Explains about parallel port interfacing, software controlling part, bi directional facility.

1. PARALLEL PORT
INTERFACING

2. General Types of Digital Data
Communication
1. Parallel
2. Serial
Parallel
Parallel communication requires as much wires as the no. of
bits in a word for its transmission. Word may be a
combination of 4,8,16,32 or 64 bits etc.
Serial
serial communication usually requires a minimum of 2 wires
for data transmission in one direction. One wire is for data
and other is for ground.

3. Parallel port identification on IBM PC
 Usually ports are found on the rear of computer
and are of the following two types;
1) Male ports
having pins coming out of port.
2) Female ports
having holes for pins.
 Parallel port is generally a 25 pin female
connector with which a printer is usually
attached.

4. Pin configuration of parallel port.

5. Grouping Of Parallel Port Pins
i. Data port
ii. Status port
iii. Control port
Data port:
It includes pin 2 to pin 9
with pin names Data-0
to Data- 9
It is usually for data
output according to old
“standard parallel port”
standard.
Address Bit no. Pin
Name
Pin
Number
Base +0
(e.g.
0×378)
7 Data 7 Pin 9
6 Data 6 Pin 8
5 Data 5 Pin 7
4 Data 4 Pin 6
3 Data 3 Pin 5
2 Data 2 Pin 4
1 Data 1 Pin 3
0 Data 0 Pin 2

6. ii. Status Port
Status port is an
input only port
i.e. Data can’t be
output on this
port but it can
only be read.
Address Bit
no.
Pin Name Pin Number
Base +1
(eg 0×379)
7 Busy Pin 11 (hardware
inverted)
6 Ack Pin 10
5 Paper Out Pin 12
4 Select Pin 13
3 Error Pin 15
2 Irq -
1 Reserved -
0 Reserved -

7. iii. Control port
Control port is a
read / write port.
For printer purposes
it is write only port.
Address Bit
no.
Pin Name Pin Number
Base +2
(eg 0×37A
or 37AH)
7 Unused -
6 Unused -
5 Enable bi-
directional
-
4 Enable Irq Via Ack-
line
-
3 Select printer Pin 17
(Hardware
Inverted)
2 Initialize printer Pin 16
1 Auto line feed Pin 14
(Hardware
Inverted)
0 strobe Pin 1
(Hardware
Inverted)

8. How to access parallel port pins in the software.
Parallel port is assigned a unique I/O address which is
generally among the following:
i) 378h (mostly the case)
ii) 278h (normally found when there are more than
one parallel ports)
iii)3BCh (Rare case)
How to know the port address on your pc
• Restart your computer go into BIOS. Then see the
address of the port there.
• If it is set auto hen there are two options:
i) Explicitly assign an address in the BIOS.
ii) Check the address in Windows by going to
Control panel System Device Manager  PortsPrinter
portProperties Resources Input/Output Range.

9. Difference between memory address space and I/O address
space
•In Intel family of x86 processors the memory addresses and I/O
addresses are separate
Memory addresses
They depend upon the RAM/ROM present in your PC & Address lines.
I/O
This address space is dependent upon no. of address lines available for
accessing I/O devices.
How to address above two type of addresses in C Language
 Memory addresses are ,generally, not known to a C programmer the
programmer can refer to an address by using pointer notation.
 I/O addresses are accessed in Turbo C/C++ environment by using
following Library functions
i) Inport
ii) Inportb
iii) Outport
iv) Outportb

10. Relation between bits present at the port address and voltage
levels on port pins.
• Each bit present at base address of the port has direct link
with the voltage level on corresponding port pin.
i.e. a ‘1’ in register at I/O address corresponds to logic high at
port pin.
A brief overview of logic high and logic Low levels of parallel
port.
In parallel port
• Logic high means a voltage of +5V w.r.t ground.
• Logic low means a voltage of 0V w.r.t ground.
Deviation from above ‘bit-logic level’ relation
In case of hardware-inverted pins the above described relation of
bit and logic level is reversed i.e. ‘1’ corresponds to 0V on port
pin. Examples are busy and strobe.

