1. Homework Reading Labs Tokheim, Section 13-6 S and S Extracts
National Semiconductor UART Data Sheet
Labs
Continue labs with your assigned section

2. I/O Devices Examples of I/O Devices
Serial ports
Parallel ports
Floppy, Zip, and hard disk controllers
CD-ROM/CD-RW/DVD controllers
Keyboard Interface
Mouse Interface
Variations do not change the architecture

3. Accessing I/O Devices Can’t do it under Unix Can do it on the SAPCs
Why not?
Can do it on the SAPCs
Why?
SAPCs allow us to learn about accessing I/O devices from “hands-on” experience

4. Addressing I/O Devices
Intel I/O devices have addresses assigned in an “orthogonal” space from memory addresses
Remember the M/IO# signal that is used with the address bus to select memory versus I/O devices?
Use I/O instructions for I/O device addresses
inw inb
outw outb

5. Addressing I/O Devices
The “input” instruction – direct addressing
inw $0xdd, %ax # 8 bit address
inb $0xdd, %al # 8 bit address
The “input” instruction – indirect addressing
movw $0x3f8, %dx
inw (%dx), %ax # 16 bit address
inb (%dx), %al # 16 bit address
Reads from an I/O device to a register

6. Addressing I/O Devices
The “output” instruction – direct addressing
outw %ax, $0xdd # 8 bit address
outb %al, $0xdd # 8 bit address
The “output” instruction – indirect addressing
movw $0x3f8, %dx
outw %ax, (%dx) # 16 bit address
outb %al, (%dx) # 16 bit address
Writes from a register to an I/O device

7. Addressing I/O Devices
In some processor architectures (Motorola), there is no M/IO# signal in the control bus
This is called using “memory mapped I/O”
I/O device addresses are in the same address space as memory
I/O device ports are accessed as memory addresses
Use equivalent of “move” instructions to write or read data to or from I/O device registers as memory

8. Addressing I/O Devices
COM port addresses (Neveln, Table 4.2)
* corrects an error in the text
COM Port
I/O Addresses 1
0x3F8-0x3FF 2
0x2F8-0x2FF 3
0x3E8-0x3EF 4
0x2E8-0x2EF *

9. I/O Devices We’ll discuss 2 types of I/O devices in detail:
Serial ports
Parallel ports
Covering the following aspects:
Physical connectors
Overview of interface electronics
Handshake procedures
I/O addresses assigned
Programming procedures

10. Serial Ports (COM1: and COM2:)
EIA RS-232C interface same connector as LPT1:
COM1: a DB-9 connector on back of computer with a subset of the RS-232C signals (sufficient for async use)
Requires a conversion cable (DB9 - DB25) to connect a PC to a standard RS-232C device such as analog modem
“RS-232” level signals
+3 to +15 volts is considered a logic 0
- 3 to - 15 volts is considered a logic 1
(Note: + 12 and -12 are voltages usually used) 13 1 25 14 5 1 9 6

11. Serial Port
DB9 Pin Out
Pin 1 Data Carrier Detect (DCD) Input
Pin 2 Receive Data (RXD) Input
Pin 3 Transmit Data (TXD) Output
Pin 4 Data Terminal Ready (DTR) Output
Pin 5 Signal Ground ---
Pin 6 Data Set Ready (DSR) Input
Pin 7 Request to Send (RTS) Output
Pin 8 Clear to Send (CTS) Input
Pin 9 Ring Indicator (RI) Input
Single wire for sending data and single wire for receiving data plus return path (i.e., ground)
Multiple control and status signals

12. Serial Port The “inside story” on a serial port:
Control Bus (M/IO# and W/R#)
Physical
Connector
+5V
Address Bus (16 bits)
+12V
Transmit Data
To/From
Processor
Chip
Receive Data
EIA-423
Drivers
and
Receivers
National / 16550
Called a UART
(“You-art”)
4 Status Lines
2 Control Lines
Ground Reference
Data Bus (up to 32 bits)
-12V

13. Serial Port Handshake
Connecting PC to an access server via a pair of modems
Control / Status Lines (two straight-through cables)
Data Terminal Ready indicates that PC is on and ready
Data Set Ready indicates that modem is on and ready
With Request to Send, PC tells modem to turn on its carrier
With Clear to Send, the modem indicates that carrier is on
With Data Carrier Detect, the modem indicates carrier seen
With Ring Indicator, modem indicates incoming call
DTR
DTR
DSR
DSR
RTS
Analog signals
on phone line
RTS
CTS
CTS
Remote
Access
Server PC
DCD
Modem
Modem
DCD RI RI
TXD
TXD
RXD
RXD
GND
GND

14. Serial Port Handshake Connecting two PCs via a NULL modem cable
Behaves like a pair of modems
Control / status lines are “cross-connected”
Transmit and receive data are “cross-connected”
DTR
DTR
DSR
DSR
RTS
RTS
CTS
CTS
DCD
DCD
GND
GND
TXD
TXD
RXD
RXD
(RI not normally needed)

