Re: Question About Calculating Checksum in a StreamDevice Protocol

["Zimoch, Dirk via Tech-talk" <tech-talk at aps.anl.gov>]

Hello Huishan

What exactly do you mean by "hexadecimal format"? It could mean a string like
"3E"  (which is two bytes, 0x33 and 0x45).
But as you wrote the rest of your command as hexadecimal numbers, I assume you
mean something like 2 bytes for example 1E 2F meaning either the number
0x1e2f=7727 (big endian) or 0x2f1e=12062 (little endian).

In case of big endian you need: out 55 AA 05 04 "%.2r%<sum>";
In case you need little endian: out 55 AA 05 04 "%#.2r%<sum>";

See: https://urldefense.us/v3/__https://paulscherrerinstitute.github.io/StreamDevice/formats.html*raw__;Iw!!G_uCfscf7eWS!aqWJFIEKwy7Jzr44rjp81yiD9Alo8KmGHmLA8RqsPyh-zS1Q_BhTbmSRud6CE_brwNhKTvDqOLqHLdMj4LjBH3pAiQ$ 

The "%<sum>" adds all bytes (starting from the 55 and including the value)
https://urldefense.us/v3/__https://paulscherrerinstitute.github.io/StreamDevice/formats.html*chksum__;Iw!!G_uCfscf7eWS!aqWJFIEKwy7Jzr44rjp81yiD9Alo8KmGHmLA8RqsPyh-zS1Q_BhTbmSRud6CE_brwNhKTvDqOLqHLdMj4LhtL5tDyQ$ 

If you use an 'ao' record, the value will be taken from .RVAL, so you can scale
the value from a floating point number if necessary. 
https://urldefense.us/v3/__https://paulscherrerinstitute.github.io/StreamDevice/ao.html__;!!G_uCfscf7eWS!aqWJFIEKwy7Jzr44rjp81yiD9Alo8KmGHmLA8RqsPyh-zS1Q_BhTbmSRud6CE_brwNhKTvDqOLqHLdMj4Lj9jHjoag$ 

If you use a 'longout', it will simply use the .VAL field.

Dirk

On Mon, 2026-06-29 at 12:17 +0800, Huishan via Tech-talk wrote:
> Hello everyone,
>     I have a question about writing a StreamDevice protocol for the following command: 55 AA 05 04 byte1 byte2 byte3.
>     The last three bytes of the command need to change dynamically. The first two bytes represent the current value in hexadecimal format. The third byte is a checksum, which is calculated by adding 0x05, 0x04, byte1, and byte2 using hexadecimal addition.Is there a way to have the checksum calculated automatically in a StreamDevice protocol? If so, what should I use in the protocol file as a placeholder for the calculated value?
>     If StreamDevice doesn't support calculations like this, does that mean I have to calculate the checksum manually every time before sending the command?
>     Any suggestions or examples would be greatly appreciated.Thank you!
>                                                                                                                          Huishan