:: FM Communication Help ::

[suby786]

Hey all..

my final yr project has a subsection that sends data from a PC to a BOX which contains my electronics...

basically i have VB.NET application that sends out values from the serial port... via a USB->SERIAL adapter (emulates a com port)...and at the moment, i have a physical connection of this to my PIC Rx pins..

my PC will be 1 transmitter and i will have 2 RECIEVERS. each reciever will be identical with identical PICs..

so at the momemnt i can communicate via wires to BOTH recievers, BOTH of them recieve ANY DATA...but only 1 of them will read in DATA MEANT FOR THEM (by having a coding system - headers etc)... so im sending HEADER | BYTE 1 | BYTE 2 | BYTE 3 | BYTE 4

that happens every transmission... but as i sed only 1 will understand and store it..

now i want to remove the wires... and have it wireless.. i heard bad things about the 400MHz range.. as its quite busy and told to use the 900MHz band..

so obv i cant use a transiever as i have 2 recievers and 1 transmitter... can i jus get a matched pair and duplicate the reciever ..?

in the end id want 2 way comms so wud i need

@ PC - transmitter FREQ 1 / Reciever FREQ 2
@ R1 - transmitter FREQ 2 / Reciever FREQ 1
@ R2 - Transmitter FREQ 2 / Reciever FREQ 1

Does that mke sense??? HELPPP lol

 

[tunedwolf]

Unless you require two way simultaneous traffic I would keep them all on the same frequency, and have the lowest address set to be the master.

 

[suby786]

hey mate

what do you mean keep the lowest address set to be the master?

and so your saying i need 3 IDENTICAL transmitters and recievers on each node...

but say if PC is transmitting to ALL RECIEVERS, wont its OWN reciever (the 1 at its own node) pick up the signal?

i dont need simultaneous transmission really, it would be handy though..

i will be sending commands via a user from PC to Recievers.. BUT each reciever will be sending data back to the PC node every second or so...

will the reciever get confused if it gets 2 sets of bytes at the same time... how can i sync the 2 recievers (they are isolated and are on seperate ends of the room...)

 

[mr.gone]

Man, what are you talking about? Your not just gonna unhook wires and send the data through the air. You have to modulate an FM carrier frequency. The basic circuit will require a minumum of a crytal locked frequency oscillator with a varactor diode in the tuning circuit to modulate the data onto the carrier frequency. Do you understand that much before I go on any further? Also the data from your USB or serial port wont do 400 or 900 MHz. The oscillator is resposible for that frequency. And the receiver is a bit more complicated in that the oscillator will be running at an offset frequency of the transmitter frequency and mixer and local oscillator. Then you must demodulate the data with an FM descriminator or slope detector.

 

[tunedwolf]

I am assuming that you will be using pre built rf modules...and not building your own transceivers...
You need a transceiver in each, 1 in the master, 1 in each of the slaves.
At the moment you are using your header so each slave will only respond to it's own header...
You do the same thing as you are doing now, just that your master has the lowest address (header). The master won't respond to it's own commands, because you either won't allow it in software, or it will be sending data to the slaves with a different header...so obviously won't respond

You write in some contention detection mechanism so that if both receivers send data simultaneously to the master, that neither are responded to.
Send a handshake for data received or something back to the slave, if no handshake is "heard" at the slave end, then back off for a small period of time, then re-try sending the data etc etc, keep doing so until you get the handshake.

 

[suby786]

so i can use the PRE-BUILT-OFF-THE-SHELF tranciever modules? all 3 modules need to be at the SAME FREQUENCY... and theoretically they can all communicate with each other?

So when im looing for these modules, what important characteristics am i looking for?

Ill be dealing with SERIAL DATA... what will i pass onto the radio wave... the RS232 voltage levels or TTL voltage levels... i.e. before i SEND data from my PC... do i need to pass it thru a MAX233A say, BEFORE i send it to the transmitter... and then i WOULDNT need a MAX233A on the reciever ?

This FM lark is all a bit new 2me,

 

[tunedwolf]

Well...you would no longer need the level shifters (233's) on any of the boards, Master or slaves, unless you still plan on wiring them at some later point, because you will be feeding your RF module input directly from the PIC output pin, and obviously the receive output from the RF module goes back to your input pin on the PIC. However, you will need to do a little bit of thinking about how you syncronise your data and clock into a single serial stream. One popular way is Manchester encoding/decoding, and I would suggest that you carefully look into and understand that. It means of course that you will need to write a bit more code for the PIC's, but hey, it's all learning...

As for module selection, there is a vast number of different types available, spread over quite a few frequencies too. Given that you still haven't filled out your location in your profile, I have no idea what's legal and license free where you are, so I can't recommend anything specific.

Radiometrix make half decent transceiver modules across the range, but you may not be able to get them where you are, RF Solutions Ltd is another manufacturer etc...There are loads to choose from, so find something legal to use without a license in your country and all on the same frequencies.

 

[suby786]

cheers mate,

well im from Leicester, UK.

Id still need a shifter somewhere along the lines as the MASTER is connected to a PC... i.e. ill be creating a SERIAL dongle, that incorporates a USB->SERIAL converter (as i have no serial ports on laptop) and so id plug this usb adapter in2my laptop, and on a small pcb board attached to the SERIAL interface will be a MAX233A?and the RF transrecieve module..and id need to step up the RF OUTPUT to RS232 signal levels? (which will be 0-5v??? as id be directly feeding the PIC OUTPUT to the RF INPUT on the slave side??)..