Substitute For TDR

[netsurfer802]

I have been scheduled to go on a service call tommorow morning and was informed that it would be useful to have a network tester. Unfortunately, I had lost my TDR --this may be a strange question--but what would be the difference between just hooking up a modem to an ethernet cable on one side and on the other side of the ethernet cable plugging it into a working port of another computer to see if the light on the modem comes on signifying a signal and using a TDR?

I'm hooking up some Dell computers for some company that uses lighted signs at a hospital.