Serial Port Programming: Serial Communication

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Serial Port Programming

14.1. INTRODUCTION

Serial port is a way of communication among two devices just like the

parallel port. The basic difference is that whole bytes are sent from one place

to another in case of parallel port while the bits are sent one by one on the

serial port in a specially formatted fashion. The serial port connection is a

9pin DB-9 connector with pins assigned as shown below.

1 Carrier Detect

(CD)

Data Set

2 Received Data

Ready

(RD)

(DSR)

3 Transmitted

7 Request to

Data (TD)

Send (RTS)

4 Data Terminal

8 Clear to Send

Ready

(CTS)

(DTR)

9 Ring Indicator

5 Signal Ground

(RI)

We have made a wire that connects signal ground of the two connectors,

the TD of one to the RD of the other and the RD of one to the TD of the other.

This three wire connection is sufficient for full duplex serial communication.

The data on the serial port is sent in a standard format called RS232

communication. The data starts with a 1 bit called the start bit, then five to

eight data bits, an optional parity bit, and one to two 0 bits called stop bits.

The number of data bits, parity bits, and the number of stop bits have to be

configured at both ends. Also the duration of a bit must be precisely known

at both ends called the baud rate of the communication.

The BIOS INT 14 provides serial port services. We will use a mix of BIOS

services and direct port access for our example. A major limitation in using

BIOS is that it does not allows interrupt driven data transfer, i.e. we are

interrupted whenever a byte is ready to be read or a byte can be transferred

since the previous transmission has completed. To achieve this we have to

resort to direct port access. Important BIOS services regarding the serial port

are discussed below.

INT 14 - SERIAL - INITIALIZE PORT

AH = 00h

AL = port parameters

DX = port number (00h-03h)

Return:

AH = line status

AL = modem status

Every bit of line status conveys different information. From most

significant to least significant, the meanings are timeout, transmitter shift

ready, overrun, parity error, and framing error. Modem status is not used in

direct serial communication. The port parameters in AL consist of the baud

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rate, parity scheme, number of stop bits, and number of data bits. The

description of various bits is as under.

data bits

baud rate

00-5

000-110

01-6

001-150

10-7

010-300

parity

11-8

011-600

00-N

100-1200

10-N

stop bits

101-2400

01-O

0-1

110-4800

11-E

1-2

111-9600

INT 14 - SERIAL - WRITE CHARACTER TO PORT

AH = 01h

AL = character to write

DX = port number (00h-03h)

Return:

AH bit 7 = error flag

AH bits 6-0 = port status

INT 14 - SERIAL - READ CHARACTER FROM PORT

AH = 02h

DX = port number (00h-03h)

Return:

AH = line status

AL = received character if AH bit 7 clear

INT 14 - SERIAL - GET PORT STATUS

AH = 03h

DX = port number (00h-03h)

Return:

AH = line status

AL = modem status

Serial port is also accessible via I/O ports. COM1 is accessible via ports

3F8-3FF while COM2 is accessible via 2F8-2FF. The first register at 3F8 (or

2F8 for the other port) is the transmitter holding register if written to and the

receiver buffer register if read from. Other registers of our interest include

3F9 whose bit 0 must be set to enable received data available interrupt and

bit 1 must be set to enable transmitter holding register empty interrupt. Bit 0

of 3FA is set if an interrupt is pending and its bits 1-3 identify the cause of

the interrupt. The three bit causes are as follows.

110

(16550, 82510) timeout interrupt pending

101

(82510) timer interrupt

100

(82510) transmit machine

011

receiver line status interrupt. priority=highest

010

received data available register interrupt. priority=second

001

transmitter holding register empty interrupt. priority=third

000

modem status interrupt. priority=fourth

The register at 3FB is line control register while the one at 3FD is line

status register. The line status register has the same bits as returned in line

status by the get port status BIOS interrupt however the most significant bit

164

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is reserved in this case instead of signaling a timeout. The register at 3FC is

the modem control register. Bit 3 of this register must be set to enable

interrupt generation by the serial port.

14.2. SERIAL COMMUNICATION

We give an example where two computers are connected using a serial

cable made just as described above. The program is to be run on both

computers. After that whatever is typed on one computer appears on the

screen of the other.

Example 14.1

001

; a program using serial port to transfer data back and forth

002

[org 0x0100]

003

jmp start

004

005

screenpos:

; where to display next character

006

007

; subroutine to clear the screen

008

clrscr:

push es

009

push ax

010

push cx

011

push di

012

013

mov

ax,

0xb800

014

mov

es,

;

point es to video base

015

xor

di,

;

point di to top left column

016

mov

ax,

0x0720

;

space char in normal attribute

017

mov

cx,

2000

;

number of screen locations

018

019

cld

; auto increment mode

020

rep

stosw

; clear the whole screen

021

022

pop

023

pop

024

pop

025

pop

026

ret

027

028

serial:

push

029

push

030

push

031

push

032

033

mov

dx, 0x3FA

;

interrupt identification register

034

al, dx

;

read register

035

and

al, 0x0F

;

leave lowerniblle only

036

cmp

al, 4

;

is receiver data available

037

jne

skipall

;

no, leave interrupt handler

038

039

mov

dx, 0x3F8

; data register

040

al, dx

; read character

041

042

mov

dx, 0xB800

043

mov

es, dx

; point es to video memory

044

mov

bx, [cs:screenpos] ; get current screen position

045

mov

[es:bx], al

; write character on screen

046

add

word [cs:screenpos], 2 ; update screen position

047

cmp

word [cs:screenpos], 4000 ; is the screen full

048

jne

skipall

; no, leave interrupt handler

049

050

call clrscr

; clear the screen

051

mov word [cs:screenpos], 0 ; reset screen position

052

053

skipall:

mov

al, 0x20

054

out

0x20, al

; end of interrupt

055

056

pop

057

pop

058

pop

059

pop

060

iret

165

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061

062

start:

call clrscr

; clear the screen

063

064

mov

ah, 0

;

initialize port service

065

mov

al, 0xE3

;

line settings = 9600, 8, N, 1

066

xor

dx, dx

;

port = COM1

067

int

0x14

;

BIOS serial port services

068

069

xor

ax, ax

070

mov

es, ax

; point es to IVT base

071

mov

word [es:0x0C*4], serial

072

mov

[es:0x0C*4+2], cs ; hook serial port interrupt

073

074

mov

dx,

0x3FC

;

modem control register

075

al,

;

read register

076

al,

;

enable bit 3 (OUT2)

077

out

dx,

;

write back to register

078

079

mov

dx,

0x3F9

;

interrupt enable register

080

al,

;

read register

081

al,

;

receiver data interrupt enable

082

out

dx,

;

write back to register

083

084

al, 0x21

; read interrupt mask register

085

and

al, 0xEF

; enable IRQ 4

086

out

0x21, al

; write back to register

087

088

main:

mov ah, 0

; read key service

089

int 0x16

; BIOS keybaord services

090

push ax

; save key for later use

091

092

retest:

mov

ah, 3

;

get line status

093

xor

dx, dx

;

port = COM1

094

int

0x14

;

BIOS keyboard services

095

and

ah, 32

;

trasmitter holding register empty

096

retest

;

no, test again

097

098

pop

; load saved key

099

mov

dx, 0x3F8

; data port

100

out

dx, al

; send on serial port

101

102

jmp

main

166

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