1. A graphics processor comprising: a test pixel source configured to supply a sequence of test pixels during a test operation; an input port configured to receive a sequence of external pixels from another graphics processor; a pixel selection logic circuit coupled to the input port and the test pixel source, the pixel selection logic circuit being configured to select the sequence of external pixels or the sequence of test pixels as a sequence of output pixels during the test operation; a pattern detection circuit coupled to the pixel selection logic circuit and configured to detect whether, when the sequence of external pixels is selected by the pixel selection logic during the test operation, the sequence of selected output pixels corresponds to the sequence of test pixels; and an output port coupled to the pixel selection logic circuit and configured to deliver the sequence of selected output pixels to another graphics processor, wherein the test operation is usable to determine whether the input port or the output port is connected to another graphics processor, and wherein the graphics processor is further configured to perform the following during a first test operation: output the sequence of test pixels to a second graphics processor operating in a pass-through mode such that the second graphics processor outputs via a first output port of the second graphics processor pixel data received via a first input port of the second graphics processor; determine whether the second graphics processor outputs the sequence of test pixels while operating in the pass-through mode; and detect a connection between the graphics processor and the second graphics processor in the event that the second graphics processor outputs the test patter while operating in pass-through mode.