GorbaDisplay

Project GorbaDisplay
Public transport LED display
Status	Initializing
Contact	bertrik, User:Eloy
Last Update	2023-08-04

Introduction

This project is about reverse engineering a former public transport LED display. It consists of Z panels of each X * 16 LEDs. Each panel has 16 amber LEDs vertically.

The plan is to make the display fully addressable as a bitmap display over a network connection, preferably with individually controllable brightness per pixel.

TODO:

• more reverse engineering by tracing out the connector to the individual lines of the integrated circuits
• is there a separate power supply line to the board? if so, can we power our new microcontroller from the 14-pin connector?
• measure clock frequencies (column clock, line update frequency) of original controller board
• write software to control this thing, e.g. for ESP8266
• investigate https://github.com/qisun1/ESP8266_LED_64x16_Matrix , seems to be a similar display

Hardware

14-pin header + chips

This thing consists of two main parts:

• a control board, with logic to receive (for example) text messages over a serial connection and convert them to a bitmap display on the display board
• a display board, with logic to light up each LED, consisting of 4 panels of 16x16 LEDs each

Each panel has two 14-pin connectors, one incoming and one outgoing (to the next panel)

It has a light sensor to sense the ambient light level.

Theory of operation

My guess this is probably another row-multiplexed display. The display can light up one row at a time. By quickly lighting up each row in succession, the illusion to a human observer is that all LEDs are controlled simultaneously.

The display board contains the following integrated cicruits:

• 74HC541, an octal buffer/line driver, probably buffers all signals coming in from the 14-pin connector to the rest of the electronic on the display board
• 74HC238, a 3-to-8 line decoder/demultiplexer, probably selects which row is currently being lit up
• group of 4x IRF7425, power MOSFET, probably for driving a row of LEDs with
• a whole bunch of TPIC6C596, 8-bit shift register, probably drives the columns inside one row of LEDs

The demultiplexer handles only 3 bits, enough for 8 rows. Perhaps there are two of them to handle 16 rows: one for the top 8 rows and one for the bottom 8 rows.

Each panel contains 32x16 pixels, but seems to have 4 series of 4 shift registers. Each series of 4 shift registers can drive 32 pixels (4 registers x 8 bits).

14-pin connector

My guess for the pinout so far is:

Connections

Pin	Id	Remark
1	GND	Ground
2	GND	Ground
3	?
4	?
5	COL_SEL1	A1 of row multiplexer
6	COL_SEL2	A2 of row multiplexer
7	?
8	ROW_DATA
9	?
10	COL_SEL0	A0 of row multiplexer
11	?
12	ROW_CLK	SRCK of column shift register
13	VCC	Power
14	VCC	Power

There are possibly pins for:

• row select, bit 0
• row select, bit 1
• row select, bit 2
• row select, bit 3
• row-enable
• shift register data
• shift-register clear
• shift-register data
• analog LDR value
• remote control input
• power pin
• ground pin