Simple circuit tests twisted-pair cables
Quickly test a cable containing twisted-wire pairs and detect open or reversed pairs, shorted pairs, and shorts between unrelated pairs.
Mark D Braunstein, Contel Information Systems, Fairfax, VA; Edited by Paul Rako and Fran Granville -- EDN, January 19, 2012
Originally published in the May 29, 1986, issue of EDN
Using the system shown in Fig 1, you can quickly test a cable containing twisted-wire pairs and detect open or reversed pairs, shorted pairs, and shorts between unrelated pairs. The tester consists of an active test set that plugs into one end of the cable, and a passive terminator that plugs into the other end. (An RS-449 cable is used as an example.)
if a pair is shorted,
corresponding LEDs at both ends will be
extinguished; and if any two unrelated
wires of different pairs are shorted, all
intervening LEDs in the strings at both
ends will be extinguished. For example,
if pins 4 and 6 are shorted, LEDs DA, DB,
D1, and D2 will not light.You can add a heat sink to the IC1
regulator as a safety precaution, but
normal tester operation is well within
the regulator’s power-dissipation limits.
Even with many shorted pairs, a dissipation
of 700 mW would cause no more
than 60°C junction temperature, and
the IC is guaranteed to turn itself off at
160°C. The complete tester costs less
than $50 to build.
Talkback
-
The circuit as shown does not detect shorts between unrelated pairs that have their wires connected on the tester such as
22-5 or 23-6. Perhaps if diodes Da thru Dh were moved from across the pairs to between the pairs, that would work.
As always, building and testing the circuit will confirm the behavior.
Dan Prysby - 2012-8-2 21:48:34 PST -
I find that most cables that are built prior to installation have connectors on both ends. So the system should be useful for checking a lot of cables. Cables that have the connector installed after installation can still use this circuit, but the user would need to visit both ends of the cable to verify the results. This would be more work, but still easier than using an ohm meter.
William Ketel - 2012-23-1 13:55:57 PST -
If you used 1.9V green LEDs on the right, and 2.2V red LEDs on the left, you might be able to get:
- the greens to display if the cable is good
- the reds to display if the cable is open
- nether to display if the cable is shorted
BostonEngineer - 2012-23-1 13:30:17 PST -
I would like to mention here that I appreciate the simplicity of the
design. However in the posted design it is required that both ends of
the cable plugs be available at the disposal of the user. In real life
cables are laid with their ends in remote locations. If a single user
has to test the cable it may not be easy, unless one removes the cable
entirely?!
I would like to suggest here for the benefit of the reader that such
a testing with single user is possible using a test gadget titled
"Versatile Probe" published in a Indian electronics magazine. By terminating
one end of the cable by a n-1 diode network, where n is the number of
cable lines, it should be easily possible to test out the cable wires
both for connectivity and spurious internal shorting/breaks. The n-1 diodes
terminate one side of the cable as follows: The n-1 anodes connect to
the n-1 cable terminals and the common cathode of all these diodes
connects to the last cable terminal. So there is only one direction
of current flow from the n-1 cable lines to the last cable line.
Versatile Probe tests connectivity in both directions(+ -, - +)
indicating each direction of current flow by a color of LED light
(red,green). A spurious shorting would make both the color LEDs glow
where as a break in the cable line would not produce any LED glow at
all. However a healthy cable would produce glow of only one color
depending on how you choose to connect the probes of Versatile Probe.
One has to then plug the above described terminator to one of the
test cable ends and use Versatile Probe at the other end to check
out the cable terminals.
Raju Baddi - 2012-19-1 07:49:08 PST
