EIA/TIA Recommended Standard (RS)-232C

Vicente González Ruiz

September 12, 2016


1 Introduction
2 Description
3 Null Modem

1 Introduction

Originally, the concept of DCEs and DTEs was based on two types of equipment: terminal equipment that generated or received data, and communication equipment that only relayed data. In the development of the RS-232 standard, there were reasons why 25-pin RS-232 connectors on these two types of equipment needed to be wired differently. These reasons are no longer significant, but we are left with two different types of cables: one for connecting a DTE to a DCE, and another for connecting two DTEs directly to each other.

The terms DTE and DCE are relative with respect to what part of a network you are observing. RS-232C is the recommended standard (RS) describing the physical interface and protocol for relatively low-speed, serial data communication between computers and related devices. The EIA originally defined RS-232C for teletypewriter devices. The DTE is the RS-232C interface that a computer uses to exchange data with a modem or other serial device. The DCE is the RS-232C interface that a modem or other serial device uses in exchanging data with the computer.

For instance, your PC typically uses an RS-232C interface to communicate and exchange data with connected serial devices such as a modem. Your PC also has a Universal Asynchronous Receiver/Transmitter (UART) chip on the motherboard. Since the data in your PC flows along parallel circuits, the UART chip converts the groups of bits in parallel to a serial stream of bits. To work faster, a UART chip has buffers so it can cache data coming from the system bus while it processes data going out the serial port. The UART is the DTE agent of your PC and communicates with the modem or other serial device, which, in accordance with the RS-232C standard, has a complementary interface called the DCE interface.

2 Description

1 Data Carrier Detect (DCD) indicates that the carrier for the transmit data is ON (only modems)
2 Receive Data (RxD) carries data from the serial device to the computer
3 Transmit Data (TxD) carries data from the computer to the serial device
4 Data Terminal Ready (DTR) indicates to the modem that the computer is ready to transmit
5 Ground (GRD)
6 Data Set Ready (DSR) similar to DTR. It indicates that the Dataset is ON
7 Request To Send (RTS) equests clearance to send data to a modem
8 Clear To Send (CTS) acknowledge the RTS signal of the computer
9 Ring indicator (RI) An auto answer modem uses the Ring Indicator (RI) to signal receipt of a telephone ring signal (only modems)

3 Null Modem

The original RS-232 standard only defined the connection of DTEs with DCEs, which were modems. However, if you want to connect two DTEs, such as two computers or two routers in the lab, a special cable called a null modem eliminates the need for a DCE. In other words, the two devices can be connected without a modem. A null modem is a communication method to directly connect two DTEs, such as a computer, terminal, or printer, using a RS-232 serial cable. With a null modem connection, the transmit (Tx) and receive (Rx) lines are crosslinked.

Connector 1 Connector 2 Function

2 3 Rx <- Tx
2 3 Tx -> Rx
5 5 Ground

When using a null modem, keep in mind that synchronous connections require a clock signal. An external device can generate the signal, or one of the DTEs can generate the clock signal. When a DTE and DCE are connected, the serial port on a router is the DTE end of the connection by default, and the clock signal is typically provided by a CSU/DSU or similar DCE device. However, when using a null modem cable in a router-to-router connection, one of the serial interfaces must be configured as the DCE end to provide the clock signal for the connection.