The Intel 8255 Programmable Peripheral Interface (PPI) was a key component in bridging microprocessors with external devices during the early days of digital systems. With versatile I/O ports, multiple operating modes, and ease of programming, the 8255 enabled reliable communication with displays, sensors, and controllers, making it useful in both education and industry.
8255 Programmable Peripheral Interface (PPI) Overview
The Intel 8255 PPI chip is a widely used I/O chip designed to connect microprocessors with external devices. It acts as a communication bridge for peripherals like ADCs, DACs, keyboards, and displays. Supporting both direct and interrupt-driven I/O, it provides flexibility in system design. With three 8-bit bidirectional ports (A, B, C), it delivers 24 configurable I/O lines. Its cost-effectiveness and compatibility with processors such as the Intel 8085/8086 made it a staple in early computer systems, training kits, and industrial controllers.
Features of 8255 PPI Chip
• Programmable interface – Configurable via software instructions to adapt to devices like displays, sensors, and input modules.
• Three 8-bit ports – Ports A, B, and C provide 24 lines that can function as input or output.
• Multiple operating modes –
Mode 0: Simple input/output without handshaking.
Mode 1: Strobed I/O with handshake signals for synchronized communication.
Mode 2: Bidirectional data transfer with handshaking (only on Port A).
• Bit Set/Reset (BSR) – Port C bits can be individually set or cleared for control/status applications.
• Flexible grouping – Ports may be split into 8-bit or 4-bit groups.
• TTL compatibility – Easy integration with standard digital ICs.
• Independent control registers – Each port can operate separately, in different modes or directions.
Pinout of 8255 PPI Chip
| Pin No. | Group | Signal | Description |
|---|---|---|---|
| 1–8 | Port A | PA0–PA7 | 8-bit general-purpose I/O port |
| 9–16 | Port C | PC0–PC7 | Split into PC0–PC3 (lower) and PC4–PC7 (upper); used as I/O or handshake lines |
| 17–24 | Port B | PB0–PB7 | 8-bit general-purpose I/O port |
| 25 | Control | CS’ | Chip select (active low) |
| 26 | Power | Vcc | +5 V supply |
| 27 | Control | RD’ | Read enable |
| 28 | Control | WR’ | Write enable |
| 29 | Control | RESET | Resets all ports to input state |
| 30–37 | Data Bus | D0–D7 | Transfers data/commands between CPU and 8255 |
| 38–39 | Address Pins | A0, A1 | Select internal registers/ports: 00=Port A, 01=Port B, 10=Port C, 11=Control |
| 40 | Ground | GND | Ground reference |
Architecture of 8255 PPI Chip
| Functional Block | Description |
|---|---|
| Data Bus Buffer | Acts as an interface between the CPU’s bidirectional data bus (D7–D0) and the internal 8-bit data bus of the 8255. It temporarily stores and transfers data between the CPU and internal registers or ports. |
| Read/Write Control Logic | Manages all communication between the CPU and the 8255. It interprets control signals such as RD, WR, A0, A1, CS, and RESET to determine the operation type (read, write, or control) and selects the correct port or control register. |
| Control Logic (Decoder) | Decodes the control word sent by the CPU to configure the ports in various modes (Mode 0, 1, or 2) or in Bit Set/Reset (BSR) mode. It determines how each port will operate - as input, output, or handshake. |
| Group A Control | Controls Port A (8 bits: PA7–PA0) and Upper Port C (4 bits: PC7–PC4). It supports Modes 0, 1, and 2, allowing simple I/O, handshake I/O, and bidirectional data transfer |
| Group B Control | Controls Port B (8 bits: PB7–PB0) and Lower Port C (4 bits: PC3–PC0). It supports Modes 0 and 1, allowing basic input/output or handshake-controlled operations. |
| Port A | An 8-bit I/O port that can function as input or output depending on the mode configuration. Supports Modes 0–2 under Group A control. |
| Port B | Another**8-bit I/O port** for data transfer. Operates under Group B control and supports Modes 0 and 1. |
| Port C | A split 8-bit port divided into two 4-bit groups: Upper (PC7–PC4) and Lower (PC3–PC0). These can act as independent I/O ports, control lines, or handshake signals. Individual bits can also be controlled using the Bit Set/Reset (BSR) mode. |
| Internal Data Bus (8-bit) | Connects all internal blocks of the 8255, transferring data and control information between the CPU, control logic, and ports. |
| Power Supply | The chip operates with a +5V DC supply and GND connection to power the entire circuitry. |
Operating Modes and Working Principle of 8255 PPI Chip
The Intel 8255 serves as a programmable interface between the CPU and peripherals, translating bus operations into parallel data transfers. Its operation is ruled by initialization steps and selectable modes:
Reset State
On power-up or reset, all ports (A, B, and C) default to input mode to avoid damaging peripherals with unintended outputs.
