Some Questions about My Circuit

[ling1995]

Good afternoon~everyone.I met some troubles now and I need some help.
The SCM I used is AT89C2051.(this is AT89C2051's data-sheet: http://www.kynix.com/uploadfiles/pdf8798/AT89C2051-12PU.pdf ).I drew a circuit picture by using proteus simulation software.The port of p1.6 is a signal of 5v when the input signal OUT is 26V. They are in a high level ! However,when OUT is in a low level of 0v,why the port still shows a high level? (the red dot is belong to high level while the blue is low level. Is the inside resistance of 2051 SCM's port have any relations about this question? How to solve it?

Thank you in advance!