The purpose of the Panel Decoder Test Frame was to run routine tests on each Decoder. This was generally done during light traffic because the Decoder under test was not available for regular call processing.
The purpose of the trouble indicator panel was to display via lamps the status of the failed Decoder operation. A minor alarm would also sound. If other errors occured they were lost until the trouble indicator panel was reset as it had no memory.
If a Decoder actually failed its internal relay logic checks either during setup connection to a Sender or during release of the Sender a local "very loud" gong would start ringing activating aisle pilot indicator lamps.
Since a reasonable sized Panel office only had three Decoders during peak traffic you could not afford to have one out of service.
Bildquelle: Public Domain