8255 PPI Tutorial
The 8255 Programmable Peripheral Interface (PPI) is a very popular and versatile input /
output chip that is easily configured to function in several different configurations. The
8255 is used on several of our range of cards that plug into an available slot in your IBM
PC. This chip allows you to do both digital input and output (DIO) with your PC. For
example, you may want to have your PC turn on a switch, or have a switch electronically
activate your PC to execute a program. Each 8255 has 3 off 8-bit TTL-compatiable I/O
ports which will allows the control of up to 24 individual outputs or to read 24 individual
inputs, or indeed a mixture of both input or output. For example, you could attach this to
a robotic device to control movement by use of motors to control motion and switches to
detect position etc.
Our range of cards utilising the 8255
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DC-0600 8255/8253 Card Dual 8255 (48 lines) plus 8253 Timer / Counter Chip
DC-0610 PCI 8255/8253 Card PCI version Dual 8255 (48 lines) plus 8253 Timer
DC-0620 8255/8253 Lab Card Dual 8255 plus 8253 with Prototype Area.
DC-064x Multi-8255 Card 2x to 8x 8255 (48 to 192 lines) plus Dual 8253 Timer
To use this card some knowledge of programming will be required, the simple examples
below use Microsoft's QuickBasic. This language is fairly easy to obtain and was
supplied with DOS 6.22. By no means is the use of these cards limited to programming
with any particular language, but the language does need to be able to input and output to
ports.
Ports vs. Memory
Addressing ports is different to addressing memory. Ports have port addresses and
memory has memory addresses, port address 1234 is different to memory address 1234.
Our range of cards use port addresses and cannot be set to use memory addresses. Sorry
to go on about this but there is a lot of confusion surrounding this subject.
Theory of Operation
The Base Address
Our range of cards plug into any available 8 or 16-bit slot (also known as an AT or ISA
slot) on your PC's motherboard, just like a sound card or disk drive controller card does.
Your CPU (Central Processing Unit) communicates with cards by knowing the card's
address and sending data to it. By physically using jumpers on the card, we can assign a
set of addresses to the card, then in software, we can tell the CPU what these addresses
are (more about this in the Programming section).
Table 2: DC-0600 Addresses
Option 1 : default
(JP2 Linked)
Option 2
(JP2 Open)
8255 Port
Address [Hex
(dec)]
Address [Hex
(dec)]
Port 1A
300H (768)
360H (864)
Port 1B
301H (769)
361H (865)
Port 1C
302H (770)
362H (866)
Port 1 Control
Reg.
303H (771)
363H (867)
Port 2A
304H (772)
364H (868)
Port 2B
305H (773)
365H (869)
Port 2C
306H (774)
366H (870)
Port 2 Control
Reg.
307H (775)
367H (871)
For notation purposes, a number with an H next to it denotes hexadecimal notation and
plain numbers will denote denote plain decimal.
8255 Configuration
The first thing that must be done, before the chip can be used, is to tell it which
configuration is required. The configuration tells the 8255 whether ports are input or
output and even some strange arrangements called bi-directional and strobed, but these
'funny' modes go a little beyond the scope of this tutorial. The 8255 allows for three
distinct operating modes (Modes 0, 1 and 2) as follows:
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Mode 0: Ports A and B operate as either inputs or outputs and Port C is divided
into two 4-bit groups either of which can be operated as inputs or outputs
Mode 1: Same as Mode 0 but Port C is used for handshaking and control
Mode 2: Port A is bidirectional (both input and output) and Port C is used for
handshaking. Port B is not used.
For most applications using this range of cards Mode 0 will be used.
Each of the 3 ports has 8 bits, each of these bits can be individually set on or off, it's a bit
like having 3 banks of 8 light switches. These bits are configured in groups to be inputs
or outputs allowing their function to either read data into the computer or control data out
of the computer. The various modes can be set by sending a value to the control port. The
control port is Base Address + 3 (i.e. 768+3 = 771 Decimal). The table below shows the
different arrangements that can be configured and the values to be sent to the
configuration port.
TABLE 3: 8255 Control Register Configuration ( Mode 0: )
Control Word [Hex(Dec)]
Port A
Port B
Port C
80H (128)
OUT
OUT
OUT
82H (130)
OUT
IN
OUT
85H (133)
OUT
OUT
IN
87H (135)
OUT
IN
IN
88H (136)
IN
OUT
OUT
8AH (138)
IN
IN
OUT
8CH (140)
IN
OUT
IN
8FH (143)
IN
IN
IN
As mentioned the Control port is Base Address + 3. Port A is always at Base Address;
Port B is Base Address + 1; Port C is Base Address + 2. Thus in our example Ports A, B
and C are at 768, 769 and 770 (Decimal) respectively. By writing say, 128 to the Control
port will then configure the 8255 to have all three Ports set for output. This can be done
using QuickBasic's OUT statement, for example:
110 BaseAddress = 768
120 PortA = BaseAddress
130 ControlPort = BaseAddress + 3
140 OUT ControlPort, 128
150 OUT PortA, 1