TroiaDoc - PixelRoom - PixelModule

A pixel module receives a serial stream of data and generates the PWM signal for 32 pixels out of it.
The name (of the circuit board, the firmware, ...) of the new pixel module is "PixelModule2".
A fast optocoupler, which is connected to form a 10mA current loop with the input lines, is used for the serial input. The data is transmitted in NRZ coding using the 8N1 protocol with 38400 bps.
The outputs for the LEDs switch the ground and are current regulated to 20mA. This is done using two AllegroMicro A6276. The brightnes control of the LEDs is done via PWM with 128 steps and at least 100Hz.
The supply voltage for the pixel module are the same 5V that are also used to drive the LEDs.

Protocol

The protocol used on the serial line uses 32 bytes to specify the new brightness values of the 32 connected LEDs. Valid values are in the range 0..127, only 7 bits are used. For synchronisation, the most significant bit is set in the first byte (which conatins the brightness value for the first LED).
The data for the Frames may arrive as fast as possible as permitted by the serial transmission with 38400 bps.
As soon as a valid frame is received, this frame is displayed.
At any time, if no (new) frame is received within a time of 5 seconds, the emergency lighting is activated. This makes it possible to switch into emergency lighting mode very easily and very fault-tolerant (every pixel module acts on its own) by not sending frames any more.

Because pixel modules are only used in the pixel boxes, we can do wihtout plugs and connect everything to solder pads for reasons of cost (and for simplicity). In a case of failure, the entire pixel box will be replaced anyway and the defect box can be repaired.
A circuit board contains two pixel modules incl. the RJ45-Jack. The supply voltage is connected using two cleraly marked solder pads on the back side. For the LEDs, there are also 64 already correct placed solder pads on the back side of the circuit board.