Carlos Manuel Rodrigues Machado Autonomic Ubiquitous Computing

The Teleswitch network implementation is divided into 3 layers:
Physical Layer
• Physical Layer;
• Data Layer;
• Network Layer.
The network hardware of the Teleswitch consists in a signal RS485 driver SN75LBC176DR
connected to the USART microcontroller hardware. An extra microcontroller IO is used to activate
the transition of the RS-485 driver. The physical signal is very robust to electromagnetically noisy
environments, because it uses a differential balanced line in combination with a twisted pair wire.
The reception circuit of the RS-485 driver is always active, allowing the microcontroller to
send and receive at the same time and this way it enables a later data collision detection. The bus
speed was set to 115.2kbps which is fast enough for sending home events (light switching or
movement detection, etc) and slow enough for allowing the execution of nearly 700 instructions
between receiving bytes interrupts on the microcontroller that runs at a 8 MHz clock. A faster CPU
speed was not used because it requires more expensive clock circuit hardware and increasing
device final cost.
Data Layer
The data layer is responsible for receiving and sending the bytes from and to the RS-485 bus.
It may detect some physical errors like: data overrun and framing errors. These errors are directly
obtained by the USART controller of the atmega329. The data integrity is checked in the network
layer.
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