• Serial:
  • One bit is sent and received at a time
  • Originally involved a terminal (Data Terminal Equipment) and a modem (Data Communications Equipment)
• Parallel:
  • Multiple bits are sent and received simultaneously
  • Originally used for printers

• The (Almost) Simplest System:
  • One wire (i.e., circuit) is used to transmit a sequence of bits by raising and lowering 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

• 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"

• The Standards Organization:
  • The Electronic Industries Association (EIA)
• The Timeline:
  • 1960 - Original adoption
  • 1969 - Third version (RS-232C)

• DB-25 Connector:
  • 
• DB-9 Connector:
  • 

• 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

• 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)

• 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

• History:
  • Designed to replace serial ports and parallel ports
• Why Serial?
  • Speed advantages outweight the fact that only one bit is transmitted at a time
• Today:
  • A standard that replaces almost all common ports (including power)
  • Managed by the USB Implementers Forum (USB-IF)

• usb4java (usb4java.org):
  • A complete library for accessing USB ports
  • Fairly complicated
• jSerialComm (fazecast.github.io/jSerialComm):
  • A library for RS-232 communication using a USB port
  • Easy to use

    SerialPort[] ports = SerialPort.getCommPorts();
    
    for (SerialPort port:ports)
    {
      String description = port.getPortDescription();
      String path = port.getSystemPortPath();
      System.out.println(description + "\t" + path);
    }
  

    SerialPort port = SerialPort.getCommPort(path); 
    port.openPort();
    port.setComPortTimeouts(SerialPort.TIMEOUT_READ_SEMI_BLOCKING, 0, 0);
    InputStream is = port.getInputStream();