Initialization
The CPU must send a control word that configures each port as input/output and selects one of the four operating modes. Until this is done, ports remain inactive.
Operating Modes
Bit Set/Reset (BSR) Mode
• Applies only to Port C.
• Allows individual bits to be set or cleared for control/status tasks.
Mode 0 – Simple I/O
• Basic input/output without handshaking.
• Used for straightforward transfers such as LEDs, switches, and displays.
Mode 1 – Strobed I/O
• Adds handshake signals (STB, ACK, IBF, OBF) via Port C.
• Ensures synchronized CPU ↔ peripheral data transfer.
Mode 2 – Bidirectional I/O
• Available only on Port A.
• Supports two-way transfer with handshake control, useful for high-speed or asynchronous devices.
Read/Write Operations
• Write: The CPU places data on the system bus, and the 8255 decodes the address lines (A0, A1) to direct it to the correct port’s output latch.
• Read: External devices put data on port lines, which the 8255 latches and makes available to the CPU during a read command.
Synchronization
• In Mode 0, data transfers occur directly without handshakes.
• In Modes 1 and 2, handshake signals from Port C coordinate readiness and acceptance, preventing data loss during high-speed or asynchronous transfers.
Interfacing Considerations of 8255 PPI Chip
When designing systems with the 8255, careful interfacing ensures reliability and prevents damage to both the chip and external devices:
• Default Input State – On reset, all ports default to inputs. This avoids conflicts but also means outputs are inactive until configured. The CPU must always send a control word to properly define direction and mode before attempting communication.
• Output Drive Limits – The 8255’s ports can source or sink only limited current (a few milliamps). Directly driving heavy loads like lamps, solenoids, or relays is unsafe. Instead, buffer or driver ICs such as the ULN2803 (Darlington array) or open-collector gates like 7406 are commonly used. These provide higher current capability and protect the PPI.
• Motor Control – For DC motors or stepper motors, the 8255 ports should not connect directly. Instead, outputs must be routed through transistor stages or H-bridge driver circuits. This arrangement allows bidirectional current flow while isolating the PPI from inductive voltage spikes.
• AC Load Switching – Interfacing with AC appliances requires isolation for safety. Mechanical relays or solid-state relays (SSRs) driven through buffer stages ensure that the 8255 only handles control signals, while the actual high-voltage load is safely switched externally.
• Port C Restrictions – Port C’s bits are not always freely usable as general I/O. In Modes 1 and 2, several pins (e.g., STB, ACK, IBF, OBF) are automatically reserved for handshake control. You must account for these reserved lines to avoid conflicts when mixing general I/O with handshaking.
Advantages of 8255 PPI Chip
• CPU Compatibility – The 8255 works seamlessly with processors like the Intel 8085, 8086, and their compatibles. Its design matches standard bus protocols, making integration straightforward without extra glue logic.
• Flexible Port Configuration – With three 8-bit ports (A, B, C), users can configure them as input, output, or a mix depending on the application. The ability to switch between simple I/O (Mode 0) and handshake-driven communication (Modes 1 and 2) allows the same chip to handle a wide variety of tasks.
• Single-Supply Operation – Operating from a standard +5 V supply, the 8255 is easy to power in TTL-based systems. No special regulators or multiple voltage levels are required, simplifying board design.
• Reliable Parallel Data Transfer – The chip provides stable and predictable 8-bit parallel communication, reducing timing uncertainties. This reliability makes it suitable for driving displays, reading sensors, and managing control signals in actual systems.
• Educational Value – Because it is well-documented and widely available, the 8255 has been a key teaching tool in microprocessor labs and training kits. You can quickly understand I/O interfacing concepts through practical experiments with this device.
Applications of 8255 PPI Chip
• Educational Systems – Training kits and lab boards frequently include the 8255 to demonstrate peripheral interfacing concepts. You can practice programming different modes and observe actual interaction with external devices.
• Display Control – The chip drives visual output devices such as seven-segment LEDs, LCD modules, and alphanumeric panels. With its multiple I/O lines, it can refresh displays or send control commands to driver ICs.
• Keyboard Interfacing – Matrix keyboards in early terminals and personal computers were often scanned using the 8255. By configuring some lines as row drivers and others as column sensors, it efficiently detected key presses.
• Motor Control – Stepper motors and DC motors can be controlled when the 8255 is paired with transistor stages, Darlington arrays, or H-bridges. This made it useful in robotics, positioning systems, and automation projects.
• Data Acquisition – When connected to ADCs (Analog-to-Digital Converters) and DACs (Digital-to-Analog Converters), the 8255 provided a complete interface for measurement and control tasks. This enabled microprocessors to handle signals in scientific and industrial equipment.
