I want to make a RS-232 path to send the command from the PC connected to VE8900T to monitor connected to VE8900R.

When I set follow video path and connect the RS232 cable, I couldn't make it work.

Are the pin assignment different on both side?

You could use the pin assignment on VE8900T as below:

T--->2

R--->3

G--->5

Then use the pin assignment below on VE8900R:

T--->3

R--->2

G--->5