Asynchronous Serial Communication and standards
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This document discusses asynchronous serial communication and standards. It provides an overview of serial communication concepts such as synchronous vs asynchronous communication, baud rate, protocols, and error checking methods. It then describes several common serial communication standards including RS-232, RS-422, and RS-485. Each standard is compared in terms of wiring, signaling type, voltage levels, data rates, cable lengths, and number of drivers and receivers supported. The document also discusses UARTs and how they are used to implement asynchronous serial communication with microcontrollers.
Asynchronous Serial Communication and standards
- 1. Asynchronous Serial Communication and Standards Dr. N. Mathivanan Visiting Professor Department of Instrumentation and Control Engineering National Institute of Technology, TRICHY, TAMILNADU INDIA
- 2. Parallel Communication Serial Communication Cable Use large number of wires Use less number of wires Cable length Can’t use lengthy cables. EMI limits data rate Use long shield cables, protected from EMI Communication modes Only single shared path is available. Hence, can be only half-duplex Can have separate paths for transmission and reception. Hence, can be full-duplex Communication error Bits get corrupted due to capacitance effects between cable wires Only one bit is communicated at a time Data rate It is faster. Latest techniques offer faster / comparable rates. E.g.: PCI-Ex, SATA Serial Communication - Features Dr. N. Mathivanan
- 3. Serial Communication Terminologies • Mark and Space: o Logic ‘H’ is Mark, Logic ‘L’ is Space • BAUD rate: o Baud rate & bit rate (bps) are different. o No. of time the serial communication signal changes (voltage level, frequency or phase angle) in one sec. If for each bit voltage level changes once, then bit rate = baud rate. • Channel: o Pathway between communicating devices o Physical wire, media (propagating radiated energy) • Protocol: o Set of rules defining procedures – no. of bits, framing, formatting, error detection methods, and control of comm. hardware Dr. N. Mathivanan
- 4. • Network o Connecting multiple devices to same media for resource sharing • Point-to-point, Multi-drop, Multipoint o Point-to-point: Interface between devices having peer relationship o Multi-drop: One transmitter communicating with multiple receivers o Multipoint: Several devices communicating with several receivers Interfacing using serial interfaces involves conversion of parallel data bytes to stream of serial bits at transmitter end and detection, collection & conversion of bits to bytes at receiver end. Transmitter & receiver need to be synchronized for communication. • Serial Communication Formats o Synchronous communication o Asynchronous communication Dr. N. Mathivanan
- 5. • Synchronous communication • Transmitter & receiver use common clock • Used for short distance, high volume, in blocks (instead of individual characters) and fast transfer • Standards merge clock & data, eliminate separate wire for clock (e.g. NRZ and bi-phase Manchester encoding) • Synchronous serial communication interfaces: I2C, SPI, Microwire, USB, IEEE1394 (FireWire) Dr. N. Mathivanan
- 6. • Asynchronous communication o Framing Start bit, data bits, parity bit, stop bits – Popular format 8N1 o Character oriented Standard communication speeds: 110, 150, 300, 600, 1200, 2400, 4800, 9600, ………., 115.2 kbaud • Serial communication – Error checking methods o Checksum method o Cyclic redundancy check o Error flags Dr. N. Mathivanan
- 7. • Signal encoding o RZ, NRZL, NRZL, Manchester, Manchester Differential • Compression o E.g.: Run-Length encoding, Huffman encoding Dr. N. Mathivanan
