triggering a midi message by moving a fader

hello again,
this time I'd like to know how moving a fader could send just one single midi message, no matter how long I move this fader. The same midi message would be sent again only when I move another fader.
Do you see what I mean?
This is for selecting a channel group automatically by detecting which fader is moved.
If I have a midi control surface with 16 faders, and if I move a fader among the first 8, then channel group is 1 and if the fader is between 16 and 24 then this is the second channel group.

edit : Ok maybe it's confusing so I'll try to explain it more in detail :
I have 16 faders (two groups of 8 faders), and I need midi translator to guess which fader
is being moved (fader NEW) and which fader was previously moved (fader OLD).

If fader NEW and fader OLD are from the same group, then no midi message is triggered.
If fader NEW is from first group and fader OLD from second group then "90 47 7F" is triggered.
If fader NEW is from second group and fader OLD from first group then "90 46 7F" is triggered.

This is because 90 47 7F is the message that a mackie control sends to select the next 8 channels
(and 90 46 7F is for the previous 8 channels).

So basically what I want to do is automatically select the right group of channels by moving a fader...

You can create a translator for each fader with the same outgoing message.

For group 1
So 90 xx pp incoming
and 90 46 7F outgoing

for group 2
90 xx pp
and 90 47 7F

where xx is whatever the incoming fader message is

Set a variable for each group, and then just have a rule that says

if g0==0 then exit rules, skip outgoing action
g0=0
for group 1
and
if g0==1 then exit rules, skip outgoing action
g0=1
for group 2.

I think that'd work.