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, RS-485, and current loop.

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 Centronics, PS/2 type bi-directional, 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 efficiency. 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.

SERIAL COMMUNICATION INTERFACES  

(1) RS-232 INTERFACE

 

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 state or simply a single signal that can be positive or negative in respect to ground.

 

RS-232 Communications FAQ.

What is the maximum length of cable that can be
used in RS-232 communication?

What are some limits of RS-232 communications?

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Q: What is the maximum length of cable that can be used in RS-232 communication?

A: The maximum cable length per RS-232 specification is 50 feet. There is a ground capacitance between the signal wire and SIGNAL GROUND. The specification for RS-232 states that capacitance seen by a driver circuit shall not exceed 2500 pF. The capacitance increases with cable length, and 2500 pF corresponds to approximately 50 feet of cable. In practice using lower baud rates, RS-232 can operate reliably on cables longer than 50 feet.

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Q: What are some limitations for RS-232 communication?

A: There are three big factors that limit the application of RS-232 communication. One of which, distance, was described in the previous question's answer. The other factors are described as follows:

DATA RATE- Data rate limitations are directly related to cable capacitance. RS-232 specifies a fairly high input impedance for the receiver circuit (the impedance is between 3000 ohms and 7000 ohms). This impedance limits the rate at which cable capacitance can be charged and discharged. This leads to a rounding or distortion of the signal edges. At high data rates, this distortion becomes a large part of the signal. RS-232 specification states that the highest data rate without error allowable is 20,000 bits per second.

NOISE- Noise can distort and change the value of a binary signal on a cable. Two sources of noise are :

INTERNAL- If SIGNAL GROUND is not a perfect conductor (0 ohms), the resulting current causes a voltage drop in the SIGNAL GROUND wire which is seen by the receivers for all wires.

EXTERNAL- Signals picked up by fluorescent lights, motors, radio transmitters.

Both of these types of noise are further exaggerated by long cables, which make better "antennas" to unwanted noise on the RS-232 line. External noise can be reduced by using shielded cables, and the wire gauges that used in typical RS-232 cables are usually heavy enough to make voltage drops in the RS-232 SIGNAL GROUND insignificant.

(2) RS-422 INTERFACE

 

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.

 

(3) RS-485 INTERFACE

 

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.

 

(4) CURRENT LOOP

 

Current loop is a type of serial communication used primarily in industrial applications for its high immunity to noise. It is now mostly obsolete because of its slow speeds and the better performances of RS-422 and RS-485. If you find yourself with the need to interface with current loop equipment, Quatech's DCL-624 or DS-225 for ISA, or the SCL-6100 for Micro channel will accommodate your needs.

The first PC-compatible parallel printer ports were unidirectional, allowing 8-bit data transfer only from the host to the peripheral. These early Standard Printer Ports (SPP) implemented eight data lines and used nine handshaking lines, four output from the host and five input to the host. Later came the PS/2 type bi-directional parallel port; this bi-directional port simply added the capability to read 8-bit data from the peripheral to the host. Both the SPP type port and the bi-directional port implemented three registers for the control and monitoring of the data and handshaking lines; these are the data port, status port, and control port. The SPP type parallel ports are most commonly used for printers, plotters, keys, etc.