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I mentioned on another thread about using a computer and VGA monitor as a kind of Heath-Robinson TDR setup to find a break in a run of cable. Here's the promised demo.
The hacked VGA test cable has a pair of flying leads connected in parallel with the green channel coax:
Connected to the flyleads are two reels of phone cable - one is a full 100m reel and the other an identical cable but of unknown length, to simulate a broken connection at an unknown distance. One pair in each cable is connected, with the far end of the cables left open-circuit. I.e. there is no DC continuity between any of the conductors connected to the flyleads. One could imagine that the shorter cable represents one broken conductor or pair in the full length cable:
I used MS Paint to simply draw a vertical green line against a black background. This is the result on the VGA monitor. The impedance mismatch at the far end of each cable sends back a reflection which reaches the monitor delayed by the bothway transit time. From left to right the bright bar is the direct signal, the first dim bar the reflection from the shorter cable and the second from the longer cable:
I used the cursor x,y pixel position display in MS Paint to measure the delay in pixel-clock cycles between each reflection and the direct signal. The actual clock speed is unimportant, we need only the ratio of the two. This turned out to be 0.79, indicating that the shorter cable is 79m in length:
Fun eh?
The hacked VGA test cable has a pair of flying leads connected in parallel with the green channel coax:
Connected to the flyleads are two reels of phone cable - one is a full 100m reel and the other an identical cable but of unknown length, to simulate a broken connection at an unknown distance. One pair in each cable is connected, with the far end of the cables left open-circuit. I.e. there is no DC continuity between any of the conductors connected to the flyleads. One could imagine that the shorter cable represents one broken conductor or pair in the full length cable:
I used MS Paint to simply draw a vertical green line against a black background. This is the result on the VGA monitor. The impedance mismatch at the far end of each cable sends back a reflection which reaches the monitor delayed by the bothway transit time. From left to right the bright bar is the direct signal, the first dim bar the reflection from the shorter cable and the second from the longer cable:
I used the cursor x,y pixel position display in MS Paint to measure the delay in pixel-clock cycles between each reflection and the direct signal. The actual clock speed is unimportant, we need only the ratio of the two. This turned out to be 0.79, indicating that the shorter cable is 79m in length:
Fun eh?
