Muratec F-525, Field Engineering Manual

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2.6 Network Control Unit (NCU) PCB
The NCU PCB provides the connection to the telephone line. It consists of the interface circuit, ring
signal detector and telephone control circuit.
NCU PCB block diagram for USA
NCU PCB block diagram for GBR
Major components of the NCU
DP relay
Connects the telephone line to the fax.
S relay
Used to connect the telephone line at seizure state.
OH & Tone detector
Detects the On-hook condition of the second telephone unit.
H relay
Disconnects the Tel1 and Tel2 line from PSTN.
24V generator
Supplies 24 volts to the relays for OH detection of optional handset or external telephone line.
Ring signal detector
A photo coupler that detects an incoming ring.