9-3

Fax and Facsimile  

Fax is the abbreviation for facsimile. There are four major parts in a normal facsimile machine, the 

scanner, encoding and decoding device, modem, and printer. Before a page can be sent, it is first 

scanned, the bit-mapped data is encoded with data compression, and then it is transmitted across the 

phone line by an internal modem module. The remote facsimile receives the data with its internal 

modem, decodes it back to bit-mapped image data and prints it on paper. 

Modem as a fax  

Modems can also be designed to include a fax transmitting and receiving function similar to a fax 

card. Since the modem’s interface with the computer is the standard serial RS-232 interface, this 

interface is used for both modem and fax operations. Since the data throughput on a serial RS-232 

interface is limited, only compressed fax image data should be carried through this serial interface. 

Fax image coding and decoding must be done in the computer. Modem/Fax, also called faxmodem, 

can be either an external stand-alone unit or a plug-in card. External stand-alone units can be 

connected to any computer with a standard RS-232 serial port.  

EIA Class 1 Fax Commands  

The EIA Class 1 fax commands are a set of AT fax commands defined by 

EIA/TIA(Telecommunications Industry Association) for controlling faxmodems from a computer 

through the serial RS-232 interface. Faxmodems and fax software supporting this standard will 

work together. Class 1 commands control the details of how the modem does on-line negotiation. 

The Class 1 command set is also called the TIA-578 standard.  

Constriction of Using Internal Fax/Modem and Phone2 

The internal fax/modem is using the same circuit of POTS port phone2, that is, when POTS port 

phone2 is in use, you can not dial out/in a modem call. On the other side, when internal fax/modem 

is in use, if you pick up phone2, you will hear the noise of modem carrier or handshaking.