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9 pin D-SUB male connector layout
9 pin D-SUB male connector
at the computer
9 pin D-SUB female connector layout
9 pin D-SUB female connector
at the at the cable
This RS232C DE-9 (usually miscalled DB-9) port is available at most of PC computers and many other devices. This document features description of how PC serial mouse works.

The RS-232 (also called serial, COM port)  is  still  common interface and most PC that are still equipped with serial interface (RS232C) connector. This PC serial port interface is single ended (interconnect  two devices only), the data rate is less than 20 kbps. RS232 is the voltage loop serial interface with full-duplex communication represented by voltage levels with respect to system ground. A common ground between the PC and the associated device is necessary. Hot-plug isn't supported, but sometimes allowed.

DE-9 Pin Signal Name Dir Description


 
1 DCD <-- Data Carrier Detect
2 RXD <-- Receive Data
3 TXD --> Transmit Data
4 DTR --> Data Terminal Ready
5 GND --- System Ground
6 DSR <-- Data Set Ready
7 RTS --> Request to Send
8 CTS <-- Clear to Send
9 RI <-- Ring Indicator

Note: Direction is DTE (Computer) relative DCE (Modem)

Nowdays RS232 PC serial port is often hidden and presented as the RS232 header on the motherboard only.
 

PC serial port pinout signals

Since PC serial port is based on RS-232 standard, you may find signal details in the RS-232 interface pinout document

Standard RS232 data packet

RS232 data is usually sent as a packet with 7 or 8 bit words, start, stop, parity bits (may be varied). Sample transmission shown on picture: Start bit (active low, usually between +3v and +15v) followed by data bits, parity bit (depends on protocol used) and finished by stop bit (used to bring logic high, usually between -3v and -15v).

Sample RS232 serial port device. How serial mouse works

Typical PC mouse controlling system has the following parts: sensors -> mouse controller -> communication link -> data interface -> driver -> software. Sensors are the movement detectors which sense the mouse movement and button swiches which sense the button states. Mouse controller reads the state of those sensors and takes acount of current mouse position. When this information changes the mouse controller sends a packet of data to the computer serial data interface controller. The mouse driver in the computer received that data packet and decodes the information from it and does actions based on the information.

PC RS232 serial mouse voltage levels:

Mouse takes standard RS-232C output signals (+-12V) as its input signals. Those outputs are in +12V when mouse is operated. Mouse takes some current from each of the RS-232C port output lines it is connected (about 10mA). Mouse send data to computer in levels that RS-232C receiver chip in the computer can uderstand as RS-232C input levels. Mouse outputs are normally something like +-5V, 0..5V or sometimes +-12V. Mouse electronics normally use +5V voltage.


Serial device hardware implementation

PC serial mouse uses typically DTR and RTS lines for generating +5V power for microcontroller circuit in the mouse. Because typical optomechanical mouse also needs power for 4 leds in the optocoupler movevement detectors, there is not much power to loose. A typical approach is to use diodes to take current from DTR and RTS lines and then feed it through resistor to all of the (infrared) leds in the movement detectors. The positive power supply usually taken from RTS and DTR lines (just after the diodes and before the resistor going to leds). The negative supply for transmitter is taken from TD pin. Typical PC serial port mouse takes 10 mA total current and operates at voltage range of 6-15V. The data itself in sent using standard asynchronous RS-232C serial format:

              Start D0  D1  D2  D3  D4  D5  D6  D7  Stop
   Logic 0      ___ ___ ___ ___ ___ ___ ___ ___ ___
  +3..+15V     |   |   |   |   |   |   |   |   |   |
               |   |   |   |   |   |   |   |   |   |
               |   |   |   |   |   |   |   |   |   |
   Logic 1     |   |   |   |   |   |   |   |   |   |
  -3..-15V  ___|   |___|___|___|___|___|___|___|___|____

Serial mouse pinout explanation

Pin Signal Description
shell Protective Ground  
3 TD Serial data from host to mouse (only for power)
2 RD Serial data from mouse to host
7 RTS Positive voltage to mouse
8 CTS  
6 DSR  
5 Signal Ground  
4 DTR Positive voltage to mouse and reset/detection

RTS = Request to Send CTS = Clear to Send DSR = Data Set Ready DTR = Data Terminal Ready

When DTR line is toggled, mouse should send one data byte containing letter M (ascii 77) to identify itself. To function correctly, both the RTS and DTR lines must be positive. The lines DTR-DSR and RTS-CTS must NOT be shorted. Implement the RTS toggle function by setting the RTS line negative and positive again. The negative pulse width is at least 100ms. After a cold boot, the RTS line is usually set to a negative level. In this case, setting the RTS line to a positive level is also considered an RTS toggle.





 

RS232 serial data parameters and packet format

1200bps, 7 databits, 1 stop-bit

Data packet is 3 byte packet. It is send to the computer every time mouse state changes (mouse moves or keys are pressed/released).

        D7      D6      D5      D4      D3      D2      D1      D0
 
1.      X       1       LB      RB      Y7      Y6      X7      X6
2.      X       0       X5      X4      X3      X2      X1      X0      
3.      X       0       Y5      Y4      Y3      Y2      Y1      Y0

Note: The bit marked with X is 0 if the mouse received with 7 databits and 2 stop bits format. It is also possible to use 8 databits and 1 stop bit format for receiving. In this case X gets value 1. The safest thing to get everything working is to use 7 databits and 1 stopbit when receiving mouse information (and if you are making mouse then send out 7 databits and 2 stop bits).

The byte marked with 1. is send first, then the others. The bit D6 in the first byte is used for syncronizing the software to mouse packets if it goes out of sync.

LB is the state of the left button (1 means pressed down); RB is the state of the right button (1 means pressed down); X7-X0 movement in X direction since last packet (signed byte); Y7-Y0 movement in Y direction since last packet (signed byte)
 

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Pinouts.ru > RS-232 and other serial ports and interfaces > Pinout of PC serial port (RS-232 DE9) and layout of 9 pin D-SUB male connector and 10 pin IDC male connector
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