Parts and Designs
Our design uses four pre-built ISM band radio modules, two (transmitter and receiver) at either end to form a wireless terminal. The terminals are not identical, as they must transmit at different frequencies. Figure 6 diagrams a single terminal of the pair. The transmit and receive antennas are separate. If the frequencies you use are too close together, you may have to isolate the transmitter and receiver from each other with grounded metal shield cans; and you may need to create some separation between the two antennas. Some experimentation with separation before you finalize your packaging should show you what’s necessary.
FIGURE 6: Complete wireless RS-232 terminal
Figure 7 shows the Ramsey Electronics modules. The larger boards are the receivers; the 916 MHz modules are on the left. The four pads you connect to are on the undersides of the boards. Ramsey rates them for operation up to 2.4 Kbps.
You’ll want to package the terminals well enough to make sure that they can take the casual abuse PC equipment gets, and you’ll need to make connections between the printed wiring board (PWB) components, such as the voltage regulator and transceiver, and modules such as the radios, connectors, and power adapter. The ONE PASSircuit prototyping boards help solve this problem, having the layout of a solderless prototyping boards, but being PWBs you solder to. They also have shapes that fit the Radio Shack enclosures. You’ll want to mount the radio modules to the PWB; we suggest insulated standoffs.
Insulated wire of the lengths suggested for the modules you use should do for the antennas.
FIGURE 7: Radio modules