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5: Serial I/O

5: Serial I/O. CET360 Microprocessor Engineering by J. Sumey. Introduction. serial , i.e. bit-at-a-time , interfacing techniques are useful when parallel interfacing limitations become problematic distance limitations due to crosstalk cabling costs

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5: Serial I/O

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  1. 5: Serial I/O CET360Microprocessor Engineeringby J. Sumey

  2. Introduction • serial, i.e. bit-at-a-time, interfacing techniques are useful when parallel interfacing limitations become problematic • distance limitations due to crosstalk • cabling costs • was popularized in the 1960s by teletypes and modem technologies • standardized as RS-232C in 1969 by the EIA • used by many I/O peripherals • mice, printers, digital cameras, PDAs, GPS receivers,POS terminals (cash drawers & scanners) • predecessor to USB 11001010000100101110

  3. Serial Interface Components • at a minimum, a serial interface requires: • transmit / receive registers • holds parallel data before/after serialization • PISO/SIPO shift registers • handles serialization/deserialization • clock circuitry (Baud rate generator) • timing signals used by shift registers • referred to as UARTs or SCI • Universal Asynchronous Receiver/Transmitter • Serial Communication Interface (Motorola) • also: COM ports on PCs

  4. Example: S12 SCI

  5. Serial Data Transmission Issues • a pair of devices communicating over a serial link must agree on a number of parameters… • communication type • data coding • transmission (Baud) rate • clocking method • error handling • handshaking

  6. 1 – Communication Type • refers to how the 2 devices communicate with each other • simplex – fixed, 1-way over a single channel • half-duplex – 2-way over a single, shared (unidirectional but reversible) channel • full-duplex – 2-way over a pair of unidirectional channels

  7. 2 – Data Coding • choice of binary code used to represent alphanumeric data • ASCII – American Standard Code for Information Interchange • EBCDIC – Extended Binary Coded Decimal Interchange Code (IBM) • proprietary – ex: credit card scanner • also may refer to the logic levels used • 1=mark, 0=space

  8. KEY NUL = Null SOH = Start Of Header STX = Start of Transmission ETX = End of Transmission EOT = End of Text ENQ = Enquiry ACK = Acknowledge BEL = Audible Bell BS = BackSpace HT = Horizontal Tab LF = Line Feed VT = Vertical Tab FF = Form Feed CR = Carriage Return SO = Shift Out SI = Shift In DLE = Data Link Escape DCx = Device Control x NAK = Negative Acknowledge SYN = Synchronization char. ETB = End Transmission Block CAN = Cancel EM = End of Medium SUB = Substitute ESC = Escape FS = Field Separator GS = Group Separator RS = Record Separator US = Unit Separator DEL = Delete ASCII Chart!

  9. 3 – Baud Rate • speed at which bits (per sec.) are sent over a serial data link • may (asynchronous) or may not (synchronous) include “overhead” bits • also: bit rate = number of data bits per sec. • common baud rates:110, 300, 1200, 2400, 4800, 9600, 19.2k, 38.4k

  10. 4 – Clocking Method • determines how receiver is synchronized to transmitter • synchronous • same clock source is either sent along with or encoded within a continuous data stream, ex: Ethernet LAN • bit rate = baud rate • asynchronous • no common clock, Tx & Rx run from separate clocks • data bits are framed with Start/Stop bits • Start bit: a fixed 0 bit prefixing the data bits • Stop bit: a fixed 1 bit suffixing the data bits • bit rate < baud rate (because of overhead)

  11. Asynchronous Format • line is held in inactive mark state (1) when no data to send • data bits are sent/received LSB to MSB • characters may be sent back-to-back • but not a requirement! • ex – 2 chars using 8 data, 1 stop, no parity: • challenge: show waveform for the message “Hi”

  12. 5 – Error Handling • deals with how errors are detected and/or corrected • parity • each sent character may have an odd or even parity bit added to it, directly before the stop bit • based on the total number of 1s in the character (including parity bit) • used for simple error detection, only 50% accurate • checksum • better method based on 1’s complement of binary sum • 8-bit performance: 255/256 accuracy, also 16-bit, 32-bit • cyclic redundancy check (CRC) • more sophisticated error detection mechanism based on polynomial math, 99.99%+ accurate

