Ringing instructions (Ringing-RI) -- When modem sent the switchboard received Ringing call sign, signal to the effective (on state), informed terminals have been shouting.

②Send and receive data lines :

Send data (Transmitted data-TXD) -- through TXD serial data terminals will be sent to modem, (DTE→DCE)。 

Receiving data (Beijing data-RXD) -- through RXD line terminals receive data from modem made to the serial，(DCE→DTE)。

③ Earth

There are two lines SG, PG-- signal to the signal line and the protection, without direction. 

The control signal line when effective, and when the order expressed interface signaltransmission process. For example, only when DSR and DTR are in the effective (on) status, can be transmitted between the DTE and DCE operation. If the DTE to send data, the advance will buy into effective DTR line (on) status, such as CTS line received effective (on) of the answer, can send serial data at TXD line. This sequence of the provisions of Half of the two-way work lines of communication particularly useful because duplex communication can be determined by the DCE to receive directions to the direction of this line can begin sending.

The MAX220–MAX249 contain four sections: dual charge-pump DC-DC voltage converters, RS-232 drivers, RS-232 receivers, and receiver and transmitter enable control inputs.

Dual Charge-Pump Voltage Converter

The MAX220–MAX249 have two internal charge-pumps that convert +5V to ±10V (unloaded) for RS-232 driver operation. The first converter uses capacitor C1 to double the +5V input to +10V on C3 at the V+ output. The second converter uses capacitor C2 to invert +10V to -10V on C4 at the V- output.

A small amount of power may be drawn from the +10V (V+) and -10V (V-) outputs to power external circuitry (see the Typical Operating Characteristics section),except on the MAX225 and MAX245–MAX247, where these pins are not available. V+ and V- are not regulated,so the output voltage drops with increasing load current.Do not load V+ and V- to a point that violates the mini-mum ±5V EIA/TIA-232E driver output voltage when sourcing current from V+ and V- to external circuitry. 

When using the shutdown feature in the MAX222,MAX225, MAX230, MAX235, MAX236, MAX240,MAX241, and MAX245–MAX249, avoid using V+ and V-to power external circuitry. When these parts are shut down, V- falls to 0V, and V+ falls to +5V. For applications where a +10V external supply is applied to the V+ pin (instead of using the internal charge pump to generate+10V), the C1 capacitor must not be installed and the SHDN pin must be tied to VCC. This is because V+ is internally connected to VCC in shutdown mode.

RS-232 Drivers

The typical driver output voltage swing is ±8V when loaded with a nominal 5kΩ RS-232 receiver and VCC =+5V. Output swing is guaranteed to meet the EIA/TIA-232E and V.28 specification, which calls for ±5V minimum driver output levels under worst-case conditions. These include a minimum 3kΩ load, VCC = +4.5V, and maximum operating temperature. Unloaded driver output voltage ranges from (V+ -1.3V) to (V- +0.5V).

 Input thresholds are both TTL and CMOS compatible. The inputs of unused drivers can be left unconnected since 400kΩ input pull-up resistors to VCC are built in (except for the MAX220). The pull-up resistors force the outputs of unused drivers low because all drivers invert. The internal input pull-up resistors typically source 12µA, except in shutdown mode where the pull-ups are disabled. Driver outputs turn off and enter a high-impedance state—where leakage current is typically microamperes (maximum 25µA)—when in shut down mode, in three-state mode, or when device power is removed. Outputs can be driven to ±15V. The power-supply current typically drops to 8µA in shutdown mode. The MAX220 does not have pull-up resistors to force the outputs of the unused drivers low. Connect unused inputs to GND or VCC. 供电多通道RS-232驱动器接收器英文文献和中文翻译(3):http://www.751com.cn/fanyi/lunwen_49444.html