This network cable tester is designed for testing network cable with CAT 5 connectors, RJ-45. It can be modified to test any 8 conductor cable by replacing the RJ-45 jacks with the mating connectors of the cable to be tested.
Schematic of the cable tester
The cable tester operation is mostly digital circuitry. The only analog component is the oscillator that uses a NE555 timer as a clock source for the TTL logic. Detailed information for this part can be found here: NE555 Datasheet.
The circuit operates from a single 9V battery. The voltage of the battery is regulated to +5V with the LM7805 linear regulator, U6. This provides to correct voltage level required by the TTL logic parts. The capacitors on the output of the regulator stabilize the +5V power and prevent power glitches when the circuitry is switching. Detailed information on this part can be found here: LM7805 Datasheet.
The NE555 timer, U1, oscillates around 6Hz and provides a clean square wave clock signal that is used by the 74LS393 counters. The counters output binary count sequences that begin with a count of 0 and count to 7, also known as a 3-bit counter. Detailed information on this part can be found here: 74LS393 Datasheet
The output of the first counter goes to the inputs of the 74LS138 decoder U4, The decoder has 8 outputs that are set by the ‘count’ from the 3-bit counter. Only one output of the counter is ‘low’ at any time, all other outputs are ‘high’. The internal gates in the decoder select which output is low based on the binary value of the inputs. In this circuit the outputs drive the cathodes of the LEDs. This means that only one LED will be able to turn on at a time. Detailed information on this part can be found here: 74LS138 Datasheet.
The second counter goes to the inputs of the 74LS138 decoder U3. This decoder operates exactly like U4, but the outputs are inverted with the 74LS240 8-bit inverter/buffer. This causes the signals going to the resistors to be the opposite of the outputs on U4. Only one output goes ‘high’, while all other outputs are ‘low’. These outputs provide the drive to the anodes of the LEDs. This that only one LED will be able to turn on at a time. Detailed information on for the 74LS240 can be found here: 74LS240 Datasheet.
The results of the decoders driving the LEDs causes only one LED to be illuminated when the correct pins of the cable are selected, provided the cable is wired correctly and not damaged. The circuit will sequence through all of the wires in the cable in order. One wire is selected by U3 and tested against all of the wires driven by U4, one at a time. This is done by only one output of U4 going ‘low’ for each of the 8 counts of U2A. The correct illuminated LED for each wire verifies that the wire selected by U4 is corrected connected to the wire selected by U3. When all 8 wires have been tested, U4’s binary input advance one count, and the sequence for U3 repeats.
A network cable to be tested is connected to the 2 RJ-45 jacks. Initially, it doesn’t matter which end of the cable is connected to the RJ-45 jacks. Be sure they are completely seated into the jacks for accurate cable testing. Poorly connected cables will show a false failure.
The test begins when the push button switch is pressed and held. This powers up the tested and begins the testing cycle. It is important to hold the push button down during the entire test.
As the test runs, the LEDs will illuminate in sequence from LED1 to LED8 for a correctly wired cable. They will blink once quickly and pause as each wire is tested. It is important to note that the LEDs sequence with a very regular pause between each wire for a correctly wired cable. Shorted wires will illuminate more than one LED in the step which has the shorted wires. A broken or disconnected wire will fail to illuminate any wire in the step which has the open or broken wire.
It will help to see the intended operation of the cable tester by testing a known good cable and observing the LEDs as the illuminated in the correct sequence. It is a good practice to do this each time the cable test is first used. Another good practice is to swap the connectors of the cable and re-run the test. This test will be more thorough and catch more difficult to detect errors.
This tester is fairly easy to build as a wired prototype or built using a bread board. It can also be made into a printed circuit board with the proper CAD program, such as Eagle from Cadsoft (which is a free download for the light version – Eagle download).
The tester is does a good job catching opens, shorts and miswires. It is also easy to use and very portable. Although an enclosure is not required, it does nicely finish the project and turns it into a professional piece of test equipment.