Well I tried it out. All three squiggle lines and I can not change the length beyond 2.5. It is pretty just not what I expected.

Serpentine stitch looks like a zigzag but instead of having pointed edges it has rounded ones.

I know the Janome 7700 does not have a serpentine stitch, however the 8200 and 8900 does. I'm not sure about your dc5100. I know you can program the stitch into the 7700 so you probably can program it into the 5100 if you do not have it. I got the directions off the internet and will look for them again if you are interested.

On my Janome I do the serpentine stitch and it looks fine. I use these settings:
Mode 2. Stitch No. 26. Width 7.0 and Length 2.5

Let me know if you try it.