25 Pin

1.CD.......Carrier detect


Carrier Detect



2.RXD......Received data


Receive Data



3.TXD......Transmited data


Transmit Data



4.DTR......Data terminal


Data Terminal Ready



5.GND......Signal graund


System Ground



6.DSR......Data set ready


Data Set Ready



7.RTS......Request to send


Request to Send



8.CTS......Clear to send


Clear to Send



9.RI.........Ring indicator


Ring Indicator




2-Cable pinout for: 9 Pin to 25 Pin Serial

3-This adapter will enable you to connect a 25 pin serialcable to a 9 pin connector at the computer.


RS232 serial connector pin assignment


The RS232 connector was originally developed to use 25 pins. In thisDB25 connectorpinout provisions were made for a secondary serial RS232 communication channel.In practice, only one serial communication channel with accompanying handshakingis present. Only very few computers have been manufactured where bothserial RS232 channels are implemented. Examples of this are the Sun SparcStation10 and 20 models and the Dec Alpha Multia. Also on a number of Telebit modem models the secondary channel is present. It can be usedto query the modem status while the modem is on-line and busy communicating.On personal computers, the smaller DB9 version is more commonly usedtoday. The diagrams show the signals common to both connector types inblack. The defined pins only present on the larger connector are shown in red.Note, that the protective ground is assigned to a pin at the large connectorwhere the connector outside is used for that purpose with the DB9 connector version.

The pinout is also shown for the DEC modified modular jack. This type of connector has been used on systems built byDigital Equipment Corporation; in the early days one of theleaders in the mainframe world. Althoughthis serial interface is differential (the receive and transmit have their ownfloating ground level which is not the case with regular RS232)it is possible to connect RS232 compatible devices withthis interface because the voltage levels of the bit streams are in the same range.Where the definition of RS232 focussed on the connection of DTE,data terminal equipment (computers, printers, etc.) with DCE,data communication equipment (modems), MMJ was primarily defined for theconnection of two DTE's directly.

RS232 DB25 to DB9 converter

The original pinout for RS232 was developed for a 25 pins sub D connector.Since the introduction of the smaller serial port on the IBM-AT,9 pins RS232 connectors are commonly used. In mixed applications,a 9 to 25 pins converter can be used to connect connectors of differentsizes. As most of the computers are equipped with the DB9serial port version, all wiring examples on this website will use that connectoras a default. If you want to use the example with a DB25, simplyreplace the pin numbers of the connector according to the conversion table below.

RS232 DB9 to DB25 converter

RS232 serial loopback test plugs

The following RS232 connectors can be used to test a serial port on your computer.The data and handshake lines have been linked. In this way all data will besent back immediately. The PC controls its own handshaking. The first test plugcan be used to check the function of the RS232 serial port with standard terminalsoftware. The second version can be used to test the full functionality ofthe RS232 serial port with Norton Diagnostics or CheckIt.

RS232 loopback test plug for terminal emulation software

Testing occurs in a few steps. Data is sent on the Tx line and thereceived information on the Rx input is then compared with the original data.The signal level on the DTR and RTS lines is also controlledby the test software and the attached inputs are read back in the software to see if thesesignal levels are properly returned.The second RS232 test plug has the advantage that the ring-indicator RI input linecan also be tested. This input is used by modems to signal an incomingcall to the attached computer.

RS232 null modem cables

The easiest way to connect two PC's is using an RS232 null modem cable. The onlyproblem is the large variety of RS232 null modem cables available. For simpleconnections, a three line RS232 cable connecting the signal ground and receiveand transmit lines is sufficient. Depending of the software used, somesort of handshaking may however be necessary. Use theRS232 null modem selection tableto find the right null modem cable for each purpose. For a Windows 95/98/ME DirectCable Connection, the RS232 null modem cable with loop back handshaking isa good choice.

RS232 null modem cables with handshaking can be defined in numerous ways,with loopback handshaking to each PC, or complete handshaking betweenthe two systems. The most common null modem cable types are shown here.

Simple RS232 null modem without handshaking (Null modem explanation)


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