- 8. • Serial communication modes o Simplex, Half duplex, Full duplex • Communication medium o Generally, metallic cables for distance up to 50 m, o Beyond 50 m and up to 4 km, signal is converted to differential and sent thro’ twisted pair cable, o Above 4 km, telephone line / fibre optic cable (has several advantages) used. • Serial communication – Classification of devices o DTE (Data terminal equipment) Devices used as source / destination for the data, E.g.: PC o DCE (Data communication or Data carrier equipment) Devices used in between DTEs, E.g.: Modem Dr. N. Mathivanan
- 9. Asynchronous Serial Interface Standards • RS-232, EIA/TIA-232-F o The standard defines physical & electrical connection, voltage levels, noise margin, speed, and distance. o It is asynchronous interface, o It provides point-to-point interface and peer relationship between devices. • Pins & Signals o DCE devices have 25-pin socket, DTE devices have 25-pin plug o IBM modifies the RS-232 25-pin definition to 9-pin serial port o Applications use 2, 4 or all signals for communication o Signals classified into: Data, Control, Common & Timing signals Dr. N. Mathivanan
- 10. o Data Signals TxD – pin no. 3 in DTE, pin no. 2 in DCE RxD – pin no. 2 in DTE, pin no. 3 in DCE o Control Signals RTS, CTS – used by DTE for hardware handshaking, DTE activates RTS when it has data to send, waits till CTS is activated by DCE DTE starts transmitting, keeps RTS activated till the end of communication DTR, DSR – used by DTE for hardware handshaking DTE activates DTR to inform that it is on-line, ready to establish comm. DCE activates DSR to inform that it is on-line, ready to establish comm. DCD – used by modem (DCE) Indicates DTE when it connects with another modem or detects carrier tone. RI – used by modem Indicates the presence of ringing signal on communication channel o Common Signals: SG Dr. N. Mathivanan
- 11. • Signal voltage levels: o RS-232 voltage levels different from TTL, CMOS voltage levels o Uses bipolar voltages, o Driver output and receiver input voltage profiles - noise margin +/- 2 V o RS-232 uses single-ended signal transmission o Interfacing RS-232 devices to TTL devices require RS-232 driver/receivers (E.g.: MAX232, MC1488 and MC1489A) o Advantages of MAX232 Dr. N. Mathivanan
- 12. • Linking embedded systems / PCs by phone line using modem o Local PC monitors RI input, Remote PC activates RI o Local PC activates DTR, Local modem responds by activating DSR, Local PC monitors DCD, Modem asserts DCD if it receives carrier signal o Data transfer begins using RTS, CTS handshaking o Data transfer takes place via TxD, RxD o When transfer is completed, disables DTR, modem inhibits DSR, DCD signals. Dr. N. Mathivanan
- 13. • Communication between two DTEs • Null Modem wiring without handshaking, with handshaking, with loop-back handshaking Dr. N. Mathivanan
- 14. • Daisy chaining • RS-232 Characteristics o Uses single-ended signaling (unbalanced transmission) o Peer-to-peer communication o Networking – daisy-chaining o Speed Max. – 19.6 kbps, o Distance Max. – 20 m o Drawbacks: noise immunity low, short distance, low speed o Still popular and widely used for communication within 20 m. Dr. N. Mathivanan
- 15. • RS-422 Standard o Characteristics Uses differential signaling (balanced signaling) Can be networked, master-slave, multi-drop, 1 driver & up to 7 receiver Communicates up to 1.2 km at 100 kbps and up to 10 m at 10 Mbps Backward compatible to RS-232 Apple Macintosh computers include RS-422 ports (LocalTalk) Does not define connector, pin configuration o Differential signaling (balanced transmission) Dr. N. Mathivanan
- 16. • RS-422 driver-output and receiver–input voltage profiles • Pins designated as ‘A’ (or ‘+’) and ‘B’ (or ‘-’) • Signals are defined based on voltage at ‘A’ w.r.t. ‘B’ • Common mode voltage within +/- 10 V. • RS-232, TTL, RS-422 voltage levels - Illustration Dr. N. Mathivanan
- 17. • RS-422 half-duplex (2-wire), full-duplex (4-wire) networks o Master transmitter can drive up to 7 slave receivers o Echo cancellation logic needs to be implemented Dr. N. Mathivanan
- 18. • RS-485 Standard • Uses differential signaling (balanced transmission) • Drivers & receivers have enable inputs, voltage profiles similar to RS-422 • Common mode voltage range: +12 V to -7V • Speed: 100 kbps if distance is 10 km, 10 Mbps if distance is less than 20 m • Multi-point communication Dr. N. Mathivanan
