leuro-m20-bbb

Hardware interface to Leurocom M20 using
Beaglebone Black mmap’ed GPIO with a cavalier approach.

Overview

leuro-m20-bbb reads framebuffer updates (see related project
caprica
) sent over a Unix domain socket, then writes the framebuffer
to an attached Leurocom M20 monochrome LED display by bit-banging
GPIO pins on a Beaglebone Black. In order to achieve acceptable
refresh times, GPIO set and clear registers are written directly
using mmap-ed pages from /dev/mem.

Build

Edit the socket, screen configuration, and user defines
then compile leuro-m20-bbb using make:

$ make

Install

Add a non-system account for the caprica user, then install to
the path:

# useradd -r -s /usr/sbin/nologin caprica
# install -s -o root -g root -m 0700 leuro-m20-bbb /usr/bin
# install -o root -g root -m 0700 leuro-m20-bbb.hwinit /usr/bin
# install -o root -g root -m 0644 leuro-m20-bbb.service /etc/systemd/system
# systemctl enable leuro-m20-bbb.service

Test

Install the socat utility and run the supplied script sendpage.sh:

# apt-get install socat
# ./tests/sendpage.sh ./tests/panels.bin
# ./tests/sendpage.sh ./tests/blank.bin

Hardware Setup

Connect from the Beaglebone Black
P9 expansion header through a ~500 Ohm resistor[1] to the
non-inverting inputs[2] on the “LED-ADAPTER-KARTE” or directly
to the “LED-OPTO-KARTE”. Inverting inputs can be tied together
and connected to a ground pin (eg P9.1, P9.2) or run separately
back to a ground point on the Beaglebone Black. If the display
has a “LEDDIMMER” board, remove it and the connecting ribbon
completely.

• [1] On the “LED-OPTO-KARTE” board, A2231 optocouplers
  decode balanced lines from a Leurocom PCI card through a
  220 Ohm series resistor. To get the recommended
  2.5 mA “ON” current from a Beaglebone Black 3.3V GPIO pin,
  an additional ~500 Ohm series resistor is required on each input.

• [2] Balanced inputs are labeled X/XN where X is the inverting input,
  and XN is the non-inverting input. X connects to the A2231
  cathode, XN connects to the anode. For more information on the
  A2231, see the
  manufacturer website.

Display Mapping

The Leurocom M20 (viewed from the front-side) is arranged in rows
of LED panels labeled “LED - M20CC” (12x12 pixels in this case),
daisy-chained right to left. Each LED panel has two shift registers
“a” and “b”, arranged in 6 rows of 3 columns, each 4 bits wide.
Groups of 3 or 4 LED panel rows are driven from interface cards
labeled “4fach-Verteiler PCI IO -VM2003”.
Interface cards are on the right side of the display, connected in a
daisy-chain from bottom to top. Each collection of rows, numbered 1-4
on the interface card, is connected top to bottom.
Framebuffer updates from caprica describe the desired image as a buffer
of 32 bit pixel values packed little-endian[3] from left to right,
top to bottom.

• [3] Beaglebone Black typically runs little-endian

Hardware Reference

LED-ADAPTER-KARTE

Breaks-out opto-coupler inputs to screw terminals. Header pins:

LED-OPTO-KARTE

Decodes balanced line inputs to 5V logic and connects to
dimmer board and first interface card input. In this case, the
opto card passes all logic through to the interface card
without change. The dimmer connection appears to provide
pwm of the DI signal using HA and HB signals - these were not
necessary since the Beaglebone Black can generate clean PWM
internally.

Dimmer Header - 10 pins

Interface Card (“PCI”) Header - 20 pins

Interface Card (4fach-Verteiler PCI IO -VM2003)

The interface card provides an array of buffers for distributing
input signals to 4 LED panel rows and the next interface card in the
chain. It also includes an address latch for stepping across the
display.

In/Out Header - 34 pins

M20 CC LED Panel

The M20 CC LED panel is a 12x12 pixel LED display driver with
2 shift registers and logic to propagate signals along a chain
of panels.

In/Out Header - 26 pins