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PRESENTED BY
MITUL MODI(ME MSA 199)
M S UNIVERSITY, BARODA
2012-13
BIOS Serial Data Transmission INT Services
and
Serial Modem Port Control
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Data Transmission Technology
Data transmission technologies are used to connect two devices directly.
These technologies used to provide a physical connection that can be used to
carry many higher level programming
Beside a network card, PARALLEL AND SERIAL PORT provide a way to get
data IN and OUT of microprocessor.
Parallel Port:
1. Transfer 8 bit data at a time.
2. Most common use is to drive printer.
3. LPT1, LPT2,LPT3 : Logical Designators
4. 25 pin D Connector.
Serial Port:
1. Transfer one bit at a time.2. Most common is to connect Modem.
3. COM1, COM2, COM3, COM4 : Logical Designator.
4. 9 pin D Connector.
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Steps for Data Transmission:
When program wants to send something (data) out through port it
1. First load data in data segment register of microprocessor.
2. Initialized serial port.
3. Load necessary parameter in al register for serial data transmission.
4. Use serial data transmission interrupt (INT 14h) to transmit data to port.
Basic Interrupt Processing
Mechanism which causes the processor to stop execution of current program and
transfer control to ISR handler and resume current execution on completion ofISR.
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Step: 2 Initialization of Serial Port:
Execution of any interrupts using BIOS INT 14h. We have to perform
following assembly instructions.
mov ah,CODE ; code is loaded in AH
mov al, PARAMETER ;
int 14h
Code Description
00h Serial Port Initialization
01h Transmit Character
02h Receiving Character
03h Status
Serial Port for Communication:
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Step:3 Necessary Parameter for Communication:
Parameter
Bit Description
0 Word Size 00= 5 01 = 610 =7 11 =81
2 Stop Bit 0 = 1 stop bit
1 = 2 stop bit
3 Parity 00 = None
01 = Odd11 = Even
4
5 Baud Rate : 000 = 110 baud
001 = 150 baud
010 = 300 baud
011 = 600 baud
100 =1200 baud
101 = 2400 baud
110 = 4800 baud
111 = 9600 baud
6
7
Baud rate: a data transmission rate (bits/second) for modems.27 November 2012 5
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Port Initialization Code : (00h)
Setting parameters for necessary communication like word size, stop bit, parity
and baud rate etc. & load accordingly parameter to AL.
Code (i.e. 00h) is loaded in AH register.
INPUT OUTPUT
AX 00h Parameter AX RS232 Status Modem Status
DX Port No. ( 0-3) DX
Port Transmission Code : (01h)
Code (i.e. 01h) is loaded in AH register.
Character is to be transmitted and loaded in AL register.
INPUT OUTPUT
AX 01h Character AX RS232 Status Modem Status
DX Port No. ( 0-3) DX
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There are several many serial transmission standards available for use in
microprocessor including RS232, USB.
Advantages of Serial Bus:
1. Signal Integrity , Less cross talk, lesser noise2. Low cost
Disadvantages of Serial Bus:
1. Higher clock rates require for given bandwidth.
RS232 Interface:
Used for asynchronous serial data transmission.
Nominal Logic Levels : Logic 0 : +12 V Logic 1 : -12 V
allows signals are to be transmitter over greater distance.
specifies maximum cable length 75 feet at 9600 bps.
Common Signals : Pin Desciption
1. Data Carrier Detect : DCD 6. Data Set Ready : DSR2. Receive Data : RD 7. Request To Send : RTS
3. Transmitted Data :TD 8. Clear To Set : CTS
4. Data Terminal Ready : DTR 9. Ring Indicator : RI
5. Signal Ground : GND
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DB 25 Pin Description:
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Modem:
Contraction of modulation and demodulation.
Used to send and receive serial data over telephonic line.
Consists of three basic hardware and software components i.e. data pump, controllerand UART .
Audio signal are also known as carrier signal.
Basics of modems:
RS 232 data terminal equipment (DTE) Modem, data communication
equipment (DCE) telephonic line
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Steps for Data Receiving:
When a character arrives
1. Port places an interrupt message in interrupt request queue.
2. Processor stops what it is doing, save the state of flags and address on stack.
3. Transferring control to interrupt handling program.
4. Interrupt handling program changes processor to I/O addressing mode.
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BIOS Serial Port Status: (AH Reg.)
Bit - 7 Bit - 6 Bit - 5 Bit - 4 Bit - 3 Bit -2 Bit - 1 Bit
0
Time
Out
TSRE THRE Break
Detected
Frame
Error
Parity
Error
Overrun
Error
RDA
Received DataAvailable
Transmit Holding Reg. Empty
Transmit Shift Reg. Empty
Occurs if received char. w/ovalid stop bit
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BIOS Modem Port Status: (AL Reg.)
Bit - 7 Bit - 6 Bit - 5 Bit - 4 Bit - 3 Bit -2 Bit - 1 Bit 0
RLSD RI DSR CTS Change
inRLSD
Trailing
EdgeRI
Change
in DSR
Change
in CTS
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Ring Indicator
Data Set Ready
Clear to Send
Received Line
Signal Detect
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Program Code for transmission of data:
.data
strg1 dw R, E, S, I, S, T, E, R
.code.startup
mov ax, @ data ;
mov ds, ax ;
mov ds, [0000h] ; Select Port 1
mov cx, 08h ; Set count equal to String Length
mov ah, 00h ; Serial port initializemov al, 0e7h ; Parameter set up
int 14h ; BIOS Interrupt
LOOP1: lea si, strg1 ; Load effective add. of string
mov ah, 01h ; Code for Transmission
mov al, [si] ;
int 14h ;inc si ;
loop LOOP1 ;
.end
Segment Initialization
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Program Code for receiving of data:
.data
strg2 dw 10 dup(?)
.code.startup
mov ax, @ data ;
mov ds, ax ;
mov ds, [0000h] ; Select Port 1
mov ah, 00h ; Serial port initialize
mov al, 0e7h ; Parameter set upint 14h ; BIOS Interrupt
mov cx, 08h ; Set count equal to String Length
LOOP1: mov ah, 02h ; Code for Receiver
lea di, strg2 ; Load effective add. of string
int 14h ;
mov [di], al ;inc di ;
loop LOOP2 ;
.end
Segment Initialization
Received character copied to destinationAdd.
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