can't download program into EP2C5 via jtag

i can't download program into EP2C5 via jtag,so i test it use Quartus II tool named jtag chain debugger ,Quartus II know the device is EP2C5 TMS TCK TDO is ok ,but there is a Fork on TDI,then i use oscilloscope to test Circuit,

i found the wave of TMS TCK TDO is good ,but the TDI is not good ,

the wave of TDI is in the attachment

high level is 3.3V ,that right,but the low level is 1.68V (the half of 3.3),not 0V

,can anyone tell me why?