• Industrial Automation – The 8255 found use in controlling traffic signals, elevator logic, and process monitoring panels. Its ability to reliably manage multiple inputs and outputs made it a low-cost solution for embedded control systems.
• Retro-Computing – Classic machines like the IBM PC/XT and MSX computers employed the 8255 for peripheral interfacing. It was also used in printers and expansion cards, cementing its place in early personal computer history.
8255 PPI Chip Comparison with Other PPIs
8255 vs. 8155
The Intel 8155 combines multiple functions in one package: it offers a small block of static RAM, a programmable timer, and general-purpose I/O ports. This made it suitable for compact systems where memory and timing control were needed. In contrast, the 8255 focuses entirely on programmable I/O, with no built-in memory or timing. Its simpler design made it cheaper and easier to program when the application did not require integrated RAM or timers.
8255 vs. 8259
The 8259 Programmable Interrupt Controller serves a very different purpose: managing hardware interrupts to help the CPU respond quickly to external events. While the 8255 handles parallel I/O data transfer, the 8259 coordinates interrupt signals. In many microprocessor-based systems, the two chips were used together, 8255 for interfacing with devices like keyboards and displays, and 8259 for managing the interrupt requests generated by those devices.
8255 vs. Modern GPIO Expanders
Today’s systems often use I²C or SPI-based GPIO expanders (such as MCP23017 or PCF8574). These devices provide additional I/O pins with fewer connections, saving board space and reducing pin count on the CPU. However, they operate serially, which can be slower compared to the direct parallel access of the 8255. While the 8255 requires more bus lines, its parallel structure allows faster transfers and makes it highly valuable in educational environments, where direct control of individual pins and understanding bus timing are important for learning.
Troubleshooting & Common Issues
Working with the 8255 can sometimes lead to system faults if design rules are not carefully followed. Common issues and remedies include:
• Uninitialized Ports – After reset, all ports default to input mode. If the CPU does not send a proper control word, outputs remain inactive or behave unpredictably. Always program the control register before attempting to read or write data.
• Incorrect Control Words – Misconfigured control words may assign the wrong directions or modes to ports, locking out expected signals. Cross-check control word values against datasheet tables to ensure proper bit settings.
• Handshake Failures – In Modes 1 and 2, Port C provides needed handshake signals (STB, ACK, IBF, OBF). Missing, miswired, or misinterpreted connections lead to stalled or lost transfers. Carefully verify both the wiring and the logic level expectations of connected devices.
• Overloading Outputs – Each port pin can only handle small currents. Driving LEDs directly is possible with resistors, but motors, relays, and lamps require external buffer stages such as transistor arrays or driver ICs. Ignoring this limit risks permanent damage to the chip.
• Bus Conflicts – If multiple devices attempt to drive the system bus at the same time, data corruption or hardware damage can occur. Proper bus arbitration and the use of enable signals (RD’, WR’, CS’) prevent this issue.
• Debugging Tools – When problems persist, test equipment helps isolate faults. Logic analyzers can confirm timing and control signals, while oscilloscopes can check whether the issue arises from noisy hardware wiring or incorrect software initialization.
Conclusion
The Intel 8255 PPI remains a cornerstone of microprocessor interfacing. Though largely replaced by modern GPIO expanders and built-in microcontroller I/O, it continues to serve as an active teaching tool. Its clarity in demonstrating parallel data transfer, port configuration, and handshaking makes it invaluable for anyone.
Frequently Asked Questions [FAQ]
What is the control word in 8255 and why is it important?
The control word is an 8-bit instruction sent by the CPU to configure the 8255’s ports and modes. Without it, all ports remain in their default input state. It defines whether each port acts as input or output and selects between Modes 0, 1, 2, or Bit Set/Reset.
Can the 8255 directly drive motors or relays?
No. The 8255 outputs can only source or sink a few milliamps, which is insufficient for motors or relays. External driver circuits, such as transistor arrays or H-bridges, must be used to handle higher current safely.
Why is the 8255 still used in education today?
The 8255 provides a clear, hands-on way to learn about microprocessor I/O, control words, and parallel data transfer. Its simple architecture helps students understand core concepts before moving on to modern microcontrollers.
What happens if you use Port C in handshake modes?
In Modes 1 and 2, some Port C lines are reserved for handshake signals (like STB, ACK, IBF, OBF). These pins cannot be used as general-purpose I/O during those modes, which you must account for to avoid conflicts.
How does the 8255 differ from modern GPIO expanders?
Unlike I²C/SPI expanders that use serial communication, the 8255 works with a parallel bus, enabling faster transfers but requiring more pins. This makes the 8255 less space-efficient but valuable for actual control and learning bus timing.