Explore the fundamentals of the RS232 protocol, a classic serial communication standard for connecting computers to peripheral devices like modems. This article covers key topics such as the meaning behind RS232, how data is transferred between DTE and DCE devices, serial connectors, COM port pinouts, RS232 cables, and the differences between null modem and straight-through cables. Despite modern alternatives, RS232 remains relevant due to its long-distance capabilities and cross-manufacturer compatibility.
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The RS232 protocol is a popular serial interface that is used to connect computers to peripheral devices such as modems. We will take a look at the serial port pinouts used to implement RS232 as well as some additional reference information concerning the protocol.
The RS232 protocol transmits data of wires employing signal levels that differ from the standard 5V in order to minimize signal interference. It performs asynchronous transmission at a constant rate that is synchronized with the start pulse signal’s level. Distances of up to 20 meters are the limit for reliable data transfer using the RS232 interface.
Data transfer standards are developed by the Electronic Industry Association (EIA). The prefix RS denotes a Recommended Standard, and all of the EIA standards begin with those characters. The formal specification of RS232 is that it is an interface which uses serial binary data exchange to communicate between DTE and DCE devices. DTE is the acronym for Data Terminal Equipment and DCE represents Data Communication Equipment. The basic example of these two types of equipment defines a computer as a DTE device with a modem filling the role of DCE.
Serial communication is implemented by the transmission of serial data between the DTE and DCE. For instance, a computer (DTE) might send the binary data “11011101” serially to the modem (DCE) which then replies by sending “11010101” back to the DTE device.
The RS232 protocol specifies the operation mode, electrical standards, number of bits, and voltage levels to be used when transferring data between a DTE and DCE.
Serial communication devices make use of 9 or 25 pin D-type connectors for their cabled connections. They are commonly designated as DB-9 or DB-25 with the number used to differentiate between the pin counts. Various manufacturers’ names may replace the DB in the specifications. The plugs contain sockets and pins, with each pin numbered and labeled. A serial pinout diagram is presented below.
The RS232 protocol uses a 9 pin serial port that can have either male or female connectors. The most recent version of the protocol is known as RS232C.
RS232C retains the features of RS232 but uses 25 pins rather than a 9 pin serial pinout. Whether a DB9 serial pinout or a 25 pin connection is used, only three of the pins are required to connect terminal devices.
RS232 manages communication flowing between the DTE and DCE using serial pinouts of either the DB9 or DB25 variety. These D-sub connectors can terminate with an RS232 female pinout or DB25 or DB9 male connector pins. Each pin in a 9 or 25 serial connector pinout has its own distinct function. You can also learn RS485 pinout.
Functional Description:
In addition to defining electrical characteristics, RS232 specifies the signals used in serial cable pinouts and serial ports. Familiar items such as timing signals and ground are included in these specifications.
Following is a list of the signals used in an RS232 COM port pinout:
Protective Ground -This signal is connected to the chassis ground of the metallic connector.
Common Ground - Zero reference voltage level for all the control signals.
TxD (Transmit Pin) - To transmit data from DTE to DCE.
RxD (Receive Pin) - Sends data from DCE to DTE.
DTR (Data Terminal Ready) - DTE is ready to accept the request.
DCD (Data carrier Detect) - DCE accepts a carrier from a DTE located at a remote location.
DSR (Data Set Ready) - DCE is prepared to send and receive the information.
RI (Ring Indicator) - Detects the incoming ring tone on the telephone line.
RTS (Request to Send) - DTE call for DCE to send the data.
RTR (Ready to Receive) - DTE is geared up to receive data coming from DCE.
CTS (Clear To Send) - DCE is in a ready state to accept data coming from DTE.
These signals are the primary RS232 signals, but the protocol allows for secondary signals as well. They include secondary DTE, RTS, DCD, TxD, and RxD. The secondary signals are used to optionally connect DTE and DCE equipment.
Null modems enable serial communication between DTE and DCE devices. An RS232 null modem pinout links the Tx pin of a male connector with the Rx pin on an RS232 female and the Rx male’s pin to the female’s Tx pin.
Using the RS232 protocol you can connect two computers that do not have modems by using a null modem cable. This highlights one of the original uses of the RS232 protocol, which was developed in order to let teletype machines communicate with each other through their modems.
The other type of RS-232 Cable is the Straight-through cable. It is a one to one connector, It transmits a pin of one device that is connected to the transmit pin of another device and the receiver pin of one device is connected to the receiver pin of another device.
Modern hardware designs use innovative serial communication protocols like USB, Ethernet, and Wi-Fi.
But still, RS232 has proven to be used. The reason is, RS232 signals spread over longer distances. Moreover, it has better noise immunity. It is proven to be compatible across different manufacturers for interfacing computer and modems.
