REDLION VOLTAGE LEVELS TO RECEIVE DATA

[B J]

Hello all, Actually I am sending a character from my DSP processor to my hmi (REDLION ) with voltage levels +5v and -5v ( I checked my voltage levels using oscilloscope ) and i wrote a program in crimson to receive the character ( used portinput() function ) whatever i send the character with my DSP , the character was not receiving on my REDLION display and nothing it was showing, but the same thing is working with PC for which the voltage levels are +10v and -10v and my redlion display is showing the character i send from my hyper terminal . Is there any restriction on voltage level for the REDLION for communicating. hope some will solve this problem, thanks, B J RAJENDRANATH