15. Serial Port Handshake
Bits are sent on TXD and RXD serially (one at a time)
Bit Rate needs to be specified
When the sequence starts and stops has to be specified
How the bits are serialized has to be specified
ASCII character sent
With LSB first in time
Optional
Parity Bit
Arbitrary time since
last character sent
Bit value = 0
+12V D0 D1 D2 D3 D4 D5 D6 D7
-12V
Bit value = 1
One Start Bit
One or Two Stop Bits
Bit Duration = 1 / Bit Rate

16. Parallel Serial Conversion
UART performs double buffered, bidirectional, parallel-to-serial / serial-to-parallel conversion:
Overrun Error
Receive Holding
Register
Data Ready
Receive Shift
Register
RXD
(Serial)
Data Bus
(Parallel)
TXD
(Serial)
Transmit Holding
Register
Transmit Shift
Register
THRE
TX Empty

17. Accessing the Serial Port
PC specification allows up to four serial ports
COM1: base address is 0x3f8
COM2: base address is 0x2f8
Write
Read
0x3f8 D7 D6 D5 D4 D3 D2 D1 D0 D7 D6 D5 D4 D3 D2 D1 D0
0x3fb
Set
Brk
Stk
Par
Evn
Par
Par
Enb #
Stop
Len
Sel 1
Len
Sel 0
DLAB
Same as Write
0x3fc
Loop
Out2
Out1
RTS
DTR
Same as Write
- - -
0x3fd RX
ERR TX
EMP
THRE
BRK
Int
FRM
ERR
PAR
ERR
OVRN
ERR
Data
RDY
- - -
0x3fe
DCD
CHG TE RI
DSR
CHG
CTS
CHG
DCD RI
DSR
CTS

18. Accessing the Serial Port
PC specification allows up to four serial ports
COM1: base address is 0x3F8
COM2: base address is 0x2F8
Each has up to eight port addresses
Usually use six of these addresses
Base: Receive buffer on read / Transmit buffer on write
Base Interrupts and FIFO buffer
Base+2: Interrupt ID
Base+3: Line control (set up by Tutor for us)
Base+4: Modem control
Base+5: Line status
Base+6: Modem Status

19. Accessing the Serial Port
Don’t want to use hard coded numbers!
Look at $pcinc/serial.h for symbolic constants
#define COM1_BASE 0x3f8
#define COM2_BASE 0x2f8
#define UART_TX 0 /* send data */
#define UART_RX 0 /* recv data */
. . .
#define UART_LCR 3 /* line control */
#define UART_MCR 4 /* modem control */
#define UART_LSR 5 /* line status */
#define UART_MSR 6 /* modem status */

20. Accessing the Serial Port
Examples:
Send an ‘A’ out on COM2: (port mtip connected to)
ps 2f8 41 (ASCII A = 0x41)
And you will see:
ATutor> (Character A then prompt)
Read a character from COM2:
pd 2f8
02f c1 03 0b ff ff ff ff ff ff ff ff

21. Accessing the Serial Port
Port access support for C programs
Can use functions specific to the PC
We have our own library ($pcinc/cpu.h)
Look at example $pcex/echo.c
Function prototypes are in cpu.h
void outpt(int port, unsigned char outbyte);
unsigned char inpt(int port);
Port address < 0xFFFF
Unsigned char is the 8-bit character
Example for COM2:
outpt(0x2F8, 0x41);

22. Accessing the Serial Port
Construct addresses using symbolic constants
unsigned char status;
outpt(COM1_BASE + UART_TX, ‘A’);
status = inpt(COM1_BASE + UART_LSR);

23. Parallel Port (LPT1:) LPT1: a DB25 connector on back of computer
Data appears on pins 2-9
Control/Status on pins 1 & 10-17
Pins are ground
“TTL” level signals
0-1volts is considered low and a logic 0
3-5volts is considered high and a logic 1
Very simple interface to understand and use
Provides 8 bits of output (one byte at a time)
No transformation of data
Simple handshake protocol 13 1 25 14

24. Parallel Port The “inside story” on a parallel port:
Control Bus (M/IO# and W/R#)
+5V
Physical
Connector
Address Bus (16 bits)
8 Data Lines (D0–D7)
To/From
Processor
Chip
5 Status Lines (Busy)
Interface
LSI Chip(s)
4 Control Lines (Strobe#)
Ground Reference Lines
Data Bus (up to 32 bits)

25. Parallel Port Printer Handshake
Data byte sent to parallel data port (all 8 bits at once)
1. Printer indicates ready for next data byte (Busy = 0)
2. PC sets up data bits on data lines D0-D7
3. PC tells printer that data is ready Strobe# = 0
4. Printer acknowledges or “acks” (Busy = 1) and takes data
5. PC sets Strobe# =1 to be ready for next cycle
Signals on Pins
One Handshake Cycle
D0-D7
(from PC)
Byte N-1 2
(Data valid for Byte N) 2
Byte N+1
* * *
Strobe#
(from PC) 5 3 5 3
* * *
Busy
(from Prtr) 4 1 1
* * *

