1. Lecture 13 Basic I/O Interface
Presented By
Dr. Shazzad Hosain
Asst. Prof. EECS, NSU

2. I/O Port Address Decoding
I/O port address decoding is very similar to address decoding
Any I/O device is addressed by a port number
Reading or writing to an I/O device is to read or write to that port address

3. INS Instructions
INSB – Input String Byte, from I/O device to memory location
Table 4-14: From Brey’s Book

4. Example
Read 50 bytes of data from an I/O device whose address in 03ACH and store the data in LISTS array
Example 4-7: From Brey’s Book

5. OUTS Instructions
OUTSB – Output String Byte, from string memory location to I/O device
Table 4-15: From Brey’s Book

6. Example
Transfer data form memory array (ARRAY) to an I/O device at I/O address 3ACH
Example 4-8: From Brey’s Book

7. Decoding 8-Bit I/O Addresses
Decode 8-bit I/O ports F0H – F7H using 74ALS138 decoder
0000 = F0H
= F7H

8. Decoding 8-Bit I/O Addresses
Develop an I/O port decoder, using a 74ALS138, for the 8-bit I/O port addresses 10H, 12H, 14H, 16H, 18H, 1AH, 1CH and 1EH

9. The Programmable Peripheral Interface (PPI)
82C55 is a very popular low-cost interfacing component found in many applications
It sits between microprocessor and the I/O devices
It has three ports
Port A
Port B
Port C
Also has an internal command register

10. Why Peripheral?
I/O Devices

11. Interfacing 82C55
Connect 82C55 so that it functions at 8-bit I/O port addresses C0H (port A), C2H (port B), C4H (port C), and C6H (command register).
= C0H
0010 = C2H
= C4H
= C6H

12. Interfacing 82C55
Draw the diagram for interfacing 82C55 for the following addresses 8C (Port A), 9C
(Port B), AC (Port C) and BC (command register).
CBA
011

13. References
Chapter 9 The Intel Microprocessors – by Barry B. Brey