11. Demonstration of control of light Bulb through parallel port
Schematics of circuit

12. The Software Controlling part
#include<stdio.h>
void main(void)
{
outportb(0x378,0x01); //turn on the bulb
getch(); //wait for any key to be pressed
outportb(0x378,0x00); //turn off the bulb
getch(); // wait for a key to be Pressed
}
C program to control ON/OFF Through keyboard
for (int i=1; i<=5,i++);
{
outport b (0x378,0x01); //turn on the bulb
delay (1000); // wait for 1000msec=1sec
outportb(0x378,0x00); //turn off the bulb
}
C program to have the bulb blink for five times

13. Data input through status port by using DIP switches

14. Range of voltage for logic high and logic low
Above circuit might have been connected as shown
below
But it will give you incorrect results. The
reason is the range of voltage levels that are
considered high or low.
• Parallel port is TTL(transistor transistor
logic) compatible in which
Logic high is a voltage between 0 to 0.8 V
Logic low is a voltage between 2.0 to 5V
• Any voltage in between the above rage is
undefined.
• A voltage higher than the logic high or
lower than the logic low can damage your
port or any other TTL circuit.

15. Some considerations of control port
Control port can be used both for input or out put. However there are some deviations
Open Collector Output
In some hard wares control port pins are open collector. Its equivalent circuit is as
follows:
Logic States of Open Collector Pin
i) Logic low (0V).
It is actually a short circuit to ground as
transistor is conducting.
ii)Logical high ( Open Circuit)

16. How to set a Logic High Voltage level on Open Collector
• You should connect an external resistor with pin to get logic high voltage
which depends upon the supply voltage you use.
• In case of parallel port don’t use Vcc greater than +5V.

17. Introduction to buffering for parallel
interfacing
• Buffers are normally used for purpose of load isolation.
• For digital interfacing we use digital buffers or inverters like
CD4050 (buffer)
CD4049 (inverter)
OR
74 LS245
74 LS244 TTL
Why we need Buffer? Introduction to Source & Sink Currents
Source current
In high output level the port pin or any TTL circuit can provide a certain
maximum amount of output current while maintaining logic high level. This
current is called source current.
• For parallel port this may be up to a minimum of 1mA among various ratings.
• Beyond this current, your voltage level decreases. This must be avoided.

18. Sink current
In case of a pin being in logic low state there is a minimum amount of current
that the pin can absorb called sink current.
• A current larger than sink current can result in severe damage to the logic
device.
• The amount of the sink current is usually higher than corresponding source
current for the same device.
• The usual range is about 10mA for parallel port.
Advantage of buffers
• Buffers have better ratings of the source and sink currents.
• They provide protection to your costly circuits in the event a pin is over
loaded by excessive source/sink Current or an over voltage applied.
It is always a good practice to use buffers for your costly logic devices for both
data in and out.

19. Demonstration of use of Buffers in Data Output

20. Software Code for output of numbers
//program for the out put of the binary no.’s from 0 to 255)
void main (void)
{
for (int i=0;i<256;i++)
{
outportb(0x378,i)
delay(100);
}
}
Software code to implement Moving Light
char k=1;
int i;
for(i=0; i<8; i++) {
outportb(0x378, k<<i);
delay(500); }
k=128;
for(i=0; i<8; i++){
outportb(0x378, k>>i);
delay(500);
}

21. ADVANCED TECHNIQUES IN PARALLEL PORT
INTERFACING
PARALLEL PORT
INTERFACING
PART#2

22. Introduction to Bi-directional Parallel Port
• These days starting from PC’s equipped with Pentium processors, the parallel
ports have Bi-directional read/write facility.
• Bi-directional means that you can read byte (8 bits) from parallel port which
traditionally was an output only port in the old PC’s.
• In order to check whether your port is configured for it, restart your PC and go
into BIOS see the portion related to the parts. You will find port address as
well as its type, usually the options are as follows
SPP standard parallel port not bi-
directional
Bi-
directional
standard parallel port with bi-
directional facility
EPP Enhanced Parallel port
ECP Extended Capabilities parallel port

23. How to configure for Bi-directional Facility?
To accomplish data in through data port you should:
1. set your port to bi-directional, EPP or ECP.
2. write a 1 in bit 5 of control port by using following commands
Example Code:
x = inportb(0x37A); //read control port byte
y = x | 0x20; //perform a bit-wise OR with
//1 in position5 to set bit 5 of x 1
//and store the result in y as 0x20 = 00100000
outportb(0x37A,y); //send byte to control port
Now you can give any external 8-bit digital input on data pins (pin2 to pin 9)
and read the data by using
z=inportb(0x378);

24. Introduction to EPP and ECP
Before we go into details of the EPP & ECP let us see why it was designed.
Printer Hand shake used in standard parallel port (Centronics
Standard)
According to this standard , following hand shake was used:
All above signaling, was done by software in which PC had to wait for a response
from printer before outputting the next signal. It was a slow process with data rate
of 50KB/sec to 150KB/sec.

