Dell 2135CN, Service Manual

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6 – 15
Chapter 6 Principle of Operation (Fax)
Version 1 2008.02.01
After the telephone number is entered, the NCU automatically performs steps 1 and
2 of the line connection procedure described in 2.2 Telephone Call Connection
Mechanism.
If on the receiving end, step 6 is automatically performed to answer.
The following is the line connection procedure between two FAXes based on the steps 1 to 5 of 2.2
Telephone Call Connection Mechanism. At the receiving FAX, step 6 is also automatically performed.
1) When an AT command (a modem control command) is sent from the control circuit to the modem,
the hook switch is activated, and a state is obtained that is identical to that when the handset of a
telephone is lifted. A dial tone (400Hz/48VDC) is sent from the local switch. The modem's speaker
emits the dial tone as an audible sound.
2) After image scanning, the telephone number (a previously stored number, number entered by
pressing phone buttons, etc.) is automatically dialed and transmitted to the local switch.
3) Steps 3, 4, and 5 for establishing a connection via the switches are identical to those for telephone.
6) The receiving party's FAX automatically answers when it receives the call signal, and the hook
switch is activated. The local switch on the receiving party side receives a response signal and
stops sending tones to the sending and receiving parties, thereby establishing a communications
path between the both parties.
In the case of a telephone call, only voice conversion between the two parties follows. For FAX,
preparation for delivery of image data is required that includes the following types of exchanges:
• The sending FAX indicates that the transmission is a FAX transmission.
• The receiving FAX indicates that it is ready to receive and also its communications capacity.
• The sending FAX then sends data in accordance with the receiving FAX's communications capacity.
Once mutual preparation is completed, image data sending and receiving is started. Image data is
modulated into an analog signal by the A/D converter at the sending FAX, then sent from its NCU.
Image data received by the NCU of the receiving FAX is demodulated into a digital signal by its A/
D converter and then sent to the control circuit. When image data reception is completed, the FAX
automatically disconnects the line (hook is OFF).
In summary, the NCU automatically executes a series of such operations from hook switch ON to
hook switch OFF.
The control circuit also retains other important functions such as data
compression and memory. With data compression, any part of the scanned image
data that consists of continuous white or black pixels is encoded into a single
element, thus compressing the volume of data.
Memory temporarily stores data during transmission and reception.