This is an image from a faulty djm350. Green is S2 line (compromised), yellow SCAN2 (healthy).

However green trace is didactic as you can see each step is a scanned row, width about 667us. All 7 rows are scanned in about 4.67ms.

Scanning is used both for keys (KEYn lines) and LEDs (Gn/GRn lines).

Oh, by the way, the fault was a LED that was internally broken and conducted electricity in reverse direction, thus compromising the scan matrix.