System Diagram
The diagram shown in Figure 3 shows the elements of the system, including the radios and the interfaces to the processor and sensor. The processor would typically be a PC, but the idea is just as useful with an embedded processor such as the Javelin Stamp. The Javelin Stamp also simplifies integrating and testing the wireless link, because it’s far simpler than a PC running a large operating system and therefore easier to peel apart and discover what’s going wrong.
FIGURE 3: Wireless link block diagram
The two directions of transmission use completely identical equipment with the exception that they use different frequencies. The transmitters and receivers typically have digital interfaces expecting a 0 to 5 V signaling, while PCs use RS-232 ports with ±12 V signaling. Fortunately, converting between the two signaling levels is trivial, requiring only one transceiver chip (such as a Texas Instruments MAX232 or Intersil HIN232) at either end. Hosted on the Javelin Stamp Demo Board, the Javelin Stamp has both direct and shifted levels available, and so can interface to either signaling approach.
Figure 4 shows how a radio module connects. The modules we chose have asynchronous interfaces, meaning signal edge transitions may occur at any time so long as the data rate is within the capabilities of the module. There’s no clock signal coming from the module as a result — the only connections are data, power, and the antenna.
FIGURE 4: Radio module interfaces
Figure 5 shows how you could connect a Javelin Stamp to loop a data stream from a transmit pin out to the transmitter, onto a radio wave, through the receiver, and back into the Javelin Stamp. That complete loop forms one half of the project; a second loop on a different frequency provides the other half. If your application only needs to transmit data in one direction, such as to a PC from a sensor that accepts no commands, you only need one transmitter and one receiver.
FIGURE 5: Half duplex loop back