DDB signal (Opto-input) DDB signalInput L3 (DDB: 34) Aided 1 Scheme Rx (DDB: 493)Input L4 (DDB: 35) Aided 1 COS/LGS (DDB: 492)The Aided 1 Scheme RX signal corresponds to a 'channel-receive' signal for scheme 1. The Aided 1 COS/LGS signalcorresponds to a 'channel out of service' or 'loss of guard' signal ('Loss of guard' is the inverse signal to 'guardreceived').As well as the default mapping, it is possible to map the signals to other inputs if required.The window during which the unblocking logic is enabled starts 10ms after the guard signal is lost and continuesfor 150ms. The 10ms delay gives time for the signalling equipment to change frequency.Note:If the Z1 Ext Scheme setting is set to operate for channel failure, the Zone 1 extension logic will be invoked if a channel failureis detected.This scheme type also provides Loss of Guard logic as described below.System condition Permissivechannel received Loss of guard Permissivetrip allowedAlarmgeneratedHealthy Line No No No NoInternal Line Fault Yes Yes Yes NoUnblock No Yes Yes, during a 150 ms window Yes, delayed on pickup by 150 msSignalling Anomaly Yes No No Yes, delayed on pickup by 150 msP446SV Chapter 8 - Carrier Aided SchemesP446SV-TM-EN-1 179
