pointer when you over hover a debug message property in the sidebar.
How to post data values
In order to make code readable and usable it is necessary to surround your code with three backticks (also known as a left quote or backquote ```
)
```
code goes here
```
You can edit and correct your post by clicking the pencil icon.
See this post for more details - How to share code or flow json