When we think of modern computing, sleek interfaces and complex software come to mind. But dig just a little deeper, and there's a fascinating world of simpler, more direct communication protocols that laid the foundation for everything we use today. One of those is the asynchronous serial interface—raw, byte-by-byte communication between machines. And at the heart of many classic systems enabling this type of communication lies the uPD71051C, a dedicated UART (Universal Asynchronous Receiver Transmitter) chip from NEC.

In this DIY project, we’re going to build a Retro Serial Terminal Controller using the uPD71051C as the core communication interface. This isn’t just a conceptual guide; it’s a full-on, specific project—a tribute to an earlier era of digital communication, brought to life with real components.

This article contains no code, no formulas, and no tables. It’s a hands-on journey, focused on creating a tangible project: a working terminal device that can interact with other vintage or modern serial equipment, such as an old modem, a data logger, or even a vintage computer that still uses RS-232 communication.

Why the uPD71051C?

The uPD71051C is a classic UART IC that handles the job of converting parallel data from a CPU into serial form and vice versa. It's one of those chips that allowed microprocessors to communicate over serial lines without needing complex firmware. Although it's no longer a staple in modern designs, it offers a great hands-on learning experience and brings real-world utility in retro computing, embedded development, and hardware experimentation.

Some reasons to base a project around it:

● It handles low-level serial tasks in hardware, reducing the complexity of design

● It supports standard RS-232 style interfaces

● It’s well-suited to working with vintage CPUs or TTL-based systems

● It introduces the builder to fundamental digital communication techniques

The Goal of the Project

Our project is to build a retro-style serial terminal controller. It will consist of:

● A basic keypad input (hexadecimal or ASCII)

● A simple character display (LED, LCD, or even a 7-segment style setup)

● The uPD71051C UART chip handling the communication logic

● A buffer or register array to hold received/transmitted characters

● A control system that mimics a simple terminal interface

● RS-232 interface to connect to a PC, terminal, or other device

This device will be able to send and receive characters over a serial line, display them locally, and be used as a standalone communication interface—perfect for testing legacy equipment or learning about data flow in serial protocols.

Components You’ll Need

Here’s a breakdown of the main components used in this project:

● uPD71051C UART chip – The heart of the project

● Parallel keypad (4x4 matrix or custom) – For user input

● Alphanumeric LCD (16x2 or similar) – To display sent and received data

● RS-232 transceiver (MAX232 or similar) – To convert TTL to RS-232 voltage levels

● Microprocessor or latch/buffer system – To manage data flow (optional)

● EPROM or hardwired logic for minimal control logic

● Discrete logic chips (e.g., 74LS series) – To manage state and control lines

● Crystal oscillator (1.8432 MHz or suitable) – For the UART baud rate clock

● Voltage regulators (5V) – To power logic circuits

● Capacitors, resistors, headers, and switches

● Prototyping board or custom PCB

● Power source (regulated DC)

Step 1: Planning the Terminal Logic

The idea is simple. When the user presses a key on the keypad, the system sends the corresponding character over the serial interface. When a character is received via the serial port, it is displayed on the LCD. The uPD71051C manages the TX (transmit) and RX (receive) operations.

Since we aren’t using a full microcontroller here, we’ll simulate the controller using a combination of static logic and basic control chips. The UART operates by writing and reading to specific data registers. Control logic will help us detect when data is ready to be sent or received and manage communication timing.

For those comfortable with vintage microprocessor systems, the architecture is reminiscent of how 1980s computers interfaced with serial peripherals: a minimal set of dedicated hardware, a clear purpose, and elegant simplicity.

Step 2: Wiring the UART Core

Begin by placing the uPD71051C on your prototyping board. Wire up the power supply, ground, and connect a 1.8432 MHz crystal oscillator to the clock input. This provides a standard base frequency that allows for accurate baud rates (like 9600, 19200, etc.).

