Checksum curiosa - E1.20 RDM (Remote Device Management) Protocol Forums

sjackman · September 19th, 2006

The spec states that...

Quote:

The Checksum field is the unsigned, modulo 0x10000, 16-bit additive checksum of the entire packet’s slot data, including START Code.

However, the checksum covers a maximum of 255 bytes, each with a maximum value of 255, so the maximum possible checksum is 65025, or 0xfe01. So the checksum is in fact not modulo at all.

Cheers,
Shaun

sblair · September 19th, 2006

Shaun,

If I recall correctly the Message Length max is 255. That field points to the Slot # of Checksum High. So I believe the max packet is 257 bytes.

If each of those bytes was 255, then the Checksum would be 65535. So in that case it wouldn't roll over.

It would be best for everyone to implement it as a modulo anyway as there will be less problems when/if there is any expansion to it in the future.

sjackman · September 19th, 2006

The 257 byte maximum includes the two bytes of the checksum, which clearly aren't covered by the checksum. The checksum only covers 255 bytes.

Brian Sidebotham · October 3rd, 2006

Just because you can't get to the roll over point doesn't mean you can't say it's modulo. By it's very definition of being a 16-bit number it has to be modulo 0x10000

September 19th, 2006	#2
sblair Administrator Join Date: Feb 2006 Posts: 438	Shaun, If I recall correctly the Message Length max is 255. That field points to the Slot # of Checksum High. So I believe the max packet is 257 bytes. If each of those bytes was 255, then the Checksum would be 65535. So in that case it wouldn't roll over. It would be best for everyone to implement it as a modulo anyway as there will be less problems when/if there is any expansion to it in the future. __________________ Scott M. Blair RDM Protocol Forums Admin

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September 19th, 2006	#3
sjackman Task Group Member Join Date: Sep 2006 Posts: 26	The 257 byte maximum includes the two bytes of the checksum, which clearly aren't covered by the checksum. The checksum only covers 255 bytes.

October 3rd, 2006	#4
Brian Sidebotham Junior Member Join Date: Jul 2006 Posts: 4	Just because you can't get to the roll over point doesn't mean you can't say it's modulo. By it's very definition of being a 16-bit number it has to be modulo 0x10000