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USB to Serial Communication Protocols

Differences between RS232, RS422, RS485

USB to serial information article explaining the fundamental differences between RS232, RS422, and RS485 serial protocols. Briefly describes USB serial adapters.

What is serial communication protocol? Serial communication is the term used in telecommunication and computer science to define the process of sending data sequentially over a communication channel, one bit at a time. Serial communication protocols are rules that govern how this communication will take place. In today’s technology, the use of USB connectivity provides a broader and more functional USB Serial communication for RS232, RS422, and RS485 protocols and devices.

Types of USB to Serial Communication Protocols

RS232: This protocol defines the meaning, electrical characteristics (voltage level, signalingRS232 mini serial adapter rate, maximum load capacitance), and timing of signals between data terminal equipment and data circuit-terminating equipment. In other words, it is an interface for connecting modems and data receiving devices (GPS receivers, data loggers etc) to computers. However, RS232 does not define elements such as framing or encoding of characters, bit rates for transmission and error detection protocols.

Voltage levels: The voltage levels corresponds to logical one and logical zero levels and valid signals are in the range of -3 to +15 volts or -3 to -15 volts. The invalid ranges are -3 to +3 volts.

  • Limitations of RS232: Some of the limitations include:
    • It does not address the possibility of connecting a DTE directly to DTE or a DCE to a DCE.
    • The handshaking of the protocol is intended for setup and takedown of communication circuit only, which is not suitable for the flow control in many devices.
    • The devised multi-drop has limitations in compatibility and speed.
    • Increased power consumption and complicated power supply design are major problems caused due to large voltage swings.
    • There is no specified method for sending power to a device.

    DTE: Data Terminal Equipment
    DCE: Data Circuit-Terminating Equipment


    RS422 and RS485 mini serial adapterRS422: The standard only defines the signal levels of serial interface. Specifying the electrical characteristics of balanced voltage digital interface circuit, it provides for data transmission, using differential or balanced signaling, terminated or non-terminated transmission lines and point-point or multi-drop. The advantages of this RS422 standard include high differential and driver rates (10 megabits per second at 12 meters), differential receiver, and a differential driver. The standard is referenced with other standards to specify the complete DTE/DCE interface for a balanced voltage circuit to transmit data.

    Applications: It is used as extender for the RS232. Used in broadcast automation systems and post-production linear editing to remotely control the players/recorders in the central apparatus room.

    RS485: It defines electrical characteristics of drivers and receivers for use in balanced digitalrs232 to rs485 converter multipoint systems. The protocol configures local networks and multi-drop communication links with data transmission speed of 35 M bit/s up to 10 m and 100 kb/bit/s at 1200 m. RS485 spans relatively large distances due to its differential balanced line. They need to be put in transmit mode explicitly by asserting a signal to the driver. Using four wires, it can be made full-duplex.

    3 Differences between a RS422 and RS485

    • Max receiver common-mode voltage: RS422 is -7v to +7v and RS485 IS -7 TO +12V.
    • Receiver input impedance: RS422 is 4k ohm and RS485 is 12k ohm.
    • Min generator load: RS422 100 ohm and RS485 is 60 ohm.

    3 Differences between RS232 and RS485

    • Mode of operation: rs232 is single-ended and rs485 is differential.
    • Total number of drivers: rs232 is 1 and rs485 is 32.
    • Min generator load: RS422 100 ohm and RS485 is 60 ohm.
    RS232 - RS422 - RS485 Design and Engineering

    Although there are marked differences in the various serial communication protocols, the design and engineering of adapters has opened doors to various combinations and applications widely used in the computer and telecommunication industries. Below are examples that vary is design or concept that have been engineered for different applications.

    Industrial isolated serial adapter Selectable serial protocol adapter USB to Serial Ethernet adapter
    Industrial style isolated serial adapter supporting RS422 and RS485. Selectable serial protocol adapters for all signal variations, incorporates USB cable connection. RS232 over Ethernet DB-9 to TCP/IP, a device server adapter.