Burroughs Punched Card Reader Restoration

This project documents the restoration of a vintage Burroughs punched card reader by interfacing it with a Raspberry Pi Pico using custom circuitry and firmware. The goal is to non-destructively digitize standard IBM 80-column punch cards and make the device usable with modern systems.

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🧠 Overview

The Burroughs reader uses optical sensors to detect holes in a punch card as it passes over a light path. Analog outputs from these sensors are conditioned by op-amps (LM324) and fed to the Raspberry Pi Pico's GPIOs.

The Pico:

• Detects card presence and movement via the card detect and odometer lines
• Samples data from the sensors at precise intervals
• Buffers and displays punch card contents
• Controls a feeder motor to move cards automatically
• Exposes reset and feed control via physical switches

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🔌 Hardware Integration

Inputs to the Pico

Signal Name	GPIO	Purpose
D0–D12	0–9, 11, 12	Punch card data bits (12 bits)
Odometer (Clock)	18	Pulses during card feed to time samples
Card Presence	19	Low = card inserted
Feed Switch	26	Toggles automatic feed
Reset Button	27	Triggers a full system reboot

Outputs from the Pico

GPIO	Signal	Description
20	Feeder Pulse	Pulses the card feeder motor
21	LED (Red)	Optional status indicator

Analog Front-End

Each sensor LED pair is connected to a photodiode, buffered by an LM324 op-amp configured as a comparator. The output is digital (3.3V logic) and connected directly to the Pico’s GPIOs.

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⚙️ Firmware Logic

The firmware is written in C using the Pico SDK.

Key Concepts

• Sampling is triggered by a rising edge on the odometer signal.
• Sampling is delayed initially (START_COUNT = 4 pulses) before reading the first column.
• Each column is sampled every COLUMN_READ_COUNT = 8 odometer ticks.
• A card consists of 80 columns, each with up to 13 punch positions.
• BITMASK (0x1BFF) masks out the relevant GPIO bits.
• If odometer pulses exceed TOO_MANY_CLOCKS (700), a warning is issued.

Dual-Core Operation

• Core 0 handles all IRQs, data sampling, and user interaction.
• Core 1 monitors the feed switch and toggles the feeder motor.

Interrupt Handlers

• Card Detect (GPIO 19):
  • Falling edge → start of a card
  • Rising edge → end of card
• Odometer (GPIO 18):
  • Increments jiffies (clock counter)
  • Triggers sampling logic
• Reset Button (GPIO 27):
  • Triggers a soft reboot via the ARM AIRCR register

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⌨️ Terminal Controls

• Press ' ' (spacebar) → Visual dump of card columns (* = punched, _ = blank)
• Press 'a' → Raw hexadecimal output for each column
• Press 'e' (if enabled) → EBCDIC-decoded stream

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🛠️ Restoration Steps

1. Verified sensor outputs and rebuilt the logic board.
2. Designed the logic interface to match 3.3V GPIO of the Pico.
3. Created the firmware with sampling and timing control.
4. Added physical controls for feed and reset.
5. Implemented dual-core handling for motor and main loop separation.
6. Tested punch card reading using real cards and visual/hex output.

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⧉ Files

• burroughs.kicad_sch - Logic schematic (see image)
• main.c - Complete firmware code for Raspberry Pi Pico
• README.md - This file
• CMakeList.txt

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📷 Schematic Overview

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🧪 Example Output

Card n. 1
*_*__*__...    ← Top punch row
*____*___...   ← Zone punches
**_***__*...   ← Rows 0–9
...

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🧠 Future Improvements

• Full EBCDIC decoding table
• Add optional direct UART communication (instead of USB) via UART0

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👥 Contributors

• Made during "Electric Dreams 2025" by Associazione Verde Binario