Issue 160 November 2003
RF Made Simple

Start • Dead-Easy Wireless • Dead-Easy Application • Results • Sources and PDF

RF is magical and mysterious, and so are RF engineers. I truly believe that RF engineers are really disguised holdovers from the time of Merlin and Camelot. Look at what the RF engineering wizards have done to us. Do you own and operate a cell phone? Is your pager a constant companion? I’m from Tennessee, where blackberries grow on thorny vines along fencerows. I’ll bet you get e-mail with your Blackberrys. What about that cordless phone in your kitchen? These days, you don’t even need to have wire to get on an Ethernet LAN. The RF guys are even messing around with our food. I went out to eat recently, and the waitress took my order with a wireless Palm PC. If cranking magnetic fields around my food isn’t bad enough, RF engineering has gone so far as to leave its mark on the musical arts. Ask yourself this: When was the last time you saw a celebrity using a microphone? Was it wireless? You bet!

Not to be left out of the enchantment, this month I’ve got my eye on some new data radios and a new embedded modem. Both the data radios and the modem are purportedly “easy” to put into operation. You know me, when someone claims something is “easy,” I try it out and tell you how it went.

EASY-RADIO

Low Power Radio Solutions (LPRS) has claimed that its easy-Radio allows you to ignore the radio link and treat it as if it were a wire. A simple TTL or RS-232 data source is all that’s required to move data from point A to point B, and vice versa, using a pair of easy-Radio transceivers. All of the messy encoding, decoding, and checksum calculations are achieved by the easy-Radio’s internal processor.

The easy-Radio uses frequency hopping and a maximum of 20 mW of output power to achieve a range of 500-m line-of-sight. My 900-MHz easy-Radios hop at 25 times per second over 25 channels, which, under the control of the internal microcontroller, is transparent to the data radio user.

Using frequency hopping in low-power radios like the easy-Radio has its advantages. By hopping at predetermined intervals between preset frequencies in a band of unlicensed frequencies, the chances of interference from other radio transmissions decreases. Because the data is dispersed over a number of channels, better data security is achieved as well. Over time, the use of frequency hopping reduces the average power level that is transmitted at a single frequency, which allows higher peak power levels than those allowed for single-band transmitters.

Interfacing to the easy-Radio module is easy, because it provides two handshake lines and a serial interface running at 19,200 bps. The easy-Radio handshake lines control the flow of data to and from the easy-Radio transceiver. If the easy-Radio is busy internally, the Busy line is asserted. The easy-Radio’s Busy line alter ego is the Host Ready input line that informs the module that the host processor is ready to transfer data out of the receive buffer for further processing.

To send data using an easy-Radio, the host processor checks to see if the module is busy. If it isn’t, the host processor feeds a maximum of 80 bytes of data to the easy-Radio via the 19,200-bps serial link that exists between the host and the module. If less than 80 bytes of data are to be transmitted when the host processor has finished its transfer, two byte times later, the easy-Radio starts processing the data it received from the host processor and activates the Busy output line.

The 2-byte time between the end of a message that’s less than 80 bytes in length and the beginning of transmission processing is called the end-of-data gap. At 19,200-bps, the end-of-data gap is 1.04 ms. If the message from the host processor exceeds the 80-byte easy-Radio limit, all of the bytes beyond the module’s 80-byte buffer maximum are discarded, and the remaining contents of the easy-Radio transmit buffer are processed for transmission.

Any easy-Radio module within range picks up the initiated transmission, drives its Busy signal active, and starts to decode the incoming message. The receiving host processor should then take the easy-Radio Host Ready line low. As soon as there is data in the easy-Radio’s receive buffer that can be transferred to the host processor, it will flow on the serial link that exists between the module and the host processor. After all of the data has been transferred out of the easy-Radio’s receive buffer, the easy-Radio deactivates the Busy output line.

The host processor can use the easy-Radio’s Host Ready line as a flow-control line. To stop the flow of data on the serial link between the host processor and the easy-Radio module, the receiving host processor raises the Host Ready line. Lowering the easy-Radio’s Host Ready line resumes the data transfer. The host processor has approximately 2 s to transfer everything out of the easy-Radio’s receive buffer before the module takes control and flushes the receive buffer.