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PROTOCOL GUIDELINES

While many RF solutions impose data formatting and balancing requirements,

Linx RF modules do not encode or packetize the signal content in any manner.

Naturally the received signal will be affected by such factors as noise, edge jitter,

and interference but it is not purposefully manipulated or altered by the modules.

This gives the designer tremendous flexibility for protocol design and interface.

Despite this transparency and ease of use it must be recognized that there are

distinct differences between a wired and a wireless environment. Issues such as

interference and contention must be understood and allowed for in the design

process. To learn more about protocol considerations we suggest you read Linx

application note #00160.

Errors from interference or changing signal conditions can cause corruption of

the data packet, so it is generally wise to structure the data being sent into small

packets. This allows errors to be managed without affecting large amounts of

data. A simple checksum or CRC could be used for basic error detection Once

an error is detected the protocol designer may wish to simply discard the corrupt

data or implement a more sophisticated scheme to correct it.

INTERFERENCE CONSIDERATIONS

The RF spectrum is crowded and the potential for conflict with other unwanted

sources of RF is very real. While all RF products are at risk from interference, its

effects can be minimized by better understanding its characteristics.

Interference may come from internal or external sources. The designer’s first

responsibility is to eliminate interference from sources under their control. This

means paying careful attention to layout, grounding, filtering and bypassing in

order to eliminate all radiated and conducted interference paths. For many

products this is straightforward, however, products containing components such

as switching power supplies, motors, crystals, and other potential sources of

noise must be approached with care. Comparing your own design with a Linx

evaluation board can help to determine if and at what level design-specific

interference is present.

External interference can manifest itself in a variety of ways. Low-level

interference will produce noise and hashing on the output and reduce the link’s

overall range.

High-level interference is caused by products sharing the same frequency in

proximity or from near-band high-power devices. It can even come from your

own products if more than one transmitter is active in the same area. It is always

important to remember that only one transmitter at a time can occupy a

frequency regardless of the coding of the transmitted signal. In most instances,

this type of interference is less common than those mentioned previously, but in

severe cases it can prevent all useful function of the affected device.

Although technically it is not interference, multipath is also a factor to be

understood. Multipath is a term used to refer to the signal cancellation effects

that occur when RF waves arrive at the receiver in different phase relationships.

This effect is a particularly significant factor in interior environments where

objects provide many different signal reflection paths. Multipath cancellation

results in lowered signal levels at the receiver and, thus, shorter useful distances

for the link.

TYPICAL APPLICATIONS

The figure below shows an example of a basic remote control receiver utilizing

the KH Series receiver. When a key is pressed on the transmitter, a

corresponding line on the receiver goes high. A schematic for the transmitter /

encoder circuit may be found in the KH Series transmitter data guide. These

circuits are implemented in the KH Series Basic Evaluation kit. These circuits

can be easily modified for a custom application and clearly demonstrate the ease

of using the Linx KH Series modules for remote control applications.

The ten-position DIP switch is used to set the address to either ground or

floating. Since the floating state is a valid state, no pull-up resistors are needed.

The data line outputs can only source about 1mA of current so transistor buffers

are used to drive the buzzer and LED. 1mA is sufficient to activate most

microcontrollers, but the manufacturer’s data guides should be consulted to

make sure.

The KH Series receiver / decoder module is also suitable for use with the Linx

OEM handheld transmitters. These transmitters are FCC certified making

product introduction extremely quick. Information on these transmitters can be

found on the Linx website at www.linxtechnologies.com.

Q1

2N2222

R2

2.2K

R4

10K

BZ1

BUZZER

VCC

Q2

2N2222

R3

2.2K

R5

10K

LED1

RED LED

R6

220 OHM

VCC

B1

CR2032 3V LITHIUM

VCC

S4

NC

1

D0

2

D1

3

GND

4

VCC

5

PDN

6

D2

7

D3

8

D4

9

DATA

10

VT

11

D5

12

D6

13

D7

14

A0

15

A1

16

A2

17

A3

18

A4

19

A5

20

A6

21

A7

22

A8

23

A9

24

NC

25

NC

26

GND

27

ANT

28

RXD-***-KH

1

2

3

20

19

18

4

5

6

17

16

15

7

8

9

14

13

12

10

11

S1

SW-DIP-10

GND

ANT1

BNC

GND

GND

VCC

GND

GND

GND

Figure 12: Basic Remote Control Receiver

Figure 13: Linx OEM Transmitters

Figure 14: Linx OEM Keyfobs