I am not having 2 different GND .I run power from main controller to its sensing module which is 24V and then down convert it to 5V and 3.3V .
if you have a choke in the 24V return which is what I have commented on you DO have two different grounds..
A choke in the return of the 24V supply will make the remote gnd 'dance' relative to the local gnd. This 'dance' will make the 232 inputs 'dance' by the same amount as the gnd 'dance'. Thus your 232 inputs will be infected by the same amount of variance that the choke eliminates.

Either end will output 232 signals relative to it's ground, the other end will see it relative to it's ground.

So, as far as I'm concerned if you have a choke in the supply return between two units, you can not communicate single ended between them. That I would never communicate single ended over any significant distance is another story.

Erik