Ericsson / Alpha-American 4200C LED Sign

This page documents the large LED sign acquired by Hackeriet, gifted by pere and asle.

Overview

This is an Ericsson-branded, Alpha-American / Adaptive Micro Systems 4200C LED sign.

It is a programmable LED display commonly used for transit, public information, or industrial messaging systems.

Model: 4200C
Manufacture Date: 2004-04-08
Power: 100–240V AC, 50/60Hz, 1.5A
Status: Fully operational, displaying test message on boot. Unknown operational method.
For indoor use only.

Microcontroller

The Microcontroller is TS80C32x2 which is compatible with Intel 8051.

Datasheet: https://ww1.microchip.com/downloads/en/devicedoc/doc4184.pdf

Intel MCS 51 MICROCONTROLLER FAMILY USER’S MANUAL: https://datasheets.chipdb.org/Intel/MCS51/MANUALS/27238302.PDF

Firmware

The device seemingly runs a custom firmware, the label on the epromm is “4200c CUST 1018-6008e”, and it displays CUST during boot.

The firmware is here:

The firmware is 128k (131072 bytes). SHA-1: e5e3bdad5e507f658a874d40483de60161152777 The Intel 8051 is designed as a Harvard architecture with two independend 16bit address buses. So the 128k dump is used as two separate ROMs of 64k each (PROM and XRAM).

Firmware Analysis

Total ROM size is 128k
The 8051 can address 2x 64k separately as PROM and XRAM
The ROM is probably used as 2x 64k for PROM and XRAM (or maybe only PROM with bank switching?)
The first 256 bytes of the lower 64k and upper 64k are similar (both contains “© COPYRIGHT ADAPTIVE MICRO SYSTEMS, INC. 1991” at the same offset)
All the ASCII strings are in the lower 64k (except for the copyright message which is in both)
According to Intel MCS 51 users manual: After reset, the CPU begins execution from location 0000H.
Ghidra can read Intel 8051 binaries.
The first three byes of low and high are 02 01 00 which is LJMP 0x0100 which suggest that both files are code and thus that PMEM is bank switched
The real program seem to start at 0x0100 in both low and high
Low has data from 0x0000 - 0xc3ef
High has data from 0x0000 - 0x8ef3
High has zero-patterns starting at 0x1000 or so which suggest that this is data and not code, but it not strings so maybe font data?
In high at 0x0100 the code does some bit fiddling with IO ports and then does an infinite loops. While low has nops at the same addresses. So this probably triggers the bank switching to low, maybe with IO port P1.

Ports

The sign has a RJ-style modular port on the back panel with:

RS-232 IN
RS-485 IN
RS-485 OUT

It also has an IR sensor hidden behind a red sticker, visible when removing a panel.

Dongle

A small dongle with resistors was included, likely serving as a RS-485 termination and bias dongle.

Communication

The sign probably uses the Alpha Sign Communications Protocol, which is:

A structured serial protocol using STX/ETX framed packets.
Allows sending:
- Text messages
- Control commands (clear, brightness, modes)
- Time-based messages
Multiple signs can be connected via a RS-485 network

Serial Configuration

Suggestion for initial connection (untested):

Interface: RS-232 IN (single device)
Baud rate: 9600 or 19200
Data bits: 8
Parity: None
Stop bits: 1
Flow control: None

RJ-12 Pinouts, from left, clip down

ground → GND TTL / db9 pin 5
RS485 (+)
RXD → TXD TTL / db9 pin 3
TXD → RXD TTL / db9 pin 2
RS485 (-)
+5V (200ma max)

Logic analyser dump -- Cable between main board and driver board chain

The driver board consists of: - 2x data lines (R channel + G channel) - 2x clock lines (R channel + G channel) - + 2 more lines

This analysis was performed on a dump with duplicate channels and some missing channels, so some blanks have been filled in by reasoning.

The total pixel surface of this board is 16 rows by 200 columns. We have 8 bits per module, with 25 modules per row. Total of 200 bits per row, per channel.

The entirety of one line is blasted out starting from the bottom row of the display, batched in groups of 8 bits (one-and-one matrix module, left to right). Once the red channel has been output, the same is done for the green channel on separate lines. Afterwards, the row selection is shifted one row up and the next line is output.

The line selection automatically wraps around, however it's suspected that the previous output latches are being reset using a pulse at the end of the _first line_ of a given frame.