2. University of Tehran 1 Interface Design Serial Communications Omid Fatemi

3. University of Tehran 2 Typical Interface Design Connect ComputeConveyCooperate Sense Reality Touch Reality Connect Transform Embedded Systems Micros Assembler, C Real-Time Memory Peripherals Timers DMA PC interfaces HCI Busses Protocols Standards PCI IEEE488 SCSI USB & FireWire CAN

4. University of Tehran 3 Outline Concept of serial communications Synchronous, Asynchronous RS-232 standard Hand shaking UART and USART chips 8250 and 8251 chips

5. University of Tehran 4 What is Serial Communications

6. University of Tehran 5 Serial Communication Types Asynchronous Synchronous Transfer: –Simplex –Half duplex –Full duplex

7. University of Tehran 6 Transfer Types

8. University of Tehran 7 Asynchronous Data Framing Idle (high): Mark Low: Space Overhead? (parity, start, stop)

9. University of Tehran 8 Data Transfer Rate Baud rate bps

10. University of Tehran 9 RS-232 Standard EIA 232 = ITU-T V.24/V.28 Specifies the interface between DTE and DCE: –V.28 : mechanical and electrical characteristics –V.24 : functional and procedural characteristics Even used in applications where there is no DCE –e.g. connecting computer to printer, magnetic card reader, robot, … etc. Introduced in 1962 but is still widely used Stand for Recommended Standard

11. University of Tehran 10 Vocabulary DTE –data terminal equipment –e.g. computer, terminal DCE –data communication equipment –connects DTE to communication lines –e.g. modem

12. University of Tehran 11 DTE Connections

13. University of Tehran 12 Mechanical Characteristics 25-pin connector –9-pin connector is more commonly found in IBM-PC but it covers signals for asynchronous serial communication only Use male connector on DTE and female connector on DCE Note: all signal names are viewed from DTE

14. University of Tehran 13 25-Pin RS232 Connector Source: Duck, Bishop & Read, Data Communications for Engineers, Addison-Wesley

15. University of Tehran 14 9-Pin RS232 Connector

16. University of Tehran 15 Electrical Characteristics Single-ended –one wire per signal, voltage levels are with respect to system common (i.e. signal ground) Mark: –3V to –15V – represent Logic 1, Idle State (OFF) Space: +3 to +15V –represent Logic 0, Active State (ON) Usually swing between –12V to +12V Recommended maximum cable length is 15m, at 20kbps

17. University of Tehran 16 TTL to RS-232 Line drivers and line receivers

18. University of Tehran 17 RS-232 Frame Format … Start bit ASCII Parity Stop bit 111101000001111 Idle A Example

19. University of Tehran 18 RS232 Logic Waveform

20. University of Tehran 19 Function of Signals TD: transmitted data RD: received data DSR: data set ready –indicate whether DCE is powered on DTR: data terminal ready –indicate whether DTR is powered on –turning off DTR causes modem to hang up the line RI: ring indicator –ON when modem detects phone call

21. University of Tehran 20 Function of Signals DCD: data carrier detect –ON when two modems have negotiated successfully and the carrier signal is established on the phone line RTS: request to send –ON when DTE wants to send data –Used to turn on and off modem’s carrier signal in multi-point (i.e. multi-drop) lines –Normally constantly ON in point-to-point lines CTS: clear to send –ON when DCE is ready to receive data SG: signal ground

22. University of Tehran 21 Flow Control Means to ask the transmitter to stop/resume sending in data Required when: –DTE to DCE speed > DCE to DCE speed (e.g. terminal speed = 115.2kbps and line speed = 33.6kbps, in order to benefit from modem’s data compression protocol) »without flow control, the buffer within modem will overflow – sooner or later –the receiving end takes time to process the data and thus cannot be always ready to receive

23. University of Tehran 22 Hardware Flow Control RTS/CTS –the transmitting end activates RTS to inform the receiving end that it has data to send –if the receiving end is ready to receive, it activates CTS –normally used between computer and modem »computer is always ready to receive data but modem is not, because terminal speed > link speed

24. University of Tehran 23 Software Flow Control Xon/Xoff –when the buffer within the receiving end is nearly full, Xoff is sent to the transmitting end to ask it to stop –when data have been processed by the receiving end and the buffer has space again, Xon is sent to the transmitting end to notify it to resume –advantage: only three wires are required (TD, RD and GND) –disadvantage: confusion arises when the transmitted data (e.g. a graphics file) contains a byte equal to 13H (Xoff)

25. University of Tehran 24 RS-232 (con) Communication between two nodes x-off x-on data transmission transmitterreceiver RTS CTS TD CTS RD transmitter data transmission Are you ready to receive? No Yes Send character Software HandshakingHardware Handshaking

26. University of Tehran 25 Null Modem Cables Used to directly connect two DTEs together Many possibilities – depending on whether and how the two DTEs handshake (i.e. doing flow control)

27. University of Tehran 26 Null Modem Cables Examples Source: Horowitz and Hill, The Art of Electronics, Cambridge University Press

28. University of Tehran 27 Other Standards

29. University of Tehran 28 8250/16450/16550 UART

30. University of Tehran 29 UART in PC

31. University of Tehran 30 Registers Transmitter holding register Receiver buffer register Interrupt enable register

32. University of Tehran 31 Registers Interrupt identification register

33. University of Tehran 32 Line Control

34. University of Tehran 33 Modem Registers

35. University of Tehran 34 Line Status

36. University of Tehran 35 Divisor Register

37. University of Tehran 36 Example

38. University of Tehran 37 Example 2

39. University of Tehran 38 Synchronous Protocols

40. University of Tehran 39 CRC In SDLC: G(X) = x**16 + x**12 + x**5 + 1

41. University of Tehran 40 8251 Block Diagram

42. University of Tehran 41 8251 Registers

43. University of Tehran 42 Mode Register

44. University of Tehran 43 Mode Instruction (Asynchronous)

45. University of Tehran 44 Mode Instruction (Synchronous)

46. University of Tehran 45 Command Register

47. University of Tehran 46 Status Register

48. University of Tehran 47 8251 Timing

49. University of Tehran 48 Summary