Thankyou Erik
I have fused your Backdoor option and if I invoke this option by P3.4 low one of the 2 sample devices entered ISP, but only through this option and never through PSEN low.But even this was possible a few times only.Now the device does not respond to ISP at all. Flash Magic says "Try adjusting oscillator frequency." I am using 7.3728 crystal.Moreover the device doesn't execute the programmed code also.At 9600 bauds the U is echoed back at the same baud rate. But in a short while( after a few bytes) communication fails.I have tried different baud rates but no use.

Has anyone used this device? Any suggestions?
Regards
Santhi