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In order for this four digit display to work it must be compatible with the 4511 BCD chip. So it was important to identify, when purchasing the display, whether the chip works with a common anode or a common cathode display. The 4511 datasheet specified that the 4511 BCD is compatible with the common cathode display.

LED’s fall into two categories: Common Cathode, where all the 8 anode legs use only one cathode which is common and Common Anode where the common leg for all the cathode is of Anode type.