SDAC gathers data mostly for amber (caution) and FWC for most red (warning). However the FWC is directly supplying signal to the lights in the MW & MC pushbuttons.
The FWC1 sends signal to Captain's MW/MC pushbuttons upper bulbs (i.e. "Master" in the MW and "Master" in the MC), but also sends signal to FO's pushbttons lower bulbs ("Warn" in MW and "Caut" in MC).
The FWC does the opposite.
All you need is a power transient or a bus bar power supply change to freeze a FWC (happens regularly during engine starts or APU coming on/offline, especially with the early MSNs). In the case of FWC1, this would be the ESS BUS (CB could be reset -- on ground-- on the OVHD panel).
Last edited by FLEXPWR; 19th November 2013 at 08:31.