Preliminary Information
The RC and FC outputs of the interfaces are used to
determine the polarity of the Tip and Ring pair which
the C.O. uses for signalling during the active (off-hook)
state of the interface. When the loop is closed by the
interface, the trunk is in the normal or unreversed
state. Some C.O.’s may reverse the polarity of Tip and
Ring, to indicate the talking state. The interface will
detect this condition and RC will output a low level.
MH88631
and Ring inputs of the interface, following by metal
oxide varistors from Tip and Ground, Ring to Ground
and Tip to Ring.
Components List (for Figure 3)
R1*, R2*
R4
= 30.9kΩ, ± 1%
1
/
4
W
= Transmit Gain Adjust Resistor
= 301.5 E3x Gain VX - 100 E3
VTR
Typical Value = 200kΩ, ± 1%
1
/
4
W
= Receive Gain Adjust Resistor
= 523.2 E3 x Gain VTR
V
R
External Circuitry Requirements
R3
The loop seize circuit is completed with the addition
of external components Q1, C5, R5 and K1.
K1, a DPST reed relay, is activated by relay control
signal RC1. When the loop seizure is required, K1 is
closed and the interface applies active termination
across Tip and Ring. The relay should have a 0.5
amp contact capability and 12 VDC operation with a
typical 500Ω coil resistance. To prevent back EMF
from damaging the relay drive transistor, (caused by
the collapsing field of the inductive coil of the relay) a
snubbing diode is provided on the hybrid and
therefore not required externally. C5 and R5 provide
relay contact noise filtering and transient noise
suppression necessary for clamping inductive spikes
created when the loop is closed during line seizure
or dial pulsing.
Q1 provides current drive for the active termination,
controlling the loop current flow of the current
modulator circuitry. Selection of a suitable transistor
for Q1 is made based on worst case conditions
which include fault conditions. A 350 volt, or higher,
rating for Q1 is necessary to meet high voltage
requirements. The Tip and Ring input protection
varistors limit any high voltage spikes to
approximately 300 volts. Under worst case
conditions Q1 must be able to handle close to
100mA of collector current and dissipate two watts
continuously. During pulse dialling, current spikes
are generated due to the inductive nature of the loop.
A 0.5 amp continuous connector current rating is
therefore recommended to provide a safe margin.
K2, R1 and R2 are required only for Ground Start
applications. K2 is the same type of relay as K1 and
is activated by RC2. Once again the snubbing diode
is provided on the hybrid. R1 and R2 provide the
-48V biasing the signalling (on-hook) state.
R6 and C6 constitute the Dummy Ringer required for
the LS/GS trunk. These components are also part of
the 600Ω input impedance.
Protection circuitry on the Tip and Ring inputs may
be required depending upon the trunk interface
application. For maximum protection it is
recommended to place fuses in series with the Tip
Typical Value = 505kΩ, ± 1%,
1
/
4
W
R5
= 510Ω, ± 5%,
1
/
4
W
R6
= 10kΩ, ± 5%,
1
/
4
W
C1, C2, C3, C4= 0.22µF, ± 10%, 200V
C5
= 0.1µF, ± 5%, 250V
C6
= 1.0µF, ± 5%, 250V
Q1
= 2N5657, NPN 350V, 0.5A, 20W
K1, K2
= 2A Reed Relay, E/M 12V
2 Form C Dip
*Note: required for Ground Start applications, not
required for Loop start applications.
2-229
MITEL [ MITEL NETWORKS CORPORATION ]
