The eos430 project implements the EOS protocol on TI MSP430 microcontrollers. If you refer to the EOS protocol specification, ignore all the stuff about dynamic addressing, configuration and group addresses for now. As far as I know, most of this is not yet implemented in HELIOS, anyway.
Here is what you need to know to understand what eos430 does:
Helios can talk to an EOS bus via a serial connection. On this EOS bus there exist up to 126 different devices, each addressed by its own device address.
An EOS device has an address from 1 to 127 (address zero is reserved for the bus master, which is the PC running Helios). It also has an 8-character board name and a 4-character firmware version.
Each EOS device has a certain number of:
Currently, only digital inputs are supported by eos430.
Helios communicates with the devices on the EOS bus using certain commands. The important two (and the only ones implemented by eos430 at the moment) are command 130 (Info), which tells Helios the name, firmware version, and number of buttons of your device, and command 191 (Poll Status), which tells Helios which buttons are pressed.
For simplicity, in eos430 one microcontroller corresponds to exactly one EOS device.
You will need one master controller and zero or more slave controllers. These run the firmwares in the eos430-master and eos430-slave subfolders, respectively.
The master controller talks to the PC over a serial port (most likely emulated via USB) and optionally to several slave controllers using I2C. The remaining GPIO pins can be used to connect controls.
The master firmware requires a MSP430 controller with an USCI peripheral. The MSP430G2553 included with the MSP430 Launchpad development board meets this requirement.
A slave controller allows you to add an additional EOS device. It is connected to the master controller using I2C.
The slave firmware requires a MSP430 controller with an USI peripheral. The MSP430G2452 included with the MSP430 Launchpad development board meets this requirement.
The USI peripheral is less sophisticated than the USCI one, so those controllers are cheaper.
The eos430 firmware will wait for incoming EOS commands addressed to its configured device address. In case of command 130 (Info), it will answer with the board name, firmware version and other information configured in eos_local.h.
In case of command 191 (Poll Status), it will call the eos_update_input_state function. You will have to adapt this function to your particular hardware. It reads the state of all your controls and stores it in a byte array in the format required by the EOS protocol.