Project Description
You don’t have to accept random wires strewn everywhere, and don’t have to plumb the attic with a rat’s nest of cables. Instead, you can use small wireless transmitters and receivers to extend a wireless link between a PC and a connected remote device. This project does exactly that, showing you how serial links work and how to integrate the radios.
The most common serial port transmissions send characters asynchronously over the wire, meaning there’s no necessary timing relationship between the signaling for one character and the signaling for the next. Figure 1 shows an oscilloscope photograph of the signal waveform for an ASCII A (hex 0x41) sent at 2400 bits per second (bps). You can see the overshoot in the signal on the transitions between states, settling to the signaling voltage after a while.
FIGURE 1: Scope trace for ASCII A
We generated the trace by monitoring the output pin from a Javelin Stamp driven with a program.
We send one character with a large delay before the next to make isolating the signal on the scope easy. One bit time is shown between the two vertical dashed lines. If you calculate that bit time as 1 second/2400 bits per second, you get 416 microseconds, the same as the duration between the two lines, as shown in the upper-right corner of the scope.
An asynchronous character, assuming we’re not sending a parity bit, consists of a start bit (a low signal), the eight data bits, sent least significant bit first, and a stop bit (a high signal). You’ll see 1.5 or 2 stop bits sent at very low data rates, but sending data that slow is very uncommon. Figure 2 decomposes those parts of the waveform.
FIGURE 2: Serial character waveform parts
These waveforms can be represented many ways.Over an RS-232 interface, a one (high in the waveform) is represented by +12 V, and a zero (low in the waveform) by -12 V.Over a 5 V digital interface, one will be +5 V and zero will be 0 V.Over a wireless link, the radio waveform will depend on the transmitter and receiver used (for more information about that, see the next section). All those representations are the same waveform, but with a different physical representation.
The serial ports on a PC use RS-232 signaling, and implement both transmit and receive paths to support full duplex operation. The wired connection uses separate wires separated by insulators. Radio signals have no direct equivalent to insulation, so they have to use one of these approaches to send two signals at the same time:
Time Division Multiple Access (TDMA) — It’s not necessary to actually send characters simultaneously in opposite directions. Instead, if you sent characters at twice the data rate of a single direction, you could alternate sending from one end with sending from the other end. That approach, an example of TDMA, lets both signal directions share the same frequency and give the appearance of simultaneous full duplex transmission.
Code Division Multiple Access (CDMA) — Pseudo random numbers give radios another way to communicate simultaneously. By shifting the radio frequency over a wider span according to a high-speed random sequence, or by mixing the data stream with the higher-rate random sequence, two independent channels can share the same spectrum without interfering with each other.
Frequency Division Multiple Access (FDMA) — Two radio links on frequencies sufficiently separated won’t interfere with each other, much as TV channels 2 and 4 transmit independently and simultaneously.
Each approach has its advantages and disadvantages. TDMA uses the least spectrum, but requires synchronization among all transmitters so only one is on at a time. CDMA requires relatively complex transmitters and receivers, but has much better immunity to noise. FDMA is very simple to implement, requiring only tuning the channels to different frequencies, but makes it more complex to implement many simultaneous transmissions.We were only interested in making one wireless RS-232 link work at a time, so we chose FDMA, operating the two directions at separate frequencies.
You can’t choose frequencies at random — nations allocate frequency usage and regulate what equipment is allowed to radiate. Most frequencies are allocated for specific purposes, and many require licensed operators. Bands of spectrum are allocated for Industrial, Scientific, and Medical (ISM) applications in most countries (although not the same bands in all countries); under ISM rules, the design of the equipment is certified, after which anyone can operate the equipment in its approved configuration. In the United States, for example, there are ISM bands at 916 MHz, 2.4 GHz, and several other frequencies. Small, inexpensive radio modules are widely available for use in the ISM bands, making it easy to build radio links operating legally in the ISM bands.