RS232 Connector Pin Assignment

RS232 Connector Pin Assignment
RS232 Cable Information
DB 9 and DB 25 Connection

The original pin layout for RS232 was developed for a 25 pins D sub connector.  In this pin-out provisions were made for a secondary communication channel. In practice, only one communication channel with accompanying handshaking is present. For that reason the smaller 9 pin version is more commonly used today.
The diagrams show the signals common to both connector types in black. The signals only present on the larger connector are shown in red.
Note: The protective ground is assigned to a pin at the large connector, in DB9 version protective ground connector to encloser.
For the DEC modified modular jack. Although is differential (the receive and transmit have their own floating ground level) still possible to connect RS232 compatible devices with this interface.

RS232 DB 9 Pin Assignment

RS232 DB 25 Pin Assignment


DEC MMJ Pin Assignment





DB9 to DB25 converter

The original pin layout for RS232 was developed for a 25 pins D sub connector. From IBM-AT machine popular 9 pins connectors are commonly used. RS232 DB9 to DB25 converter cable support mixed applications for 9 to 25 pins.




RS232 DB9 to DB25 converter





RS232 loop-back test plug
The following connectors can be used to self-test a serial port on your computer. The data and handshake lines have been linked. In this way all data will be sent back immediately. The PC controls its own handshaking.
Download test software from:

RS232 loop-back test plug for PCCOM Diagnostics






RS232 null modem cables


The using a null modem cable is easiest way to connect two PC’s.
For simple connections, a three line cable connecting the signal ground and receive and transmit lines is sufficient.
Depending of the software used, some sort of handshaking may however be necessary. Use the selection table to find the right cable for each purpose.
For Microsoft Windows use direct cable connection, the null modem cable with loop back handshaking.
Null modem cables with handshaking can be defined in numerous ways, with loop back handshaking to each PC, or complete handshaking between the two systems. The most common cable types are shown here.


Simple null modem without handshaking


Null modem with loop back handshaking


Null modem with partial handshaking


Null modem with full handshaking




RS232 printer cable
When a serial printer is connected to a PC, the handshaking is not symmetrical any more. In that case a cable is used where some handshaking lines at the PC side are looped back. On the printer side only the data lines and one handshaking line are used.


RS232 monitor cable

It is not difficult to monitor the serial communication between two devices with a PC. To do this you need the monitor cable, which is displayed in the next picture. Two sockets are connected straight through. The PC is connected to the third one.
This monitor cable taps communication from both sides. This means that if the two devices happen to talk simultaneously, the monitored information will be garbage. In most circumstances communication software works half duplex, in which case this problem does not exist.






RS 232 EIA, EDC and EMI Bulk Data Cable
Standare EIA, Extended Distance Low Capacitance and EMI/RFI Double Shielded Data Cable.