Applies To:
  • CitectSCADA

Question: What are the different Serial Communications available and what are their differences? 

The following information was taken from Quatech Inc.

ommunication, in the current context, refers to the transfer of digital data from the source to the destination. This overview will review serial and parallel communication and their protocols, as well as asynchronous and synchronous communication.

In data communication, streams of digital data are transferred between sources and destinations. If the data transfer occurs one bit at a time across a common signal path, the communication is called serial communication. Other signals may accompany the data signal for timing and handshaking. Among the common serial communication data signal electrical protocols are RS-232, RS-422, and RS-485.

In parallel communication, more than one bit at a time is transferred over separate signal paths to the common destination. Most commonly, eight bits of data are transferred at a time. Common parallel communication protocols include Centronic, PS/2 type bidirectional, EPP, and ECP.

In asynchronous transmission, the source and destination each operate on their own independent clocks. The clocks are used with serial communication to time when each bit in a stream is transferred. Because two separate clocks may vary slightly, the streams of data are short bursts, usually a single character, to eliminate the drift that may occur over time. Just as if you were to compare a clock that runs slow to a perfect clock. Over a few minutes there would be no noticeable difference; however, if you compare them after a week or more, they are obviously out of synchronization. Each character is preceded by a start signal and terminated by one or more stop signals for use by the receiver for synchronization purposes.

For synchronous communication, the source and destination clocks are synchronized to allow for continuous blocks of data to be transferred for improved effeciency. Common protocols that define synchronous communication are HDLC, SDLC, BISYNC and X.25. These protocols may define factors such as error detection, flow control and data format.



RS-232 is the most common type of serial interface used today. RS-232C defines both the signal to be compared to ground to determine the logical tate or simply a single signal that can be positive or negative in respect to ground.

  • Maximum recommended range of 50 feet (15.2 meters)
  • Maximum recommended baud rate of 20Kb/s, frequently exceeded in practice
  • Defines a standard pinout for D-25 connector and for D-9 connectors.


RS-422 defines its signal characteristics as a differential pair with no standard connectors or pin-out defined. The differential pair is one signal transmitted across two separate wires in opposite states; one inverted and one not inverted. The difference in voltage between the two lines is compared by the receiver to determine the logical state of the signal. The idea behind this is if noise is injected on the wires, both lines will be effected equally thus not effecting the difference between the two. Twisted pair type wire is recommended to best keep influences equal on the two lines. RS-422 is normally used in a "4-wire" full duplex mode for point to point communication, but can handle up to 10 receivers per transmitter.

  • Maximum recommended range of 4,000 feet.
  • Maximum recommended baud rate of 10M b/s
  • Well suited for noisy environments; differential signal provides common-mode noise rejection.
  • Receiver input sensitivity of +/- 200mV.
  • One transmitter can drive up to ten receivers.
  • 100 ohm termination placed at receiver furthest from the transmitter.


RS-485 is an upgraded version of RS-422 with the added capability to allow up to 32 devices (transmitters and receivers) to share the same connection (multidrop or "2 wire" mode). This is achieved by use of tristateable drivers usually controlled by a programmable handshake line to ensure only one driver is active at a time. This control must be taken into consideration by the software.

  • Maximum recommended range of 4,000 feet
  • Maximum recommended baud rate of 10M b/s
  • Support full duplex "4-wire" or half duplex "2-wire" communication
  • 120 ohms termination placed at two furthest points of communication link for 60 ohm parallel termination.

For more info check out their Website at WWW.QUATECH.COM .