Point to point bus ports

Serial  - RS232
  RS 232 - describes physical and electrical but NOT data encoding.
    Originally designed to connect a data terminal (DTE) to a system or 
    communication system (DCE).
 
  Asynchronous transmission, handshaking signals allowed for non-clocked
    data transmission.


  Earliest terminal consoles.
  Modems
  Mouse
  Printers
  System to system communication.
  'Dumb' sensors.

  9 pin or 25 pin connectors.
    DTR - data terminal ready. (terminal present)(modem, printer, mouse)
    DCD - communication interface connected to system or phone line.
    DSR - Data set ready - DCE ready to receive from DTE.
    RI - ring indicator - DCE has detected incoming ring on phone line.
    RTS - request from DTE to transmit to DCE. DTE->DCE
    CTS - clear to send. DCE ready to receive from DTE. 
    TxD - Transmitted data from DTE to DCE. (minimal)
    RxD - Transmitted data from DCE to DTE. (minimal)
    GND - Common ground - signals. (minimal)
    PG  - Protective ground - grounded shielding around cabling.

  Legacy (com1 com2) on IBM PCs
    Minimal lines
      1 transmit line. From DTE to DCE
      1 receive line. From DCE to DTE
      Common Ground
 
  Data often sent NRZ or NRZI at arbitrary speed. Both devices had to agree. 
    Not part of the RS-232 protocol.

  IBM PC initially allowed for 2 serial ports. 
    (one for mouse, one for modem, one for printer -  OOPS.)

  Modems - Very slow 120 Baud (part of which was timing/error)
    Eventually > 24000 Baud   
 
  Additional hardware and alternative OSes allow more ports. 

  RS232 standard was very loose. Connector, pin-out, voltage level, TX, Rx, Gnd.
    Additional status/handshaking available but not required.

  Declined in usage in late 90's. But still useful where device and control
    separated by large distances (10's to 100's of feet), most often terminals,
    also where "terminal" device fairly dumb, minimal logic.

  Because standard only covered physical/electrical, it was used to couple
    various instruments to computers, allowing the user to directly interact
    with the RS-232 port and interpret the data as they saw fit.