  13. 6 - Handshaking • additional control interface signals • RTS/CTS – Request To Send/Clear To Send • DTR/DSR – Data Terminal Ready/Data Set Ready • DCD – Data Carrier Detect • RI – Ring Indicate • used to control / throttle the transmitter • may be optional depending on hardware and software implementation

  14. Serial Communication Standards • a number of industry-wide serial standards are commonly used, some have been defined by the Electronic Industries Alliance (EIA) • 20/60ma current loop • T.T.L. • RS-232C (1969) • RS-423  EIA-423 • RS-422  EIA-422 • RS-485  EIA-485 • USB • SATA

  15. Serial Standards • 20/60ma current loop • original standard used by teletypes • I=1, no I=0 • once popular in (noisy) industrial settings • T.T.L. • simple, logic-level based • +5VDC=1, 0VDC=0 • good for on-PCB device-to-device transfer, otherwise not useful for much distance due to noise vulnerability

  16. RS-232C Standard • defines many characteristics of a serial interface • 20 signal names & functions: TxD, RxD, RTS, CTS, etc. • connectors & pinouts: DB-25, DE-9, RJ-45 • cable specs – upto 20kbps upto 50’ • voltage levels • 1=mark=off=-3..-25V, 0=space=on=+3..+25V • notice avoidance of ground level!

  17. RS-232C Standard • classifies devices as being either:Data Terminal Equipment (DTE) orData Communication Equipment (DCE) • defines which pins transmit, receive, etc. • requires interface drivers to convert logic levels • ex: Maxim’s MAX232http://datasheets.maxim-ic.com/en/ds/MAX220-MAX249.pdf

  18. MAX232/MAX232A 5V Supply! Charge Pump Inverter “Logic” side “EIA” side 18

  19. RS-232C Pinouts* *with respect to DTE device

  20. EIA-423 Standard • an improved version of RS-232 using high-performance drivers and receivers • single-ended / unbalanced like RS-232 but with better performance • provides up to 100 kbps and up to 4000' • supports multi-drop connections • up to 10 receivers on a single line

  21. EIA-422 Standard • similar to EIA-423 but with even better performance • provides up to 10 Mbps and upto 4000' • eliminates ground loop problems by transmitting bits as a voltage difference over a unidirectional wire pair • has a very high common mode rejection ratio (CMRR) • works better with long cable runs • Note: Ethernet LANs work the same way! • great for use as an RS-232 extender

  22. EIA-422 Example • RS-422 4-wire network using balanced differential drivers (generators) and receivers

  23. EIA-485 Standard • similar to EIA-422 but allows multiple receivers and drivers (up to 32 each) to share a single 2-wire differential pair in half-duplex fashion • thus, is multi-point… • readily supports low-cost local networking • ex: PLCs on factory floors, home/building automation • provides up to 35 Mbps and up to 4000'

  24. EIA-485 2-wire Network Example

  25. Universal Serial Bus (USB) • similar to EIA-422 (half-duplex, differential), but uses communication protocols to packetize data transfers • used by current PCs to support connection of modern peripherals • supports upto 127 devices, upto 16’ • 4 different transfer speeds • low speed (1.0) = 1.5Mbps • full speed (1.1) = 12Mbps • hi speed (2.0) = 480Mbps • superspeed (3.0) = 4.8Gbps

  26. References • Wikipedia Articles: • http://en.wikipedia.org/wiki/Current_loop • http://en.wikipedia.org/wiki/RS-232 • http://en.wikipedia.org/wiki/RS-485 • RS-232 tutorial: • http://www.radio-electronics.com/info/telecommunications_networks/rs232/rs232-serial-interface-basics-tutorial.php • B&B Electronics RS-422 & RS-485 App Note: • http://www.bb-elec.com/bb-elec/literature/tech/485appnote.pdf • Maxim MAX-232 IC: • http://datasheets.maxim-ic.com/en/ds/MAX220-MAX249.pdf

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