[SI-LIST] starter

Hi,
 
I am a SI starter.
 
I downloaded Pspice student and tried to model transmission line ringing. A 
7474 (pspice) driving a non-lossy transmission line. No termination. output of 
the transmission line is connected to another 7474 input. I also have a 1K 
connected to GND at the o/p of transmission line. The transmission line shiled 
is connected to GND at both ends.
 
When I simulate, I see ringing only on falling edge! Why is there no ringing on 
rising edge? The rising edge comes out as a capacitive rise time.
 
I tried removing the 7474 at the o/p of the transmission line - i.e.-only 1K 
resisitor at the o/p. This behaves same.
 
The digital IO model seems balanced on drive circuit. approx. 100OHM in both 
low and high.
 
Can anybody lead me in the right direction of thinking?
 
Thanks,
peter
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