RS232 serial cables pinout

RS232 serial cable layout

Almost nothing in computer interfacing is more confusing than selecting the right RS232 serial cable. Thess pages are intenting to provide information about the most common serial RS232 cables in normal computer use, or in more common language “How do I connect devices and computers using RS232?”

RS232 serial connector pin assignment

The RS232 connector was originaly develop to use 25 pins. In this DB25 connector pinout provisions that make for a secondary serial RS232 communication channel. In practice, only one serial communication channel with accompanying handshaking is present.

Only very few computers is manufacture where both serial RS232 channels are implementing. Examples of this are the Sun SparcStation 10 and 20 models and the Dec Alpha Multia. Also on a number of Telebit modem models the secondary channel is present.

It can be used to query the modem status while the modem is on-line and busy communicating. On personal computers, the smaller DB9 version is more commonly use today. The diagrams show the signals common to both connector types in black.

The define pins only present on the larger connector are showing in red. Note, that the protective ground is assign to a pin at the large connector where the connector outside is using for that purpose with the DB9 connector version.

The pinout is also show for the DEC modifiy modulair jack. This type of connector is using on systems build by Digital Equipment Corporation; in the early days one of the leaders in the mainframe world.

Although this serial interface is differential (the receive and transmit have their own floating ground level which is not the case with regular RS232) it is possible to connect RS232 compatible devices with this 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 primarly define for the connection of two DTE’s directly.

RS232 DB9 pinout

RS232 DB9 pin assignment

DEC MMJ pinout

DEC MMJ pin assignment

RS232 DB25 pinout

RS232 DB25 pin assigment

RS232 DB25 to DB9 converter

The original pinout for RS232 is develop 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 mix applications, a 9 to 25 pins converter can use to connect connectors of different sizes.

As most of the computers are equiped with the DB9 serial port version, all wiring examples on this website will use that connector as a default. If you want to use the example with a DB25, simply replace the pin numbers of the connector according to the conversion table below.

RS232 DB9 to DB25 converter

RS232 DB9 to DB25 converter

DB9 – DB25 conversion
DB9 DB25 Function
1 8 Data carrier detect
2 3 Receive data
3 2 Transmit data
4 20 Data terminal ready
5 7 Signal ground
6 6 Data set ready
7 4 Request to send
8 5 Clear to send
9 22 Ring indicator

RS232 serial loopback test plugs

The following RS232 connectors can be use to test a serial port on your computer. The data and handshake lines is linking. In this way all data will be send back immediately. The PC controls its own handshaking. The first test plug can be use to check the function of the RS232 serial port with standard terminal software.

The second version can be use to test the full functionality of the RS232 serial port with Norton Diagnostics or CheckIt.

RS232 loopback test plug for terminal emulation software
RS232 DB9 loopback connector RS232 DB25 loopback connector
DB9 DB25 Function
1 + 4 + 6 6 + 8 + 20 DTR CD + DSR
2 + 3 2 + 3 Tx Rx
7 + 8 4 + 5 RTS CTS

RS232 loopback test plug for Norton Diagnostics and CheckIt
RS232 DB9 loopback connector (Norton/CheckIt) RS232 DB25 loopback connector (Norton/Checkit)
DB9 DB25 Function
1 + 4 + 6 + 9 6 + 8 + 20 + 22 DTR CD + DSR + RI
2 + 3 2 + 3 Tx Rx
7 + 8 4 + 5 RTS CTS

Testing occurs in a few steps. Data is sending on the Tx line and the receive information on the Rx input is then compare with the original data. The signal level on the DTR and RTS lines is also control by the test software and the attach inputs are read back in the software to see if these signal levels are properly return.

The second RS232 test plug has the advantage that the ring-indicator RI input line can also be test. This input is use by modems to signal an incomming call to the attach computer.

RS232 null modem cables

The easiest way to connect two PC’s is using an RS232 null modem cable. The only problem is the large variety of RS232 null modem cables available. For simple connections, a three line RS232 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 RS232 null modem selection table to find the right null modem cable for each purpose. For a Windows 95/98/ME Direct Cable Connection, the RS232 null modem cable with loop back handshaking is a good choice.

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

Simple RS232 null modem without handshaking (explanation)

Simple RS232 null modem without handshaking

Connector 1 Connector 2 Function
2 3 Rx Tx
3 2 Tx Rx
5 5 Signal ground
RS232 null modem with loop back handshaking (explanation)

RS232 null modem with loop back handshaking

Connector 1 Connector 2 Function
2 3 Rx Tx
3 2 Tx Rx
5 5 Signal ground
1 + 4 + 6 DTR CD + DTR
1 + 4 + 6 DTR CD + DTR
7 + 8 RTS CTS
7 + 8 RTS CTS
RS232 null modem with partial handshaking (explanation)

RS232 null modem with partial handshaking

Connector 1 Connector 2 Function
1 7 + 8 RTS2 CTS2 + CD1
2 3 Rx Tx
3 2 Tx Rx
4 6 DTR DSR
5 5 Signal ground
6 4 DSR DTR
7 + 8 1 RTS1 CTS1 + CD2
RS232 null modem with full handshaking (explanation)

RS232 null modem with full handshaking

Connector 1 Connector 2 Function
2 3 Rx Tx
3 2 Tx Rx
4 6 DTR DSR
5 5 Signal ground
6 4 DSR DTR
7 8 RTS CTS
8 7 CTS RTS

 

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