Mitel SX-200 Documentation page 471

SECTION 9105/914
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Page 32
Dial Tone (Extension)
8.10 When the processor is informed of an off-hook condition (see
paragraph 8.07, Off-Hook - Extension), it interrogates its RAM
to find a free speech path which it checks via a diagnostic circuit on
the tone control circuit card. The tested speech path is then connected
to the line circuit that went off-hook (see paragraph 8.04, Speech Path
Accessing).
8.11 The processor searches for an idle receiver (DTMF/DP), then
dial tone. Using the card select, address and data lines, the
processor then connects the selected receiver and dial tone from the
tone control card to the same speech path as the line circuit, providing
dial tone to the extension.
Dialing internally (From an Extension)
8.12 DTMF or DP signals originate at an extension and are passed
over Tip and Ring through the line circuit to a speech path (see
paragraph 8.04, Speech Path Accessing). Detection of the dialed digits
takes place on a receiver circuit card which has been connected to the
same speech path. A receiver is connected whenever an extension
originates a call or whenever it does a switchhook flash. A receiver
caters to the reception of DTMF or DP signals.
8.13 When a DTMF digit is sent, filter circuits determine its value; a t
DP signal is decoded by a pulse counter. The receiver sets its
interrupt signal and when the scanner addresses it, the interrupt is
placed on the IZ line for that receiver circuit card. A change on the IZ
line causes the scanner to stop and to interrupt the processor via the
data bus which the processor then reads while restarting the scanner.
8.14 The processor reads the digit from the receiver and discon-
processor determines that the digit is valid but does not completely
define a number, it simply stores the digit in the volatile RAM memory.
Should the digit be considered invalid by the processor, reorder tone,
from the tone control card, is connected to the speech path. A valid
extension number causes the processor to disconnect the receiver
from the speech path and to reconnect a tone generator to provide
special busy tone is connected to the speech path.
Dialing Internally (From a Console)
8.15
Unlike an extension, the console key pad generates neither
DTMF nor DP. Instead, the microprocessor inside the console
notes which key was depressed and looks in the console ROM.to find
the corresponding 8-bit code. This code is converted by a UART to a. . _
serial string of data, complete with start and stop bits and is sent as a -.'-':;
differential signal along one of the 25 pairs of wires leading to the ":"'
interconnect card and from there to the console control card.