Then, connect the chip’s TX and RX lines to the RS-232 transceiver (such as MAX232), which interfaces with the outside world via a DB9 connector. The chip’s data and control lines (WR, RD, CS, etc.) will be controlled by simple logic—perhaps a pair of switches and some OR/NAND gates to mimic chip enable cycles.

If you prefer, you can build a small sequencer circuit that generates the appropriate write/read pulses based on button presses or reception flags.

Step 3: Input Interface – The Keypad

Your input comes from a keypad. The most straightforward approach is a matrix-style 4x4 keypad that can be scanned via logic. If you don’t want to deal with multiplexing or scanning logic, you can hardwire a set of buttons that each correspond to a fixed ASCII character.

When a button is pressed, the logic circuitry sets up the correct data on the UART’s data input pins, then triggers a WR (write) pulse to send the character. You can use a binary encoder or preprogrammed lookup ROM to simplify the translation of keypress to binary data.

Feedback is essential—so include an LED or small display to flash or indicate when data has been sent.

Step 4: Output Display

For the output, the received data from the UART is latched and displayed on an LCD module or a basic 7-segment arrangement. An alphanumeric LCD is the easiest way to show the character. Connect the data lines of the display to a latch (like 74LS373 or similar) that captures the received byte when the UART signals "data received."

Add a small delay timer or state logic to hold the character visible for a moment before allowing the next one to be displayed. This allows the system to act like a scrolling terminal in its simplest form.

You can get creative here. If you want to replicate a more vintage look, try using an 8x8 LED matrix or Nixie tube display, though that adds complexity and voltage requirements.

Step 5: Final Assembly and Integration

Now, bring everything together. Assemble your UART, keypad, display, and control logic into one cohesive system. Place it in a sturdy enclosure—ideally with a clear display window and neatly mounted buttons.

Connect the DB9 serial port to another machine, such as:

● An old PC with a serial port

● A USB-to-RS232 converter (if you want to connect to a modern computer)

● A network device with serial configuration

● Another microcontroller acting as a host

Power on the device and begin testing. Press keys on the keypad and observe them appear on the receiving terminal. Send characters back from the remote device and watch them appear on your LCD.

Congratulations—you’ve created a fully functional serial terminal controller, driven by vintage UART hardware and basic logic components.

Practical Use Cases

While this project is steeped in nostalgia, it’s also highly functional:

● Test vintage hardware that uses RS-232 interfaces

● Build a console interface for an embedded project or development board

● Create an educational exhibit showing how serial communication works

● Use as a front-end interface for EEPROM programmers, signal analyzers, or debugging tools

● Design custom commands to trigger relays or motors via serial instructions

The beauty of such a build lies in its simplicity and versatility.

Modifications and Enhancements

There’s a lot of room to grow and customize this project:

● Add buffering with shift registers or FIFO chips to queue incoming characters

● Use a microprocessor (like Z80 or 8085) to build a full vintage terminal

● Implement a minimal shell using logic state machines for command-response behavior

● Add a real-time clock to timestamp received messages

● Build a memory log using SRAM chips to store session data

Or go the other direction—make it even simpler by using just LEDs for display and a few DIP switches for input, creating a "bare metal" serial tester.

Closing Thoughts

This project takes you deep into the fundamentals of serial communication without the need for modern coding or software. It’s tactile, hardware-focused, and built on a chip that once served as the foundation of many serious computing systems.

By using the uPD71051C as the backbone of a serial terminal controller, you're not just building a tool—you’re recreating a part of computing history. You learn how data flows, how logic controls timing and sequencing, and how dedicated hardware can perform tasks we now leave to software.

Best of all, this isn’t just a nostalgic piece of tech—it’s a working instrument. Whether you're debugging retro gear, learning electronics, or just enjoying the thrill of building, this project offers a deeply satisfying journey into the heart of hardware.

So dig out that old breadboard, warm up your soldering iron, and breathe life into a UART-powered terminal that speaks the language of bits, bytes, and blinking cursors.