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Topic: a module that can receive and transmit at the same time? (Read 1 time) previous topic - next topic

firashelou

Jul 05, 2013, 08:35 am Last Edit: Jul 05, 2013, 10:38 pm by firashelou Reason: 1
hello guys,
i am using the RF module in the attached file

so my question is : is there any module similar to this one (cheap and not hard to work with) that can receive and transmit in the same time not just transmitter and receiver ? or i just have to use 2 of this one ?!
Aiming for the unreachable !

Grumpy_Mike

No. Recieving and transmitting at the same time is tricky. The two frequencies need to be seprated by at least 1MHz and you need a thing called a cavity filter to put a notch in the RX at the frequency of the TX.
Radio hams do it with repeaters but it is tricky.

firashelou


No. Recieving and transmitting at the same time is tricky. The two freq
uencies need to be seprated by at least 1MHz and you need a thing called a cavity filter to put a notch in the RX at the frequency of the TX.
Radio hams do it with repeaters but it is tricky.

ok so you suggest I just use 2 different modules of this one a better choice? or an xbee?
Aiming for the unreachable !

Grumpy_Mike

What I am saying is that you can't do this unless the frequency is widly seprated.

However I would question your need for full duplex operation. Most applications can make do with half duplex. Even the Xbee is half duplex?

CrossRoads

Xbee uses the UART, so could be full duplex?

http://www.digi.com/products/wireless-wired-embedded-solutions/zigbee-rf-modules/zigbee-mesh-module/
Hard to tell without digging into a specific model tho.

Even so, hard for a single-thread microcontroller to go both directions at the same time.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

firashelou

what do you mean duplex?  and using 2 receiver and 2 transmitter is ok?
Aiming for the unreachable !

Grumpy_Mike

Quote
what do you mean duplex?

Both directions at the same time.

Quote
and using 2 receiver and 2 transmitter is ok?

Not normally. You need the frequency for one pair to be a long way off the frequency for the other pair otherwise it will not work.

Quote
Xbee uses the UART, so could be full duplex?

No they are half duplex.

firashelou

then what do I. have to. do ?
what I need to. do for is this : I want to make a speed measurment on the car and send info and camera picture to the rc and from rc I am sending message for throtle and steering etc ... so?
Aiming for the unreachable !

retrolefty


what do you mean duplex?

Duplex describes the capability of the communications link. The arduino hardware serial port is capable of full-duplex communications meaning data can be going in both directions between the two devices (arduino chip and external device it's wired to) at the same time. However most simple RF comm modules cannot operate at full-duplex but rather only half-duplex where data can go in both directions but only one direction at any given time.

and using 2 receiver and 2 transmitter is ok?

It's possible but only if the two different RF modules operate on different RF frequency such that they won't interfere with each other if data is being transmitted in both directions at the same time.

Most applications don't in practice require a full-duplex channel but rather a half-duplex master/slave relationship where the slave only sends data when a specific request for data is first sent from the master and the master then waits for the slave device to respond to the request. In this case the Arduino can act as your 'master' under control of your sketch.

Lefty


enhzflep


then what do I. have to. do ?
what I need to. do for is this : I want to make a speed measurement on the car and send info and camera picture to the rc and from rc I am sending message for throtle and steering etc ... so?

Just use a 2.4ghz unit - perhaps an nrf24L01+ module or an a7105 module. Both of them are transceivers, so can transmit or receive.
If I understand right, you'd like to send control data from the Tx to the car. You would also like to send speed and a photo from the car to the Tx.
A basic telemetry setup. Well, the thing with standard rc transmitters  is that they don't send a continuous data stream from the Tx to the car. They only send packets at regular intervals, meaning you have the remaining time to swap the mode of each radio and transmit data back the other way.

Turnigy 9x comes with a module that transmits every 1.496ms, other examples of modules transmit intervals are: Husban 12ms, FrSky 1 way 9ms, FrSky with telemtery 1.496ms. Assuming it takes 100us for a packet to be sent (should be quicker than that, but I'll err on the size of caution), you are spending 0.1ms every 1.496-12ms to actually receive the fwd/backwards left/right signal from the Tx. That would give you anywhere from 1.396/1.496 - 11.9/12 (93.32% - 99.2%) of the time to send data packets back from the car.

So, the nrf modules have a max on-air data rate of 2Mbps, which includes packet pre-amble and transmitter id, so you have a max effective data rate of about 1.5 mbps if using the largest size packets permitted by the radio. The a7105s on the other hand are about 1/4 of that speed. An uncompressed 24bit colour, 640x480 image is 921,600 bytes (7,372,800 bits) So, you'd need about 5 seconds 'flat-out' to transmit an image of that size using the nrf24L01 modules. You'd also have to make sure you've got a camera that can buffer the data. An ov670 with a fifo seems like it could work okay. You'd sure have your work cut-out receiving the radio signal, sending the control the the car electronics and at the same time handling the transmission of the camera data back to the Tx! :smiley-eek-blue: You might be able to realistically expect to transmit that 921,000 bytes from the car to the Tx in about 6 or 7 seconds or so, if you were able to shave off every cpu cycle possible.

