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LEDs light LAN-cable tester

-December 02, 2010

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Cable and connector faults are relatively common in LANs (local-area networks). You can use the circuit in Figure 1 to test straight- or cross-link 10BaseT, 100BaseT, or gigabit UTP (unshielded-twisted-pair) and STP (shielded-twisted-pair) cables. The circuit performs a continuity test for each linked pair. An LED representing each linked pair flashes when the corresponding pair connects properly between the cable’s RJ-45 connectors. It can also determine whether the cable is a straight-link or a cross-link type.

LEDs light LAN-cable tester figure 1

IC1, a small, six-pin Microchip PIC10F200 microcontroller, performs the test. On power-up, the four I/O pins act as outputs and are driven high for approximately 0.5 second and then are driven low in the following sequence: GP0, GP1, GP2, and GP3. Once this task finishes, the microcontroller has a dead time for about 4 seconds, after which it loops to the beginning to repeat the procedure.

LEDs light LAN-cable tester figure 2

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Design Ideas
Adding a low-power BS170 N-channel MOSFET allows Pin 6 (GP3), normally an input-only pin, to act as an output, according to a previous Design Idea (Reference 1). The power-supply voltage of 5V is not critical, and you can choose any supply from 2 to 5.5V for the 8-bit PIC microcontroller. Figure 2 shows cable wiring for both types of cables and how they fit with RJ-45 connectors. For each type, the microcontroller initiates lighting. You can mount the RJ-45 socket in a separate small box to test the inside wall’s long cables.You can view the complete and fully commented source code here. It uses fewer than 256 words of memory. Because it does not specifically target the PIC10F200, it is easy to understand and adapt to other microcontrollers.



Reference
  1. Muñoz, Antonio, “Use the MCLR pin as an output with PIC microcontrollers,” EDN, Jan 10, 2008, pg 65.

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