RS232 & RS449 standards

RS232 standard(considering Motorola ACIA-Asynchronous communication interface adapter IC )

  • RS stands for Recommended Standards

  • Serial data is sent through TxD line.

  • It requires output voltage level of driving amplifier for mark (logic 1, between
    -5V & -15 V) and space ( logic 0, between 5V &15V).

  • The terminating circuit receive data on receiver side through RxD line.

  • For control signal , voltage value is same but logic is positive i.e., for logic 1 it is between 5V & 15V and for logic 0 it is between -5V & -15V.

Main control signals are:
1.Request to send(RTS):

  • Send by UART to MODEM

  • MODEM responds by asserting clear to send (CTS).

  • The purpose is to provide handshaking between modem & UART.

2.Carrier detect (CD), data carrier detect (DCD) or receive line signal detect(RLSD)

  • When MODEM detects RTS from UART one of its response is to send mark logic 1 to telephone line, it responds by asserting CD.

  • UART detects CD & interprets that remote station is online & data link is complete.

3.Data terminal ready(DTR) from DTE and data set ready(DSR) from MODEM

  • It informs data link are electrically ready to establish data link.

  • MODEM asserts DSR saying that it is ready.

4.Common signal return

  • This establish common signal reference point between MODEM & DTE.

5.Ring indicator(RI)

  • It is asserted by modem capable of detecting incoming telephone ring.

  • DTE connected to auto answer mode responds by asserting RTS line beginning process to establish link.

  • Remaining lines are used for signalling & timing control for modems requiring data synchronization.

  • Secondary data rates are smaller used for acknowledgement in opposite direction of data channel.

Electrical and physical specifications:

  • Maximum ±25 V can be applied to line.

  • Cable impedance ( 330 maximum line drivers & 7000 load circuits)

  • Maximum line capacitance 2500pf.

  • Capacitance per foot must not exceed 50pf/foot.

  • Maximum cable length 50 feet due to capacitance specifications.

RS449 interface

  • It has two connectors: 37 pin main connector & 9 pin secondary channel connector specified by RS449 channel.

  • Increased pins share same functions as RS232, with reduced line capacitance.

  • It allows data rates of 2 Mbps & cable length of 200 feet.

New control functions include:

  • Local loop(LL), data placed in TxD line is looped back through MODEM onto receiver
    side to DTE.

  • For local equipment remote loop back (RL),data sent by station is looped back to sending station.

  • In remote loopback DTE is not involved.

  • A similar control signal data mode(DM) indicates data is in normal transfer mode.

  • If DTE desires to prepare DCE to receive new signal line(NS), cause receive data line(RD) to mark state & disables carrier detect(CD) line.

  • Once MODEM receives new signal, it places receive data line & asserts CD line.

  • The send common(SC) & receive common(RC) indicates direction in which data flows.

Reference: Analog & digital communication by Michel Miller


RS-232 and RS-449 are both serial communication standards that were commonly used for data transmission in earlier computer and telecommunications systems. However, they have distinct differences in terms of signaling, electrical characteristics, and applications.

Here’s a comparison between RS-232 and RS-449:


Signaling: RS-232 uses bipolar signaling, where positive and negative voltage levels represent different logic states. It employs voltage levels between -15V to +15V for logic 1 and logic 0, respectively. It is commonly known as “UART” (Universal Asynchronous Receiver Transmitter) or “serial port” communication.

Connector and Pin Configuration: RS-232 typically uses a 9-pin or 25-pin D-sub connector, with the pinout configuration defined for specific applications.

Applications: RS-232 was widely used for short-distance serial communication between computers and peripherals, such as modems, printers, and serial mice. It is also used for console connections to network devices, such as routers and switches, for configuration and management purposes.


Signaling: RS-449 uses unipolar signaling, where a single voltage level (typically positive) represents a logic state. It employs voltage levels between 0V to +15V for logic 1 and logic 0.

Connector and Pin Configuration: RS-449 typically uses a 37-pin D-sub connector with a specific pinout configuration.

Applications: RS-449 was commonly used for long-distance data communication in telecommunications and data networking applications. It provided higher data rates and longer transmission distances compared to RS-232. RS-449 was used in applications such as data multiplexing, modem connections, and connecting terminals or computers to mainframe systems.


Signaling: RS-232 uses bipolar signaling, while RS-449 uses unipolar signaling.

Voltage Levels: RS-232 supports higher voltage levels (-15V to +15V) compared to RS-449 (0V to +15V).

Connectors: RS-232 typically uses 9-pin or 25-pin D-sub connectors, while RS-449 uses a 37-pin D-sub connector.

Applications: RS-232 was primarily used for short-distance communication between computers and peripherals, while RS-449 was used for longer-distance communication in telecommunications and networking applications.

It’s important to note that both RS-232 and RS-449 have been largely replaced by newer standards, such as USB and Ethernet, in modern computing and telecommunications systems.