Converting floating-point MMS

asked 2021-10-06 15:24:27 +0000

Zebacking
1 ●1 ●2

updated 2021-10-06 16:26:44 +0000

Hello everyone,

in a MMS trace, I've got the analogs measures in a kind of a hexa format, how to convert to wireshark show as the sender is sending?

floating-point: 0843adb762

image description

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Which protocol is that and in which field of that protocol? Sharing a capture with the info would help enormously, post it on a public share and amend your question with a link to it.

grahamb ( 2021-10-06 15:29:12 +0000 )edit

Hello, sorry for not sharing a pic, I've edited my post, the protocol is MMS from IEC 61850. Thanks

Zebacking ( 2021-10-06 16:27:23 +0000 )edit

What grahamb asked for was a network capture, not a picture.

Guy Harris ( 2023-10-13 20:50:39 +0000 )edit

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answered 2023-10-12 02:30:04 +0000

ttonway
1 ●1

updated 2023-10-12 02:30:38 +0000

The FloatingPoint Type described in IEC 9506-2 14.4.2.2. The first octet is exponent width part.

The representation of the subsequent octets is compatible with the single precision IEEE 754 floating-point representation when the number of subsequent octets is four and the value of the initial octet is eight. Compatibility with double precision IEEE 754 floating-point representation exists when the number of subsequent octets is eight, and the value of the initial octet is eleven.

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answered 2021-10-07 08:31:05 +0000

grahamb

flag of United Kingdom of Great Britain and Northern Ireland

23835 ●4 ●973 ●227 https://www.wireshark.org

Apparently MMS floating point values come in several flavours and only one of them is dissected as a float, others just show the hex bytes as displayed in your picture.

However, it looks as if that field does fit the requirements to be dissected as a float, i.e. the first byte is 0x08 and there are 4 other bytes, what version of Wireshark are you using?

Again if you can provide the capture, rather than an image, this can be looked at in more detail, i.e. using a debugger.

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This looks indeed that the MMS dissector can be improved.

For now, a trick to show the float as a real number you can use Perl:

perl -e 'printf "%.12g\n", unpack "f", pack "L", 0x0843adb762'
347.432678223

André ( 2023-10-12 18:13:58 +0000 )edit

Yes, the code for floating-point items needs to be redone.

Please file a bug about this on the Wireshark issues list, and attach an MMS network trace containing packets with floating-point data - not a picture of what Wireshark shows for that capture - to the bug.

Guy Harris ( 2023-10-13 20:50:00 +0000 )edit

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