Making Things Talk

192 MAKING THINGS TALK

Radio Transceivers

In many cases, one-way communication isn’t enough. For example, you might have

noticed in the previous example that sometimes the message doesn’t get through,

even when you’ve got the circuitry and the code fully working. In that case, you might

want the PC to be able to query the microcontroller occasionally to see the state of

the inputs. Or perhaps you’re making an application in which there’s input and output

on both sides. In that case, transceivers are essential.

There are many different kinds of data transceivers on

the market. Some connect directly to the serial I/O of the

microcontroller and send the data as is. Some, like the

Bluetooth module you saw in Chapter 2, add an additional

protocol layer on top of the data communication, so you

have to be able to implement that protocol on both sender

and receiver. The cost of transceivers varies widely.

Until recently, most digital radio transceivers on the

market implemented only the most basic serial com-

munications protocol. For example, the RTF-DATA-SAW

transceivers from Abacom do a good job of sending and

receiving serial information. They connect directly to the

serial transmit and receive pins of your microcontroller.

Any serial data that you send out the transmit line goes

directly out as a radio signal. Any pulses received by the

transceiver are sent into your microcontroller’s receive

line. The benefit is that you don’t have to learn any serial

protocol — you can send data in any form you want. The

cost is that you have to manage the whole conversation

yourself. If the receiving transceiver misses a bit of data,

you’ll get a garbled message, just like you did with the

transmitter-receiver pair in the preceding project. Fur-

thermore, any radio device in the same frequency range

can affect the quality of your reception. As long as you’re

working with just two radios and no interference, trans-

ceivers like the RTF-DATA-SAW do a fine job. There are

other companies on the market who sell similar transceiv-

ers, including Linx Technologies (www.linxtechnologies.

com) and Low Power Radio Solutions (www.lprs.co.uk).

There are an increasing number of cheap transceivers

on the market that implement networking protocols,

handling the conversation management for you. The

Bluetooth modem in Chapter 2 ignored signals from other

radios that it wasn’t associated with, and took care of

error checking for you. The XBee radios you’ll use in the

next project will do the same, and much more, as you’ll

see in Chapter 7. These particular transceivers are in the

same general price range as the plain serial transceivers

mentioned earlier. They require you to learn a bit more in

terms of networking protocols, but the benefits you gain

make them well worth that minor cost.

There’s one other difference between the serial trans-

ceivers and the networked ones: the networked modules

tend to operate at much higher speeds, both in terms of

transmission frequency and serial data rate. For example,

the Abacom modules mentioned previously operate at

315 MHz and a maximum serial data rate of 9600 bits per

second. The XBee modules in the following project operate

at 2.4Ghz and up to 115,200 bits per second. Hence, the

XBee radios can send a message at nearly 100 times the

speed. Even if you’re sending only a few bytes per second,

this means that your transceiver can spend more time

listening and less time speaking, thus reducing the chance

that it’ll miss a given message from another transceiver.

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