The serial port found on Personal Computers uses a 9-pin connector. Normally the RS232 serial port is used for external modems, and in older systems for the mouse and printer interfaces. Most computer systems have only one serial port. The RS232 pinout for a 9-pin D-Sub connector is provided in the table below. Jul 01, 2018 RS-232 (also called serial, COM port) is still common interface and most PC are still equiped with serial interface connector. Usually it is hidden and available as internal header connector only. There is 2 flavor of header (ex: Motherboard header) (AT/EVEREX) or (DTK/INTEL) witch match the IDC10. This adapter will enable you to connect a HP48 Calculator at the ) 9 pin computer serial port. 9 PIN D-SUB FEMALE to the Computer. +-+ O O O O -/ 4 3 2 1. 4 PIN HP48 to HP Serial Port. 9-Pin HP48; Carrier Detect: 1: Receive Data: 2: 2.

Introduction

RS232 - the standard of the asynchronous interface (serial port), was at one time the most popular interface for digital devices for various purposes. In the first computers, his physical presence was mandatory. Even now, the Windows operating system is capable of emulating a number of virtual COMs without having their physical implementations. Some probably remember computer mice, printers, scanners and other peripherals connected to the computer through this port.

Now the situation has changed, the computer peripherals are connected to the PC using faster USB ports. But in instrumentation, RS232 takes the rightful place, it is rare to see a digital device that can be configured by a computer without this interface. Quite often RS232 port serves as a transition link to the RS485 interface, connected via a miniature adapter.

The information in RS232 is transmitted in duplex mode

• Logical «0» - positive voltage from +5 to +15 V
• Logical «1» - negative voltage from -5 to -15 V

Due to the design features, the length of the communication line is small, usually no more than 10 meters.

Initially, the interface RS232 connector was designed as a 25-pin connector. In this DB25 connector, a secondary RS232 serial channel was also provided. But in practice, only one channel was realized. Computers in which both channels were presented were very rare, for example Sun SparcStation 10/20 and Dec Alpha Multia. Also on some modems there was a secondary channel, it signaled the status of the modem, while the primary one was busy with data transmission. In our time, the more 9-pin DB9 version of RS232 has taken root.

Pinout of the RS232 connector

On the diagram of the 25-pin RS232 connector, the black color indicates the pins common to both types of connectors. The figure and the table below show how to connect the adapter from the 25-pin connector to the 9-pin connector.

Adapter cable from 25 pin RS232 to 9 pin

Plug for RS232

Below is the RS232 connector pinout for testing computer serial port. Data and handshake lines connected. In this case, the sent data is immediately returned back and analyzed by the standard serial port verification software.

Null-modem cables for RS232

The simplest way to connect two computers together is to use a null modem RS232 cable. For a simple solution, a three-wire RS232 scheme is sufficient, where one wire is the signal ground, the second is the receiver, and the third is the transmitter. But depending on the type of software, some kind of handshake may be required. Below are the most popular types of null modem cables for RS232.

Simple null-modem cable without handshaking

This simple pinout of the cable does not allow monitoring of data transmission and reception at the «hardware level», but at the software level, monitoring is possible using XOFF analysis and XON symbols. Not all programs are able to work with such a cable. This is more a theoretical concept. There are also cables with a handshaking simulation on a «stub» and partial handshaking without the ability to control the data transmission at the «hardware level». The following is the RS232 cable wiring with full handshaking recommended by Microsoft.

RS232 null modem cable with full handshaking

Here, seven wires are used, and this RS232 pinout has become essentially standard.

RS232 serial cable layout

Almost nothing in computer interfacing is more confusing than selecting the right RS232 serial cable. These pages are intended 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 originally developed to use 25 pins. In this DB25 connector pinout provisions were made for a secondary serial RS232 communication channel. In practice, only one serial communication channel with accompanying handshaking is present. Only very few computers have been manufactured where both serial RS232 channels are implemented. 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 used today. The diagrams show the signals common to both connector types in black. 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 connector where the connector outside is used for that purpose with the DB9 connector version. Minitool partition wizard 8 1 iso.

The pinout is also shown for the DEC modified modular jack. This type of connector has been used on systems built 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 focused on the connection of DTE, data terminal equipment (computers, printers, etc.) with DCE, data communication equipment (modems), MMJ was primarily defined for the connection 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 different sizes. As most of the computers are equipped 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 serial loop-back 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 be sent back immediately. The PC controls its own handshaking. The first test plug can be used to check the function of the RS232 serial port with standard terminal software. The second version can be used to test the full functionality of the RS232 serial port with Norton Diagnostics or CheckIt.

Testing occurs in a few steps. Data is sent on the Tx line and the received information on the Rx input is then compared with the original data.

The signal level on the DTR and RTS lines is also controlled by the test software and the attached inputs are read back in the software to see if these signal levels are properly returned. The second RS232 test plug has the advantage that the ring-indicator RI input line can also be tested. This input is used by modems to signal an incoming call to the attached 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 defined in numerous ways, with loop-back handshaking to each PC, or complete handshaking between the two systems. The most common null modem cable types are shown here.

Db9 Serial Port Pinout

Db9 Serial Port