25. Introduction to EPP (Enhanced Parallel Port)
handshake
EPP was designed to do, the above described, handshaking by a separate hardware
chip instead of main processor. It is described as follows:
EPP Data Write Cycle:
The only port to be done in software is to write the Byte to be sent at Data port (EPP) whose
address is (base address+4) {eg.0x37C}
Note: this data port is different from the data port of SPP. However you can still use that
data port of SPP by usual way.
All above signals are generated automatically in hardware.

26. Description of handshake given on Last slide
i) I) The software programs writes the byte to be sent at EPP data register
(Base +4)
ii) Write is placed low by PC to indicate the printer that some data is to be sent.
iii) After placing the Write low, data is placed on data lines (pin 2 to pin 9)
iv) Wait pin is checked. It is an input from the printer. If it is low then it means
that printer is waiting for receiving data from PC. This being the case, the
PC pulls DATA STROBE low to indicate the printer that it should read data
from data line.
v) The printer reads data from Data lines and brings the WAIT line high fir a
short duration. This indicates to the PC that printer has received data, so it is
Ok to end EPP data write cycle.
vi) DATA STROBE is brought high. WRITE is also brought high.
vii) EPP write cycle Ends here.

27. EPP Address Read Cycle

28. Description of Handshake used for EPP Address Read Cycle.
a. The printer informs the PC that it has to send some data to PC by sending
signal on interrupt line (pin 10, ACK in SPP). This Depends upon software.
Any other input line may be used as well for this signaling.
b. Whenever pc is free, it responds to this signal and reads EPP address register
(Base+3).
c. ADDR STROBE is pulled low if Wait is low. This is done automatically when
processor reads the Address Register. This signals informs the printer that
host (PC) is ready to start an EPP read cycle.
d. Host waits for acknowledgement from Printer by waiting for WAIT line going
high.
e. As Wait line goes high, data is read from data liens (pin 2 to 9)
f. Addr Strobe is pulled high. This ends EPP read cycle.

29. Time-out indication in EPP
There is a possibility that host (PC) starts a write cycle but never receives
acknowledgement from printer when no printer is attatched. As this
operation is done in hardware so the PC will continue waiting & your
computer will halt. In order to avoid this a Watchdog timer scheme is
used it is described as follows.
i) When host writes data to address/Data port of the EPP, a timer is
started at the same time.
ii) Now host waits for an acknowledgement on WAIT line becoming
high. If it does not happen within about 10μsec, (depending on port),
then a time out is indicated by setting Bit0 of status port (base+1).
iii) The host stops waiting and informs the user about the problem.

30. How to setup port for Programming using EPP features
Before you can start any EPP cycle by reading/writing to EPP data and address
port, the port must be configured correctly.
In idle state, an EPP port should have its Address Strobe, Data strobe, wait and
reset lines high. So it is better to initiate these lines manually by writing
xxxx0100 to control port(base+2).
For details please consult the document “Interfacing The Enhanced Parallel
Port” found in Pdf form on “www.beyondlogic.org”.
Here you will find most of the details about interfacing SPP, EPP and ECP
with software example as well.

31. A brief introduction to ECP (Extended
Capability Port)
Enhancements in ECP
ECP supports SPP and EPP features. In addition it has following enhancements.
i) It can use DMA (direct memory access) feature in order to transfer a block of data from memory
automatically without the use of CPU.
ii) It can use (first in first out) FIFO buffers to transfer more than one byte automatically before the
attention of CPU is required.
iii) It supports data compression by using RLE (Run Length Encoding) scheme. This scheme helps
to transmit a single byte if a long string of this same byte is required to be transmitted.
iv) It supports a method of channel addressing by which you can communicate with different
modules of the target. For example with Fax, copier and printer in a single machine
containing these 3.
For details about ECP and its programming details please consult the document:
“Interfacing The Enhanced Parallel Port” found in pdf form on ‘www.beyondlogic.org”.
Here you will find most of the details about interfacing SPP, EPP and ECP with software example
as well.