Electrical Description - Mitel SX-200 Practices

Circuit Card Descriptions
loop current and responds with dial tone. Dialing may be DTMF or pulses. Dial pulses
are debounced by software to assure reliable performance.
When a call is directed to the set, a relay closes, sending ringing voltage to the set. The
ringing relay drops out when loop current flow indicates that the telephone has been
answered (off-hook condition).
Electrical Description
11.5 Line protection comprises high voltage varistors to energy dump ground from Tip and
\ Ring and fusible links incorporated in the battery feed resistors. EMI is controlled by
inductors in series with Tip and Ring.
The maximum loop resistance is 1800 ohms. The maximum loop length is 5850 m
(19,200 ft) when using 26 AWG wire, 15,240 m (50,000 ft) when using 22 AWG wire,
The card circuitry performs 2-wire to 4-wire conversion, splitting the signal on the line
into outgoing and incoming speech paths. The analog signal coming from the line is
converted to Pulse Code Modulation (PCM); the signal to be sent to the line is converted
from PCM to analog audio. These conversions are performed by a Mite1 Codec chip.
The line circuit applies ringing voltage to the appropriate line through a relay and
removes it when the telephone is answered. This is determined by monitoring the loop
current.
Loop current is provided through a pair of 200 ohm resistors. Below 900 ohms loop
resistance, active current limiting circuitry limits line power to less than I .5 watts,
Ringing for each line is controlled by relay 2.
Condition Relay 1
Idle or Talk OFF
Ringing OFF
Relay 2
OFF
ON
125 11-2
Revision 0 9109-096-125-NA
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