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.
| PCB |-| Ext | Circuit |
| |-| Card | Housing |
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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