26. Accessing Parallel Port
IBM defines up to three parallel port addresses
We will use “LPT1:” with 0x378 as base address
Base used to send data to printer (D0-D7)
Base+1 used to get status byte (with MSB = Busy# signal)
Base+2 used for control (with LSB = Strobe signal)
Can access parallel port using Tutor ‘ps’ command
ps 378 FF to set all data output bits to ones
ps to set all data output bits to zeros
Write
Read
0x378 D7 D6 D5 D4 D3 D2 D1 D0 D7 D6 D5 D4 D3 D2 D1 D0
0x379
- - -
Bsy#
Ack# PE SL
Err#
IRQ
0x37a D IQ SI
IN# AF ST D IQ SI
IN# AF ST

27. Accessing Parallel Port
Port access support for C programs
Can use functions specific to the PC
We have our own library ($pcinc/cpu.h)
Look at example $pcex/testlp.c
Function prototypes are in $pcinc/cpu.h
void outpt(int port, unsigned char outbyte);
unsigned char inpt(int port);
Port address < 0xFFFF
Unsigned char is the 8-bit character
Example:
outpt(0x378, 0xFF);

28. Accessing the Parallel Port
Examples:
Note that status port address is “read only”
pd 378
F E ps
F E {has effect on 378} ps
F E {no effect on 379}

29. Accessing the Parallel Port
Don’t want to use hard coded numbers!
Look at $pcinc/lp.h for symbolic constants
#define LPT1_BASE 0x378
#define LP_DATA /* 8 bits of data */
#define LP_STATUS 1 /* in: status bits */
#define LP_CNTRL /* in, out: control bits*/
Construct addresses using symbolic constants
unsigned char cntrl, status;
outpt(LPT1_BASE + LP_CNTRL, cntrl);
status = inpt(LPT1_BASE + LP_STATUS);

30. Strategies for I/O Driver Code
Two Basic Strategies for I/O Driver Code
Status Polling
Interrupt Driven
Uses only the port addresses on the I/O device
Ties up the entire processor for the duration of I/O
Adds an interrupt line from I/O device to processor
Allows processor to do other work during I/O

31. Status Polling Review the serial port details:
Status and Control Registers
We will look at assembly language driver to send and receive data in “full duplex” mode
Half Duplex – Sending or receiving alternately (data going only one direction at a time)
Full Duplex – Sending and receiving at same time (data going both directions simultaneously)

32. Initializing the UART Tutor does this for us on COM1: and COM2:
Select speed, data bits, parity, and number of stop bits
Turn on DTR and wait for DSR on
Half duplex mode modem signal handshake:
Transmit: Turn on RTS and wait for CTS on
Receive: Turn off RTS and wait for DCD on
Full duplex mode modem signal handshake:
Turn on RTS and leave it on
Transmit whenever CTS on
Receive whenever DCD on

33. Status Polling Loop on send/receive data to/from COM2:
(Assume Tutor has initialized bit rate and line control)
1. Turn on DTR & RTS, wait for DSR, CTS, & DCD
2. Read data ready (DR)
3. If data is ready, read a byte of receive data
4. Read transmit holding register empty (THRE)
5. If THR is empty, write a byte of transmit data
6. Jump back to step 2
Processor loop is much faster than byte transfer rate
But, hard to do other work while looping on status

34. Status Polling Assembly Code
Step 1a: Turn on DTR and RTS
movw $0x2fc, %dx # modem control
inb (%dx), %al # get current
orb $0x03, %al # or on 2 lsbs
outb %al, (%dx) # set control

35. Status Polling Assembly Code
Step 1b: Wait for DSR, CTS, and DCD
movw $0x2fe, %dx # modem status
loop1:
inb (%dx), %al # get current
andb $0xb0, %al # get 3 signals
xorb $0xb0, %al # check all 3
jnz loop1 # some missing
# all 3 are on now

36. Status Polling Assembly Code
Step 2: Read Data Ready
Step 3: If ready, read a byte from receive data
loop2:
movw $0x2fd, %dx # line status
inb (%dx), %al # get data ready
andb $0x01, %al # look at dr
jz xmit # if recv data
movw $0x2f8, %dx # i/o data addr
inb (%dx), %al # move rx to %al
movb %al, somewhere # save it somewhere

37. Status Polling Assembly Code
Step 4: Read transmit holding register empty
Step 5: If empty, write a byte to transmit data
xmit:
inb (%dx), %al # get thre
andb $0x20, %al # look at thre
jz loop2 # if tx hr empty
movb somewhere, %al # get data to send
movw $0x2f8, %dx # i/o data addr
outb %al, (%dx) # send it
jmp loop2 # and loop

38. COM Port Driver in C - Receive
#include <serial.h>
void unsigned char pollgetc() {
/* polling loop, waiting for DR bit to go on */
while ((inpt(COM1_BASE + UART_LSR) & UART_LSR_DR) == 0) ;
/* input character */
return inpt(COM1_BASE + UART_RX); }

39. COM Port Driver in C - Transmit
#include <serial.h>
void pollputc(unsigned char ch) {
/* polling loop, waiting for THRE bit to go on */
while ((inpt(COM1_BASE + UART_LSR) & UART_LSR_THRE) == 0) ;
/* output character */
outpt(COM1_BASE + UART_TX, ch); }