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the front of the board and 4-conductor TEL cord. In normal
operation, the switching relays are in 'make' state. In case of
power supply down, they will be in 'break' state, and the power-
failure direct-dial mode will be established.
- Self-diagnostic relay
A port has a self-diagnostic relay. In normal operation, the self-
diagnostic relay is in 'break' state. In self-diagnostic operation, it
will be in 'make' state. In self-diagnostic mode, Tip-Ring will be
connected to Tip-Ring of another outside-line card by way of the
backboard, and the self-diagnosis of the outside-line card can be
carried out.
The self-diagnosis of the outside-line card includes outside line
acquisition, dial pulse issue, DTMF issue, BELL reception, and
CPC detection.
- Bell signal issuing circuit
When feeding and speech paths are connected, BELL relay will be
in 'break' state. When the bell signal is issued, BELL relay will be
in 'Make' state.
- HOOK detecting circuit
When BELL signal is not output or when dial pulse is received,
this circuit distinguishes whether SLT is off hook or on hook.
When SLT is off hook, DC loop is formed and current flows into
U52. At this time, the collector of U52 (3, 4, and 5 pin), namely,
HOOK signal changes from H to L and is detected by CPU by way
of CODEC
and the current flow into U52 (3, 4, and 5 pin) ceases; the collector
of U52, namely, HOOK signal changes from L to H and is detected
by CPU in a similar manner.
- Ring trip circuit
While BELL signal is output, this circuit detects SLT off-hook by
the hardware, and makes BELL relay in 'break' state.
ASIC. When SLT is on hook, DC loop is interrupted
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