RF Transmitter/Reciver

[mackrackit]

Sounds like random noise. There is RF every where.

I think the internal "slicing" capacitor is wondering around giving false data.

If this is the case, depending on what you are going to use this for the solution can pretty easy.

If you are sending commands for a remote control the easy way is to have something like

SERIN ReciverPIN,T1200,[9],Counter
IF Counter = ??? THEN 
XYZ 
ELSE
QRS

If you are sending data like what you are now, you may need to set up a way to verify the data. Some sort of encoding / checksums...

The Linx web site has some info on this and dhoustion has a protocal on his web site.

[Bruce]

I'm not familiar with the Parallax modules, but if they use the Linx LR series, then this receiver will output random noise even if the transmitter is powered down.

Look at the examples on dhoustons' website, and others he has posted here. He knows what he's doing.

You might also try a search here. I have posted several code examples for RF communications that are very simple to implement, and work quite well with PBP & noisy receivers.

It's just not that simple to get a reliable wireless connection up & going, and it's for sure not going to be anywhere near as simple as a 'wired connection', but there are enough threads here 'with code examples' that cover this if you look hard enough.

[dhouston]

Like Bruce, I'm unfamiliar with the modules you chose. From your description it seems like they are superregenerative receivers. See

• http://davehouston.org/rf-noise.htm

for an explanation of the noisy output. It can be caused by environmental factors or come from internal noise sources (e.g. resistors).

Using SerIn with this type of receiver creates two problems. One is that SerIn samples the data at the mid-point of the bit period. If the output is high at that time, you get a 1 bit and, if low, you get a 0 bit. Random noise can give you either state. The other problem is that you need to set the receiver AGC and ATC (Automatic Gain Control & Automatic Threshold Control) and you need to signal the receiver that some data is on the way. The sync bytes recommended by several people who post here are intended to serve those purposes. EDIT: If you look at the bottom picture at the above referenced link, you can see how a signal very much like the sync bytes only gradually emerges from the noise. There is a better way but it depends on the type of data that you wish to transmit.

I would only use SerOut/SerIn if I wanted to send text or lengthy data packets. And these receivers are not the best choice for either. FSK gets away from noise and the receiver has a CD (Carrier Detect) output that tells you when data is coming. FSK modules cost more than ASK but not as much as the ones you've chosen.

For short data packets, I prefer ASK and the NEC protocol illustrated in this thread

• http://www.picbasic.co.uk/forum/showthread.php?t=6261

The long lead-in pulse sets AGC & ATC and signals that a data packet follows. Since the output is only monitored after seeing the long lead-in, random noise is much less of an issue. Instead of sampling at mid-bit, the receive routine measures the length of the spaces between pulses so it's less likely to be fooled by noise or its absence at mid-bit. (It's even better to measure between rising edges but that requires interrupts.) And each byte is sent twice in bitwise complementary form so error detection is built-in.

What is the nature of the data that you want to transmit?

[Fredrick]

What is the nature of the data that you want to transmit?

I just buy them to play and try to learn something so i don´t have any nature of data to transmit.

[mackrackit]

I just buy them to play and try to learn something so i don´t have any nature of data to transmit.

COOL! Whats next?

[Fredrick]

COOL! Whats next?

I don´t know but now i will try to get this to work better.

[Luciano]

Hi,

From page 3:
http://www.parallax.com/Portals/0/Do...ceive-v1.0.pdf


Calibration
When initiating communication between the RF modules, a sync pulse should be sent
to re-establish the radio connection between the modules. Sending several characters
can accomplish this, however sending a pulse (which maintains a high state during the
synchronization) is more efficient.

See page 6 for the BS2 code samples.


* * *

RSSI PIN (Received Signal Strength Indicator)

This line will supply an analog voltage that is proportional
to the strength of the received signal.


Can you test that and post the results?

(Do not connect the PIC microcontrollers during this test).


Step 1: Connect a digital voltmeter to the RSSI PIN of the receiver.
(Make sure the multimeter is set to DC volt and that you measure from the RSSI PIN to GND).
Power on the receiver and make sure the transmitter is powered off.
Q1: What voltage do you get on the RSSI PIN of the receiver?


Step 2: Power on the transmitter.
Q2: What voltage do you get on the RSSI PIN of the receiver?
(The transmitter and the Receiver are powered on).

Step 3: Connect +5V via a 1k ohm series resistor to the DATA pin of the transmitter.
(The transmitter and the receiver are powered on).
Q3: What voltage do you get on the RSSI PIN of the receiver?

Step 4: Connect GND via a 1k ohm series resistor to the DATA pin of the transmitter.
(The transmitter and the receiver are powered on).
Q4: What voltage do you get on the RSSI PIN of the receiver?

Best regards,

Luciano

* * *
EDIT:

These are the RF modules used by Parallax:

LR Series RF Transmitter Module Data Guide
http://www.linxtechnologies.com/Docu...Data_Guide.pdf

LR Series RF Receiver Module Data Guide
http://www.linxtechnologies.com/Docu...Data_Guide.pdf

[dhouston]

I just buy them to play and try to learn something so i don´t have any nature of data to transmit.

