I have a HDI-7 Elite CHIRP. It is ok for what it is, which is basic. As an ex-commercial user, I've certainly used a lot more capable sounders.  It came with the 83/200/455/800 xducer. I have since fitted a HST-DFSBL 50/200 which improves matters, but I lose the 455/800 downscan function.  The sounder on my last pleasureboat was an old FCV-600L, which was more than capable.  The HDI has no bottom lock and no"Shift"--this is what Furuno calls the ability to select any part of the water column regardless of range. I mostly used the 5 fathom range and shifted to suit, regardless of depth, with bottom lock as well. It appears that the HDS 7 (and 9?) have the ability to set custom upper and lower limits, which is similar to Shift in function. HDI only has a limited Zoom. 

Can anyone who has either a HDS 7 or 9 tell me if the custom upper and lower limits can be extended down so you are  displaying the third repeat echo, without having the first or second showing? That is to say, can you leave the area of the water column between bottom and surface, and start looking at just the bottom consistency as indicated by the third echo? eg, in 20 metres, you would select upper and lower between about 50 metres and 70 metres? I'd like to just use the HDI-7 as a plotter, and use the HDS, if it supports this function, as my sounder. Why don't I just buy a s/h FCV-585 or similar? Tempting, but if I stay with Lowrance combos, I could  keep backing up marks to the HDS and still have full plotter function if the HDI fails at an inopportune time. I like redundancy of electronics at sea, old habits die hard  And having one dedicated to sounding and one to plotting removes that irritation of having to select pages to get the context menu. I find this a massive PITA.