Don't even consider less than 5.8ghz if you wish to transmit video!

There's a bunch of r/c  Rx &/or Tx projects around in RcGroups. Here's a link: http://www.rcgroups.com/diy-electronics-199/

firashelou



then what do I. have to. do ?
what I need to. do for is this : I want to make a speed measurement on the car and send info and camera picture to the rc and from rc I am sending message for throtle and steering etc ... so?

Just use a 2.4ghz unit - perhaps an nrf24L01+ module or an a7105 module. Both of them are transceivers, so can transmit or receive.
If I understand right, you'd like to send control data from the Tx to the car. You would also like to send speed and a photo from the car to the Tx.
A basic telemetry setup. Well, the thing with standard rc transmitters  is that they don't send a continuous data stream from the Tx to the car. They only send packets at regular intervals, meaning you have the remaining time to swap the mode of each radio and transmit data back the other way.

Turnigy 9x comes with a module that transmits every 1.496ms, other examples of modules transmit intervals are: Husban 12ms, FrSky 1 way 9ms, FrSky with telemtery 1.496ms. Assuming it takes 100us for a packet to be sent (should be quicker than that, but I'll err on the size of caution), you are spending 0.1ms every 1.496-12ms to actually receive the fwd/backwards left/right signal from the Tx. That would give you anywhere from 1.396/1.496 - 11.9/12 (93.32% - 99.2%) of the time to send data packets back from the car.

So, the nrf modules have a max on-air data rate of 2Mbps, which includes packet pre-amble and transmitter id, so you have a max effective data rate of about 1.5 mbps if using the largest size packets permitted by the radio. The a7105s on the other hand are about 1/4 of that speed. An uncompressed 24bit colour, 640x480 image is 921,600 bytes (7,372,800 bits) So, you'd need about 5 seconds 'flat-out' to transmit an image of that size using the nrf24L01 modules. You'd also have to make sure you've got a camera that can buffer the data. An ov670 with a fifo seems like it could work okay. You'd sure have your work cut-out receiving the radio signal, sending the control the the car electronics and at the same time handling the transmission of the camera data back to the Tx! :smiley-eek-blue: You might be able to realistically expect to transmit that 921,000 bytes from the car to the Tx in about 6 or 7 seconds or so, if you were able to shave off every cpu cycle possible.

Don't even consider less than 5.8ghz if you wish to transmit video!

There's a bunch of r/c  Rx &/or Tx projects around in RcGroups. Here's a link: http://www.rcgroups.com/diy-electronics-199/


well that's a lot of technical info :S which i am not very good at but

first about the RF links well i need it to be quick which mean the camera should let me get a fast picture to be able to driver the car and to get kilometer/hour measurement i don't want it to be slow to get that

and about the camera, do you have any suggestion of a camera which i can use and not very hard to use it which will let me get video ?
Aiming for the unreachable !

MarkT

Mobile phone technology used to be full-duplex analog audio, and this was one of the reasons why the first
mobile phones were large and cumbersome.  I can't remember if the move to GSM did away with the need to be duplex
(still uses separate uplink and downlink channels). 

Unless you need proper duplex its much easier to avoid it.  Its like shouting at the top of your voice whilst
simultaneously listening to the quietest noise you can imagine.  The ratio of transmit to receive power can be
fifteen orders of magnitude or more.
[ I won't respond to messages, use the forum please ]

Grumpy_Mike

You can not use an arduino to send video, and a picture will take ages to send.
It sounds like you do not need to send and recieve at the same time..

firashelou

#13
Jul 06, 2013, 01:08 am Last Edit: Jul 06, 2013, 08:27 am by firashelou Reason: 1

You can not use an arduino to send video, and a picture will take ages to send.
It sounds like you do not need to send and recieve at the same time..


how can i do it then ?
and why you see that it sounds that i don't need to send and receive in the same time ? when i throttle i need the kilometer to run and be displayed on my 7 segment
can you please explain more ?

and thanks a lot for your time
Aiming for the unreachable !

Grumpy_Mike

Using half duplex, that is sending then recieving quickly makes it feel like you are sending and recieving at the same time but you are not.
Imagin send for 10ms and recieve for 10ms and you think it is doing it at the same time butit is not.

Sending video use a specific video transmitter notthearduino, it is not fast enough.

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