Introduction to RS-232

Usually the RS-232 interface appears in 9-pin (DB-9) or 25-pin (DB-25) Type

Generally, there are two groups of RS-232 interfaces on the PC, called COM1 and com2

RS-232C interface signal

RS-232C Gauge Standard Interface has 25 lines, 4 lines, 11 control lines, 3 timing lines, 7 standby and undefined lines, commonly used only 9, they are

1. The data set ready-DSR status indicates that the modem is in usable state.

2. Data Terminal ready-DTR indicates that the data terminal can use

3 request to send-RTS (request to send) -- used to indicate that the DTE request DCE sends data, that is, when the terminal wants to send data, make the signal valid (on status), send

 

Send a modem request. It is used to control whether the modem needs to enter the sending status.

4. Allow sending (clear to send-CTS) -- used to indicate that the DCE is ready to receive the data sent by the DTE, which is the response signal of the request to send the signals to the RTS. When the modem has

 

Prepare the received data from the terminal and make the signal valid when sending it forward. The notification terminal starts to send data along the txd data line.

5. Data carrier dectection-DCD)

Indicates that the DCE is connected to the communication link and the DTE is ready to receive data. When the local modem receives the load signal sent by the modem at the other end of the communication link (far away)

 

To make the RlSD signal valid, notify the terminal to prepare for receiving, and the modem will decode the received carrier signal into digital data, along the receiving data line rxd to the terminal

6. Ringing-ri: When the modem receives the ringing call signal sent from the switching station, it makes the signal valid (on status) and notifies the terminal that it has been called.

7. transmitted data-txd sends the serial data to the modem through the txd terminal (DTE → DCE)

8. the received data (rxd) receives the serial data from the modem through the rxd terminal (DCE → DTE ).

9 ground line SG and PG-signal line of the signal ground and protected area, with no direction

EIA-RS-232C of electrical properties, logic and various signal line functions are defined.

On txd and rxd: logic 1 (Mark) =-3v ~ -15 V

Logic 0 (Space) = + 3 ~ + 15 V

On the control line such as RTS, CTS, DSR, DTR, and DCD:

Effective Signal (on, positive voltage) = + 3v ~ + 15 V

Invalid signal (disconnected, off, negative voltage) =-3v ~ -15 V

The above provisions describe the definition of the logical level of the RS-323C standard. For data (Information Code): the level of the logical "1" (Pass number) is lower than-3 V, and the logical "0" (empty number)

 

Higher than + 3 V; for control signals; on means that the effective signal level is higher than + 3 V, and off means that the invalid signal level is lower than-3 V, that is, when the transmission level

 

When the absolute value is greater than 3 V, the circuit can be effectively checked out, ranging from-3 ~ The voltage between + 3 V is meaningless. The voltage below-15 V or higher than + 15 V is also considered meaningless.

 

During operation, ensure that the level is ± (3 ~ 15) between v.