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An extender card is an accessory that allows access to signals generated by an electronic device that might otherwise be very difficult to probe. Extender cards are used in the testing and troubleshooting of all sorts of devices, from the internal systems of aircraft to video game consoles.

   /----------|-I--------|               |
   |  PCB     |-|  Ext   |  Circuit      | 
   |          |-|  Card  |  Housing      |
   \----------|-I--------|               |
     Figure 1: Placement of Extender Card

Figure 1 shows the placement of the extender card in relation to the PCB (printed circuit board) and device under test. Our example device consists of a rectangular box containing several vertically mounted cards. Without the extender card, the PCB would be pushed all the way into the circuit housing. The connecter on the left side of the extender card is identical to the connecter within the device that the PCB would normally attach to. The mating connecter on the right of the PCB is the means by which this PCB communicates with the rest of the device. If the device is not working properly, or if this is a new device whose functionality needs to be verified before production, it is generally good practice to make sure all the PCBs in the device are interacting correctly. In order to do this, the tester must have a way to attach oscilloscope or logic analyzer probes to key signals. Obviously, it would be impossible to do this with the PCB pushed all the way into the circuit housing.

Extender cards normally have very user friendly pins or test points at which signals may be read. Ideally, the user will be able to clip his/her probe to a test point on the extender card and not have to worry about holding it in place. In my line of work, testing normally consists of probing signals at the extender card and probing signals directly on the PCB -- and I am always relieved when I don't have to probe the PCB directly! Extender cards are usually quite robust and able to withstand the rigors of testing (being yanked in and out, having all sorts of probes and wires clipped to it, etc.). However, the PCB of a complex device often employs tiny surface mount parts and numerous delicate conductive traces. It is very easy to scratch traces or even break device pins with the sharp, unforgiving point of a scope probe. Extender cards therefore help to minimize the abuse of the PCB by allowing as many signals as possible to be accessed indirectly.

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