- 19. RS-485 networks o Multi master network, o Allows up to 32 master/ slave combination o 2-wire network is used in LAN, o 4-wire network uses 1 master, multiple slaves o Requires termination (120 Ω resistor) Dr. N. Mathivanan
- 20. Comparison of RS-232, RS-422 and RS-485 Characteristic parameter RS-232 RS-422 RS-485 1. Wiring for communication Single-ended Differential Differential 2. Signal type Unbalanced Balanced Balanced 3. Output voltage Logic 0 + 5 to + 15 V w.r.t GND + 2 to + 6 V on terminal A w.r.t B + 2 to + 6 V on terminal A wr.t B Logic 1 - 5 to - 15 V w.r.t GND - 2 to - 6 V on terminal A w.r.t B - 2 to - 6 V on terminal A wr.t B 4. Data rate 20 kbps (max) 10 Mbps at 15 m 100 kpbs at 1200m 10 Mbps at 15 m 100 kpbs at 1200 m 5. Maximum cable length 15 m 1.2 km 10 km 6. Maximum drivers 1 1 32 7. Maximum receivers 1 7 32 8. Source impedance 300 Ω 100 Ω 100 Ω 9. Load impedance 3 to 7 kΩ 4 kΩ 10 Direction Uni-direction Uni-direction Bi-direction 11 Communication type Full-duplex Half-duplex – 2-wire Full-duplex – 4-wire Half-duplex 12 Point-to-point / master-slave Point-to-point Master-slave Master-slave Dr. N. Mathivanan
- 21. Universal Asynchronous Receiver Transmitter (UART) • Basic functions of UART o Converts llel data to stream of bits, adds framing bits, transmits at set rate o Receives stream of bits, removes framing bits, converts to llel data • 16C550 type UART (most common type in microcontrollers, PC) o Inputs & Outputs are TTL compatible o Speed: 0 – 1.5x106 baud o Programmable baud rate generator, o Independent transmitter and receiver blocks o Separate 16 byte FIFOs for transmitter and receiver o 8-bit registers – 12 nos. (data, control and status registers) o Programmed in two stages: Initialization, Operation control: Transmitting characters, Receiving characters Dr. N. Mathivanan
- 22. • Registers o Divisor Latch Low (DLL) byte, High (DLH) byte registers For programming baud rate generator for transmitter o Transmitter Holding Register (THR) Data byte to be transmitted is placed in this register o Receiver Buffer Register (RBR) Data byte received by UART is read from this register o FIFO Control Register (FCR) Used to clear FIFOs, set trigger level, o Line Control Register (LCR) Control framing, allow access of DLL/DLH/THR/RBR register o Line Status Register (LSR) Indicates error in reception, status of transmitter/receiver Dr. N. Mathivanan
- 23. o Modem Control Register (MCR) Controls loopback mode, controls RTS/CTS, DSR/DTR, RI, DCD o Modem Status Register (MSR) Indicates status changes on DCD, RI, DSR, CTS inputs o Interrupt Enable, Interrupt Identification Register (IER, IIR) Control interrupt generation on THR empty, RBR full o Scratch pad register (SCR) Not used in communication, but used to hold temp. data Dr. N. Mathivanan
- 24. • 16C550D Block Diagram UART Dr. N. Mathivanan
- 25. • UART Programming (Algorithm) o Initializing Program DLL and DLH registers Define serial communication format (Program LCR register) Reset FIFOs (Program FCR register) Reset FIFO to enable transmission o Transmitting characters Wait till Transmitter becomes ready Write a byte of character into THR register If more bytes are to be transmitted, write into THR till transmitter FIFO becomes full. o Receiving characters Checks if data has been received (by polling LSR) If received, checks if any error in communication (LSR) If no error, get data from RBR. Dr. N. Mathivanan
- 26. Application - Example • Remote I/O modules o Communicate with host system through serial interfaces o Microcontroller based embedded systems o Small, intelligent, remotely powered, o Provides varieties of analog/digital, input/output interfaces Dr. N. Mathivanan
- 27. Dr. N. Mathivanan Remote data acquisition
- 28. Reference • PC Based Instrumentation: Concepts and Practice N. Mathivanan, PHI Learning, V Printing, 2014 Dr. N. Mathivanan