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The RS-232 Protocol
An Introduction with Examples in Java


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Overview
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  • Purpose:
    • Serial communications between Data Terminal Equipment (DTE) and Data Communications Equipment (DCE)
  • Original Application:
    • Communications between a terminal (the DTE) and a modem (the DCE)
History
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  • The Standards Organization:
    • The Electronic Industries Association (EIA)
  • The Timeline:
    • 1960 - Original adoption
    • 1969 - Third version (RS-232C)
Serial Communications
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  • The (Almost) Simplest System:
    • One wire (i.e., circuit) is used to transmit a sequence of bits by raising and lower the signal
    • Another wire is used as a ground
  • Problems with this System:
    • Bits can easily be lost
    • One lost bit contaminates the remainder of the transmission
Serial Communications (cont.)
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  • Reducing Errors with Timing:
    • Have a specific, carefully measured, amount of time between data bits
  • Reducing Errors with Markers:
    • Send a specific bit pattern between data "words"
Common Connectors
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  • DB-25 Connector:
    • images/db25-connector.gif
  • DB-9 Connector:
    • images/db9-connector.gif
One Popular Scheme
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  • Data Words:
    • 7 or 8 bits
    • Least significant bit first
  • Markers:
    • 1 start bit before the word
    • 1 parity bit and 1 stop bit after the word
  • Possible Parity Bits:
    • Mark - always 1
    • Space - always 0
    • Even - make the number of 1s even
    • Odd - make the number of 1s odd
    • None
RS-232C Pins/Signals (DB-25, DB-9)
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  • Clear to Send (5, 8)
  • Data Carrier Detect (8, 1)
  • Data Set Ready (6, 6)
  • Data Terminal Ready (20, 4)
  • Signal Ground (7, 5)
  • Transmitted Data, DTE to DCE (2,3)
  • Received Data, DCE to DTE (3,2)
  • Request to Send (4, 7)
  • Ring Detector (22, 9)
RS-232C Details
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  • Voltages:
    • Low (logical 1) is between -5 and -15 volts
    • High (logical 0) is between +5 and +15 volts
  • Transmitted Data:
    • Transmits when low on CTS, DTR, DSR, DCD
  • Received Data:
    • Transmits when low on RTS, DTR, DSR, DCD
The Java Communications API
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  • Supports:
    • RS232 serial ports
    • IEEE1284 parallel ports
  • Allows:
    • Port enumeration
    • Port ownership
    • Synchronous and asynchronous communications
The Architecture
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  • Makes extensive use of the observer pattern
  • Is delivered as a .jar file and a native dynamic link library (see the javacomm site)
An Example
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Reading NMEA Sentences from a Serial GPS Receiver

javaexamples/navigation/SerialGPSLogger.java
 
There's Always More to Learn
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