Specifications

7.3. HARDWARE DESIGN
standard for the bus would exist. A custom design was thus not an ideal
solution.
A CAN or RS485 system would require additional line driving circuitry
or specialised microprocessors with on-board support. We would like to save
costs and simplify the design wherever possible, so this is again not ideal.
SPI is a simple full duplex protocol requiring three bus lines. No additional
hardware is required as most microprocessors include an SPI-compatible
peripheral. Unfortunately, an additional “chip select” hardware line is required
per device attached. This would thus limit the expansion possibilities of the
uplink module and could require hardware and/or software changes when a
new peripheral is added.
I
2
C is a two wire, half-duplex protocol with a well-documented specification.
The system was developed by Philips in the 1980’s for interconnecting integrated
circuits (hence the name IIC or I2C for Inter Integrated Circuit)[16]. The bus
is constructed in a wired-AND c onfiguration and requires a master device to
initiate communication. Additional peripherals are easily added by simply
connecting them in parallel to existing peripherals. No additional hardware is
required as most microprocessors now incorporate hardware I
2
C peripherals.
Furthermore, the communication speed is scalable to suit the application.
I
2
C was thus selected as the optimal interconnect option.
7.3 Hardware Design
Figure 7.1 illustrates the proposed system component interconnection for the
uplink module. The power supply is discussed in Section 8 and the enclosure
and physical requirements in Section 9. All other hardware design relevant
to the uplink subsystem is discussed in the proceeding sections.
7.3.1 GSM Module
The GSM module should be compact, power efficient and be compatible with
South Africa’s GSM network. Various embedded devices were considered,
including the Sony Ericsson (now Wavecom) GR64 and GM47/48, Siemens
MC35 and MC39i, Motorola G18 and G20 and Telit GM862. It was important
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