Local Asynchronous Communications and RS-232 Summary

Data Transmission Fundamentals

• Bit-wise data transmission requires encoding bits into energy (electric current, radio, infrared, light), transmitting through a medium, and decoding energy back into bits.
• Transmitter and receiver must agree on the encoding scheme and transmission timing.
• Parallel transmission sends multiple bits (typically $8\,\text{bits}$ or a byte/character) simultaneously over separate channels or wires synchronized to a clock.
• Serial transmission sends bits sequentially over a single channel, reducing wire costs but lowering transmission speed.

Timing of Serial Transmission

• Synchronous communication occurs in real time with both sender and receiver connected simultaneously.
• Isochronous communication is time-dependent and assigns each data source a fixed time slot within each cycle to guarantee regular transmission opportunities.
• Asynchronous communication allows data to be transmitted at irregular intervals without pre-arranged timing coordination between sender and receiver.

The RS-232 Standard

• Standardized by the Electronic Industries Association (EIA) as RS-232-C for serial, asynchronous character transmission over copper wire.
• Standards organizations specifying communications operation include the International Telecommunications Union (ITU), Electronic Industries Association (EIA), and Institute for Electrical and Electronics Engineers (IEEE).
• RS-232 specifications and constraints:
  • Maximum connection length must be less than $50\,\text{feet}$.
  • Data is represented using voltages between $+15\,\text{V}$ and $-15\,\text{V}$.
  • MARK represents a negative voltage (logical $1$).
  • SPACE represents a positive voltage (logical $0$).
  • When idle, the transmitter puts a negative voltage (a $1$) on the wire and never leaves the wire at $0\,\text{V}$.
  • Hardware utilizes a $25\text{-pin}$ connector where a computer transmits on $\text{pin } 2$ and receives on $\text{pin } 3$ (reversed on a modem).

Character Framing and Transmission Timing

• Asynchronous characters are sent as fixed-size units preceded by a start bit (a zero / SPACE) and followed by $1$ to $2$ stop bits (a one / MARK).
• A character represented by $7\,\text{data bits}$ requires $9\,\text{bits}$ total to transmit across the wire.
• Baud rate measures the number of signal changes per second, whereas bits per second measures the number of bits transmitted per second (in RS-232, baud rate and bits per second are equal).
• A framing error occurs when speed mismatches prevent the stop bit from being received at the expected time; an intentional framing error sent by RS-232 devices is called a BREAK.
• Full-duplex communication allows simultaneous two-way data transmission using separate electrical paths for each direction.

Universal Serial Bus (USB)

• A faster serial communication standard supporting speeds up to $480\,\text{Mbps}$ under USB 2.0.
• Uses a $4\text{-wire}$ cable interface containing $2$ wires for data, $1$ for power, and $1$ for ground signal.