comedi_sv_init / measure

Hi ,
 
I am using Advantech pcl818 card. I have succesfully tried DO and DI using latest comedi and comedilib. . But the problem now is with AI function:
 
        1. Function comedi_sv_init  gives ret = 0
        2. Function comedi_sv_measure gives ret = -1
 
I always got raw data = 4095. What would be the possible failure. ?
 
I am running the system on a 586 Advantech Buiscuit PC and interfaces to PCL818 card is 
vi PC/104 to ISA bus interface card. Do you think that high CPU timing could cause the problem.?
Really appreciate your help. Thanks.
 
regards,
Saiful.