How difficult is it to produce Tx signal by a computer ?

[dtrip]

Hello people,

I am interested in flying an RC airplane using a laptop computer, instead of a Tx. I can program the computer because Im a computer programmer, no problem there.

However I fail to understand how can I send RF signals via a computer. Obviously I must interface "something" with the computer (possibly via serial port ?). But what is this "something" ? Can anybody shed some light ?

Thanks in advance,

Dimitris
Greece

[Dan Baldwin]

Probably the easiest way to produce the RF is by using the training port of a standard transmitter. When you hold down the trainer button on the transmitter it will use whatever pulse train is coming into the trainer connector. All you have to do with the laptop is produce a PPM pulse train to feed to the trainer port, and you might need to shift voltage level to whatever the transmitter wants. The pulse train will consist of a start pulse, and a pulse for each channel starting with channel 1. The pulse will be between 1 millisecond and 2 milliseconds from the last pulse. I'm not sure what width the pulses need to be, but I don't think it's critical. Probably about 150 microseconds. Some of the other guys will have a better idea.

Now producing the pulse train with the laptop is another matter. If you have a parallel port or an RS232 serial port, you can directly manipulate one of the pins on the port to produce the pulses, but most modern laptops don't have parallel or serial ports. You could continually send out serial packets on the USB port (or a USB serial adapter) that contains the position for each channel, then use a microcontroller to convert that into the PPM that the transmitter needs.

Dan

[AleG]

Hmmm, it would be very very nice to have an interface for my PDA to use as the brains of a transmitter.

[dtrip]

...most modern laptops don't have parallel or serial ports...
Dan


Thanks your idea was very helpful,

btw there are USB to serial/parallel converters, Google
"USB to serial converter" and you ll come up with many results,

A random example at $15 is this:
http://scantool.imechatronics.com/usb2serial.htm

[Brandano]

Better go the serial route, though. Most "USB to parallel" converters are actually "USB to printer" converters. No difference in the hardware, but the drivers available for them only allow for limited control, and some pins might not be implemented at all. However, getting the signal to the TX isn't as hard a problem as getting a feedback loop from the plane itself. When you fly a plane manually you constantly adjust the controls based on what your eyes tell you and on a huge number of really complex mathematical operations that your brain does in realtime. A computer might not be able to do the same. Probably the way to go is to modify a freeflight design for "computer altered" flight rather than "computer controlled" flight.

[vintage1]

To do this without any extra hardware - that is generate a PPM stream to feed a transmitter- requires some fairly direct hardware manipulation and some quite critical timing: Most people would opt for a PIC type chip and let that do all that, and simply drive it via serial USB.


Otherwise you are going to have to code up some low level drivers for port interrupts and timer interrupts. Its possible, but quite specialised.

[key40]

Dear Dimidtri,

I think the best way to go is by using a simple PIC
like 16F628.It's very easy to program it, if you have a simple pic-programer.Then you' ll be able to send commands from the laptop to the pic through the serial port( they have both one of them).PIC microcontroller will be the interface between your laptop and the transmitter.It has also a PWM function.I don't know if you have any experience with RF to make your own transmitter, otherwise you must use the training port of a ready-made TX.


Nice project... :cool:

key40

[Malc C]

wasn't there a similar thread a while back ?

Have a look at the USB sim interface from milehigh wings. It has bi-directional coms between PC and TX and might save you a lot of trouble

http://milehighwings.com/

[Endurance]

There are already a few of these devices on the market. As Malc says MileHighWings and also Endurance R/C (www.enduarnce-rc.com/pctx.html) will do the job.

There are plans for both serial and parallel versions as well.

[jeffs555]

Endurance, you need to watch your typing, you have a typo in your link. http://www.endurance-rc.com/

[Julez]

I once managed it to drive a Servo with the pulses generated by my soundcard. If it works for a servo, it might work for a module, too.

[ZAGNUT]

if your laptop has a docking port then the serial connection should be there, you just need to know the pinout.



dave

[jeffs555]

I once managed it to drive a Servo with the pulses generated by my soundcard. If it works for a servo, it might work for a module, too.
It shouldn't be too difficult to create a program to generate a standard PPM waveform on the fly and play it on the sound card. The only problem I see is that with a 44kHz sample rate, the resolution wouldn't be that great, only 44 steps from one end to the other. A good sound card with 192 kHz would give better resolution.