This document discusses serial communication using the 8085 microprocessor and 8251A USART chip. It describes the basics of synchronous and asynchronous serial transmission including start/stop bits and framing. It provides details on the RS-232 serial interface standard including voltage levels, connectors, and DTE/DCE roles. The 8251A USART chip is explained in detail, including its block diagram, pinout, registers for control, status, and data, and how it converts parallel to serial and vice versa. Modes of operation and initialization of the 8251A are also covered.

1. SERIAL I/O WITH 8085

2. Basics of serial communication

3. SERIAL I/0 MODEDEVICEMODEM(SERIAL)PROCESSOR(PARALLEL)PARALLEL TO SERIALSERIAL TO PARALLEL

5. SYNCHRONOUS TRANSMISSIONCLKTRANSMITTERRECEIVERSYNCSYNCDATA

6. ASYNCHRONOUS TRANSMISSIONStart and stop bits

7. SYNCHRONOUS BLOCK OF DATA TRANSMITTED ALONG WITH SYNC INFORMATION

8. HIGH SPEED TRANSMISSIONASYNCHRONOUSCHARACTER CARRIES INFORMATION WITH START AND STOP BITS

9. WHEN NO DATA IS TRANSMITTED RECEIVER STAYS AT LOGIC 1 CALLED MARK ; LOGIC 0 CALLED SPACE

10. TRANSMISSION STARTS WITH ONE START BIT AND ONE OR MORE STOP BITS . THIS IS FRAMINGSerial Interfaces: RS-232DB25S is a 25 pin connector withfull RS-232 functionalityThe computer socket has a femaleouter casing with male connectingpinsThe terminating cable connectorhas a male outer casing withfemale connecting pins

11. TTL to RS-232

12. Types of equipmentDTE Data Terminal EquipmentOriginally applied to CRT terminals or other input devices

13. Today, DTE mainly applies to a computerDCE Data Communication EquipmentOriginally applied to modems or similar communicationsequipment Still applies today A modem is a device that converts a digital signal (e.g. froman RS232 interface) to an analogue signal for transmissionover a traditional telephone line (MODEM: MOdulator-DEModulator)

14. VOLTAGE LEVEL +3V TO +15 VLOGIC 0VOLTAGE LEVEL -3V TO -15 VLOGIC 1YEP…. THATS RIGHT…..HERE WE USENEGATIVE TRUE LOGICBUT USUSALLY LOGIC 1  +12VLOGIC 0-12V????!!!!!!

15. TO MAKE RS 232 COMPATIBLE WITH TTL LOGIC ,

16. VOLTAGE TRANSLATORS CALLED LINE DRIVERS & LINE RECEIVERS ARE USEDRS232 pins

17. SIGNALS OF RS232

18. We are concerned about,

22. 11 bits required to send a single character (10 if one stop bits are used)

23. Bit rate (bits/sec): actual rate at which bits are transmitted

24. Baud rate: rate at which the signalling elements, used to represent bits, are transmittedDTE Vs DCEDCE MODEMDTEMICROCOMPUTERTXRX22TXRX33GND77

25. DTE Vs DTEDTEDTETXTX22RXRX33GND77*NULL MODEM CONNECTION

26. Typical System Connections77

27. SERIAL INTERFACES : 8251A USART The functions and requirement for SERIAL I/O are,Input port & Output port for interfacing

28. DATA TX MPU converts parallel to serial

29. DATA RX  MPU converts serial to parallel

30. Synchronization between MPU and slow peripheralUSART – Universal Synchronous /Asynchronous Receiver/ Transmitter It incorporates all the above features in a single chip and other sophisticated functions for serial communication. It is a programmable device i.e its functions and specifications for serial i/o can be determined by writing instructions to its internal registers8251A USART  device widely used for serial i/o

31. The 8251A Programmable Communication Interface8251A

32. 8251A- BLOCK DIAGRAM

34. 8251A- PIN DIAGRAM

36. READ/WRITE CONTROL LOGIC AND REGISTERSSIX INPUT SIGNALSTHREE BUFFER REGISTERS STATUS REG.

37. CONTROL REG

38. DATA REG.INPUT SIGNALSCS’ – CHIP SELECT

39. LOGIC 0 8251 is selected by the MPU

40. RD’ – READ SIGNAL

41. LOGIC 0  The MPU reads the status from status register or accepts input from data buffer