(2) the mechanical properties connectors

As RS-232C not defined connectors physical characteristics, and therefore there DB-25, DB-15 and DB-9 various types of connectors, Leads feet  definitions vary. The two connectors were introduced.

1）DB-25 connectors 

PC and XT machines used DB-25 type connectors. DB-25 connectors definition of a root signal line 25, pided into four groups :

①asynchronous communications nine voltage signals (including signal to SG) 2,3,4,5,6,7,8,20,22 

②Central 20mA current signal 9 (12,13,14,15,16,17,19,23,24) 

③Air 6 (9,10,11,18,21,25) 

④Protectorate (PE) 1 as equipment grounded ends (one foot) 

Attention 20mA current signal only part PC/XT IBM PC and IBM machines to provide to at plane and beyond, is not supported.

2） DB-9 connectors

In at plane and beyond, do not support 20mA current link interfaces, the use of DB-9 connectors, as a card or main board, with multifunctional I/O COM1 and COM2 two serial interface [Link the connectors. It provides asynchronous communications nine signals. DB-25 type connectors Leads feet distribution and the type Leads feet  DB-25 signal completely different. Therefore, if the allocation to the DCE DB-25 type connectors connecting equipment to use specialized cables lines.

Cable length : In communications rate below 20kb/s, RS-232C direct physical distance of 15m from the largest (50 feet). Largest direct transmission distance Note : RS-232C standards require that if the use of modem, code distortions in less than 4% of the maximum transmission distance between DTE and DCE for 15m (50 ft). It is the largest distance in yards per distortions less than 4% given the premise. To ensure that codes for less than 4% of the distortions, interface standards in the electrical properties that, the drive should be less than 2500pF load capacitance.

(3) the interface signals RS-232C

RS-232C conventional standards interface 25 lines, four lines of data, 11 Line of Control, 3 regular lines, 7 standby and missing lines, commonly used only nine roots, and they are : ①③Liaison control signal lines :   

Data ready devices (Data set ready-DSR) -- effective (on) state that could be used modem in the state. 

Data terminal ready (Data Terminal ready-DTR) -- effective (on) state that data terminals can be used.

Sometimes even to the power of these two signals, one on the call immediately effective. Two state signals effective equipment, but indicated that the equipment itself could be used, does not indicate a communication link can initiate communication can begin by following communication to the control signals.

Request sent (Request to send-RTS) -- used to express DTE DCE request sent data to be sent when data terminals, so that a signal (on state), a request sent to modem. It is used to control access to state modem.

Allow transmission (Clear to send-CTS) -- used to express DCE DTE made ready to receive the data signal is sent to request the response RTS signal. When modem is ready to receive data transmitted from terminal, and move to, so that the effective signal to inform terminals begin sending data along the lines TXD sending data.

This request received RTS/CTS contact signal is sent to 3,600-4,000 modem system between the cut over ways and means to receive. In Full two-way work system for transmission and reception of way between the cut over. In Full two-way work system for the allocation of two-way routes, so there is no need RTS/CTS contact signals to change high.

Receive line signals departure clearance (Beijing Line detection-RLSD) -- used to express DCE has connected communications links to inform DTE ready to receive data. When local modem received by the other side of communication links (distant) modem sent to the carrier signal, the signal RLSD effective, informed terminal ready to receive, and will receive from modem demodulate signals from the carrier into the digital traffic data, along RXD line toreceive data terminals. The line is also known as data carrier departure clearance (Data carrier detection-DCD) line.