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Design files, firmware and linux scripts for my /clever/ serial terminal, the Whitterm-220 - WT-220.
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README.md

        __        _______    ____  ____   ___
        \ \      / /_   _|  |___ \|___ \ / _ \
         \ \ /\ / /  | |_____ __) | __) | | | |
          \ V  V /   | |_____/ __/ / __/| |_| |
           \_/\_/    |_|    |_____|_____|\___/
       --- JBR Engineering Research Ltd 2019 ---

Design files, firmware and linux scripts for my clever serial terminal, the Whitterm-220 - WT-220.

The WT-220 is a homage to the VT-220, providing a physical RS232 terminal with the full features of a Linux computer running on a Raspberry Pi.

Raspberry Pi Hat (./whitterm-hat)

KiCad design files for the rear io-panel:

  • USB-C input for USB-PD high current (3 A) supply to RPi. LEDs indicate detected current profile from UFP. I2C lines made available to SAMD for playing with USB-C device modes.
  • MAX3232 RS232 transceiver provides RS232 level interface to RPi UART and SAMD UART.
  • SAMD21 microcontroller provides boot button control of RPi over I2C and buffered IO. Can be programmed over USB-C CDC-serial device or ISP.
  • Control of RPi power supply from SAMD. Off by default allows safe start up, only when power input suitable/ready.
  • DIP switch control of device UART connections (SAMD, RPi, RS232).
  • Board replaces laser cut acrylic back piece from original design - it is both electronically and mechanically integrated.
  • Lots of status LEDs 😃.

There where a few green wires required for the first production (1.0). I rolled these changes into the schematic but not fully updated the PCB.

Firmware (./firmware)

Arduino compatible firmware for rear panel SAMD21. Acts as I2C slave and controls supply to RPi via 'Boot' button:

Simplified State Diagram

graph TB;
    OFF-->|boot pressed 500 ms|ON;
    ON-->|I2C booted cmd recieved|BOOTED;
    ON-->|I2C recieve booted timeout 20000 ms|I2C_TIMEOUT;
    BOOTED-->|I2C recieve booted timeout 20000 ms|I2C_TIMEOUT;
    BOOTED-->|boot pressed 3000 ms|SHUTDOWN_REQUESTED;
    I2C_TIMEOUT-->|boot pressed 3000 ms|SHUTDOWN;
    I2C_TIMEOUT-->|I2C booted cmd recieved|BOOTED;
    SHUTDOWN_REQUESTED-->|I2C shutdown cmd|SHUTDOWN;
    SHUTDOWN_REQUESTED-->|boot pressed|ON;
    SHUTDOWN-->OFF;

mermaid state

RPi Files (./rpi)

I started from a base Raspbian Lite image rather than Arch for this build. Raspbian Lite is nice because I could install only what I wanted/needed. I opted for I3WM since it's a keyboard terminal.

Services

  • boot: Runs shell script that displays boot ascii logo.
  • i2c: Runs python I2C script (wt220-i2c.py) that configures case LEDs to act on RX/TX and then maintains link with I2C link with SAMD. Polls shutdown request state and acts on request by issuing system shutdown.
  • poweroff: Runs on 'shutdown.target' and issues the final I2C shutdown command so that SAMD disables RPi 5 V.

Config Files

Would make an install script for this if I was making more...

  • xinitrc -> /etc/X11/xinit/xinitrc
  • config.txt, cmdline.txt -> /boot/
  • Xresources -> ~/.Xresources
  • motd -> /etc/motd
  • wt220-sshbanner.net -> /etc/wt220-sshbanner.net Banner /etc/wt220-sshbanner.net in /etc/ssh/sshd_config
  • *services. -> /lib/systemd/system/
  • scripts -> /opt/wt220/

Packages

Need to manually install cool-retro-term

Other packages in 'rpi/dpkg-selections.txt'

Breakout (./whitterm-breakout)

DB9 GPIO breakout from V 1.0 rear panel

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