I looked briefly at the datasheet and these do use the Lynx RXM-433-LR-S chip which is a superhet design (the same one Bruce sells). This has a little better sensitivity and a little less noise than a superregenerative receiver like the RWS-434 but, in essence, it's an AM radio receiver and the noise you see in the absence of a signal is equivalent to the noise you hear on an AM radio when it's tuned between stations.

You can make use of the RSSI pin. Connect it to one of the PIC's ADC pins to get a measure of signal strength. You can use this to compare different antennas, etc.

You can also see the output signal using the techniques I've described here.

• http://davehouston.org/learn.htm

[Fredrick]

Calibration
When initiating communication between the RF modules, a sync pulse should be sent
to re-establish the radio connection between the modules. Sending several characters
can accomplish this, however sending a pulse (which maintains a high state during the
synchronization) is more efficient.

How to i know how the the period of the puls?

There is diffrent periods for BS1 and BS2 and mybe another period for my PIC?

[Luciano]

Hi,

From page 3:
http://www.parallax.com/Portals/0/Do...ceive-v1.0.pdf

===========================
Basic Stamp 1:

PULSOUT 1, 300

300 = 300 * 10 µs = 3000 µs
===========================
Basic Stamp 2:

PULSOUT 8, 1200

1200 = 1200 * 2 µs = 2400 µs
===========================

See page 339 of the Basic Stamp manual:
http://www.parallax.com/dl/docs/prod...icStampMan.pdf

Best regards,

Luciano

[Luciano]

From Linx Technologies - Linx Knowledgebase

The DATA line of the LR receiver seems to switch randomly when the transmitter is not on.

This is a result of the increased sensitivity of the LR receiver. The sensitivity is below the noise floor of the board, so it is picking up thermal noise and other random signals and outputting it as data. This is generally not a problem for off-the-shelf decoders and can be resolved in software for custom microcontrollers (see Application Note AN-00160 for protocol recommendations), but an external squelch circuit can also be used. Using a squelch circuit will allow the designer to only allow data when the received signal is above a certain threshold, but it will sacrifice the range. This allows the user to make the tradeoff between random noise and range.

Attached is a circuit schematic for a squelch circuit for the LR Series receiver (or any receiver with an RSSI output). The RSSI line will output a voltage that is relative to the strength of the received signal. Since the LR Series is On-Off-Keyed, this output will follow the data and look like a square wave, so it will be at a lower voltage when receiving a '0' and at a higher voltage when receiving a '1'. D1, C1, and R1 form a peak detector that will follow the peak voltage of the ones.

This voltage is then fed into the non-inverting input of a comparator where it is compared to a reference level set by a potentiometer. When the signal level becomes greater than the reference voltage set by the potentiometer, the comparator will release the output line. When the signal level falls below the reference voltage, the comparator will pull the output line to ground. Most comparators have open collector outputs, meaning that they can only pull the line to ground or release it. They cannot pull the line high, so a weak external pull-up resistor (R3) is needed to pull the line to Vcc when the comparator releases it. The feedback resistor (R4) is used to stabilize the output.

The output of the comparator is used to control an analog switch that will pass the received data to whatever external circuitry the application requires. When the control line is high, the data gets passed, otherwise it is not connected. This means that when the received signal is greater than the threshold, the switch is closed and the data is passed. When laying out the board, it is a good idea to place the output of the analog switch close to the device that will be using the data. A long trace or wire here has the potential to couple AC noise onto the data line while it is squelched.

A discreet voltage divider or a voltage reference IC can be used in place of the potentiometer, and the values for C1 and R1 can be adjusted to tune the response as needed.


Attached File: LR Series Receiver Squelch Circuit - 56K
http://www.linxtechnologies.com/Know...%20Squelch.jpg

APPLICATION NOTE AN-00160
Considerations For Sending Data Over a Wireless Link
http://www.linxtechnologies.com/Documents/AN-00160.pdf

[earltyso]

Hello out there,
I am looking at using the LINX TXM/RXM LR series of serial modules to send real time data for a depth monitoring system. They sure don't look as easy to implement as the LINX TXM KH2 modules with simple bits and adressing with built in encoders and decoders.
I was hoping to get away with not using any error checking or checksum, just a simple test for one transmitter to pass ...something like....

'transmitter module
main:
B0 = 12345
SEROUT2 TXPORTPIN, 84, ["TEST", B0]
pause 100
goto main

'reciever module
main:
SERIN2 TXPORTPIN, 84, [WAIT("TEST"),DEC5 B0]
pause 100
goto main

if you pass the test (which is a string), send the simple data and wait for the next time the test is sent/passed to collect the data. Another idea I had to was to create my own adressing with one of the ports for security....
Are there any other suggestions out there?
how about using an encoder chipset for serial data? I have not looked at this yet, I will do some more looking and get back with what I find.