What is the purpose of asterix (*) in Gcode?

Question

Doing some low level monitoring, the following sequence is sent to the printer:

N0 M106*36 
N1 G28*18 
N2 M107*39

This sequence just turns the fan on, homes the printer, then turns the fan off.

Question: What is the purpose of the asterix/star/(*) and the two succeeding digits on each line?

* asterisk - [Asterix](https://en.wikipedia.org/wiki/Asterix) is a character in a cartoon series. — Andrew Morton, Sep 25 '19 at 10:33
@AndrewMorton So that would make the gcode the Romans then - with Asterix being their checksum, stopping corruption in his local area? — Mtl Dev, Sep 25 '19 at 14:43
Hmmm... maybe. The Romans did build a lot of things, like a 3-d printer does. — Andrew Morton, Sep 25 '19 at 14:46

score 13 · Accepted Answer · answered Dec 02 '16 at 08:11

13

That is a checksum. It's added by the host software to the G-code, to allow some basic verification by the firmware that the G-code was transmitted unchanged. It doesn't change the meaning of the G-code, and what your sequence actually represents is just M106, G28, M107. The N0,N1,N2,... are line numbers, and the combination of line numbers and checksums is used to request a re-send of any lines that were corrupted during transmission.

answered Dec 02 '16 at 08:11

Tom van der Zanden

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Do you happen to know the checksum algorithm used here? – Carl Witthoft Dec 02 '16 at 14:30
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In Marlin, all the bytes for the individual characters are XOR-ed together, and the result should be the same as the checksum. – Tom van der Zanden Dec 02 '16 at 15:33
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Just XOR: `byte checksum = 0; byte count = 0; while(instruction[count] != '*') checksum = checksum^instruction[count++];` – Mtl Dev Dec 04 '16 at 00:17

What is the purpose of asterix (*) in Gcode?

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