- Parallel communication allows transferring multiple bits simultaneously over parallel wires, making it faster than serial communication. However, it requires multiple wires equal to the number of bits, making the implementation more complex. - It is suitable for short distance high data rate communication, but not practical for long distances due to increased cost and complexity of multiple amplifiers and excess cable length for each wire.

1. - ‘n’ data lines between I/O device and the MPU, all bits
transferred simultaneously.
- fast: ‘n’ bits/clock
- complex implementation, since, at least ‘n’ wires are required.
- Used preferably for short distance high data rate communication
- For long distance communication, ‘n’ amplifiers on ‘n’ data lines,
plus ‘n’ times excess cable length would increase cost ‘n’ times
TRANSMITTER
RECEIVER
D0
D1
D2
D3
D4
D5
D6
D7

2.  Simple I/O: No signalling is required, as the I/O device is always
available to send/receive. Nature of data is continuos.
e.g: LED connected to an o/p port; Display devices;
 Simple Strobed I/O: Valid data must be indicated with an
accompanying strobe signal. Nature of data is intermittent.
e.g: A keyboard; ADC

3.  Single Handshaking: Receiver must
confirm with a single
acknowledgement [ACK], so that
transmitter can proceed with the
next data. Nature of data is
essential which should not be lost
or corrupted during transmission.
 Double Handshaking: Used when
the receiver device is not always
ready to receive. The sequence of
events are
- STB low: to query the receiver
- ACK high: Receiver ready
- Data transfer, STB high
- ACK low: Read complete
- Next cycle

4.  8255 is a low cost TTL compatible parallel
interfacing device
Few 82C55[CMOS version]/8255 specifications
Sink Current: 2.5mA
Max Current: 4.0mA
Speed: 8MHz
 Three ports programmable in two groups:
Group A: PA0-PA7; PC4-PC7
Group B: PB0-PB7; PC0-PC3
 The 8255 is selected at four I/O addresses. In the
PC, 8255 or its equivalent is selected at 60h-63h.

6. Mode 0 [Basic Input/ Output]
 “No handshaking” required
 Input port is simple buffered input and output port is simple latched output
 Two 8-bit ports: Port A, Port B
 Two 4-bit ports: PC0-PC3, PC4-PC7
Mode 2 [Strobed Bidirectional bus]
 Single handshaking or double handshaking can be implemented with
software
 Bi-directional Data port: Port A 5-bit Data/Control port: Port C
Mode 1 [Strobed Input/ Output]
 Latched input/ output
 Used at the input port with slower peripherals OR at the output port, when
the CPU has to only occasionally send data
 Single handshaking or double handshaking can be implemented with
software
 Data port: Port A Data/Control port: PC4-PC7
Data port: Port B Data/Control port: PC0-PC3

7.  RESET resets all ports to simple input
ports, mode 0 operation.
 To program the 8255, a command byte
needs to be OUT to the port address
corresponding to the command register
 MSB distinguishes between
Command byte A and B
 Command byte B is used to set/reset
port C bits individually (Bit Set Reset
Mode)

8. Example 1:
Q. Write a program to read DIP switches
and display accordingly the LEDs.
Soln:
To read DIP switches, port A
must be simple input port and to light the
LEDs, port B must be simple output port.
The necessary control word is
1 0 0 1 x 0 0 x 90h
Let 8255 addresses be 60h-63h. The program is:
; program 8255
MVI A,90h ; control word
OUT 63h ; OUT to control register
; begin I/O
IN 60H ; read port A to accumulator
OUT 61h ; OUT to port B from accumulator
RET

9. Example: 2
Q. Write a program to set then reset PC7 till PC0 each after 1 second.
Soln: The following will be the BSR control words to set/ reset each bit of port C

10. Port C will be independent input/output port in mode 0 only. Hence, the
necessary control word to program the 8255 will be
1 0 0 0 0 0 0 0 80h
The program segment is

12.  New valid data is latched at input port from device.
 STB signal is lowered by the device, as an indicator to the 8255 of valid data.
 IBF is raised by the 8255 indicating that port contains data.
 INTR requested by 8255 to the microprocessor.
 Microprocessor issues an I/O RD, as per priority.
 8255 acknowledges to microprocessor lowering INTR flag.
 8255 lowers IBF indicating that port is empty

15. Example 3:
Write a subroutine to read
character from keyboard
and print character.
Soln:
Port A is used in mode 1
strobed input to read
character from keyboard.
Port B is used in mode 1
strobed output to print
character. The control word
to program the 8255 is
1011x10x (B4h).
PC4 must be read to check when a key is pressed (Polled I/O), or
interrupt can be enabled (Interrupt-driven). To enable INTEA, set PC4. The BSR
control word is 0xxx1001 (09h).
To print a character, OBFB must be checked for not full (not zero), then
the character can be sent. To read PC1, read port C, and AND with bit pattern
00000010 (02h) (mask all bits except PC1).
Let the addresses be FCh-FFh. The program is