Help please - Linear Scale only homing

First Post. Am totally blown away by LinuxCNC and the people who make it what it is - so thanks in advance.

I am using a 7I76e (final config includes 7I89 & 7I88) and have 3 axis working nicely with linear scales providing positional feedback and stepper control (only configured y axis linear scale in attached HAL/ini). The y axis linear scale works brilliantly, I am just having a hard time configuring homing to use the linear scale only (no switches). I can see RAWCOUNT and it would seem a small step to set HOME to the linear scale's RAWCOUNT home position (in my case -80,000). But I am still getting my head around HAL/ini/homing config and hoping to get some advice to get past my current floundering.
Any help appreciated. Cheers Pete
 

Hi Pete,
for index-only homing is set "search_vel" to zero.
linuxcnc.org/docs/2.8/html/config/ini-ho....html#_configuration
Example sniped from my config 

Thanks Chris - sry had some bid work on so just getting back to this now. Will try and report back.

Hi Andy. I dont know enough to know. Rawcounts appears to be zero in the middle of the scale if that helps. I have a 1m DC10 for x and 1.5m for y. Ditron.com. Both TTL 5um. Should I be trying to view in Halscope to understand the index pulse?

loving the feedback loop. I can push lightly on the Y gantry and the stepper reacts and pushes back to keep the position stable. All the while 'appearing' to hold 0.005mm accuracy. Am designing a mark2 machine and this really suggests to me that active compensation means I dont need literal tons of cast iron to get high precision...

Hi Chris, tried:
HOME_SEARCH_VEL = 0.000000 (as well as your other settings)

Machine behaves as before ie will home to whatever was y=0 when LinuxCNC is first initiated. Will double check / reread the homing manual and keep tinkering.
Cheers

andypugh wrote:

Is this a scale with a single index pulse in the middle of travel, or does it have an index which is high at one side of the centre and low at the other?
 

Rawcounts=0 in the middle of the scale. According to the manual:
"Encoder Zero Position: 1 every 50mm"

HOME_SEARCH_VEL=0 didn't change anything.
Any other suggestions?
Thanks

Could it be related to the encorder begin 08?
I have a 7189 (not plugged in yet) which provides 0-7 and 8 is the encoder off the 7i76e.
hm2_7i76e.0.encoder.08 is working great for positional feedback - its only homing I am having difficulty with.

relevant line from my hal file:
net y-index-enable joint.1.index-enable <=> hm2_7i76e.0.encoder.08.index-enable

It must be absolute then. I have tapped the position on the scale where Rawcounts = 0 and it returns to it irrespective of the position it was in when LinuxCNC started. It is in the middle of the scale. Negative rawcounts one end, positive the other.

hk076c1b.pic36.websiteonline.cn/upload/t...t-DCLINEARSCALES.pdf
www.dcoee.com/Download
I will try to attach pics of manual pages shortly. The relavent piece is "Encoder Zero Position: 1 ever 50mm"