RF Wireless Serial link questions

rgamage

Hello,
Just received the set of transmitter / receiver pair for serial communications. I'm concerned that they appear to be all the same frequency (there are no designators or IDs to indicate different frequencies), so the tx/rx pair on one end will interfere with each other. Just wanted to know if anyone had any experience with them.

I plan to use one pair at a PC, and the other pair at an MCU on my robot. The robot would have a tx/rx pair, and the PC would have a tx/rx pair. Ideally they would be running on two different frequencies.

I suppose I could disable the receiver while transmitting, so I wouldn't get echoed characters back, but on the side that is the PC, there is no easy way to do that.

Thanks for any feedback,

Randy

Bruce Bates

Randy -

I presume there's a PBASIC Stamp in there somewhere. Presuming that's the case, PBASIC does not support full duplex communications. The only reason I can imagine for using two different frequencies would be to perform full duplex communications.

Regards,

Bruce Bates

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
<!--StartFragment -->

rgamage

No, I'm not using a stamp, I'm using an Atmel, but that's not really the issue. If you think about the PC sending out a character, it will go to the transmitter, get transmitted, and that character will be received both by the Atmel in the robot, and also by the receiver at the PC side. So the PC will see an echo of everything it sends, unless it's on a different frequency. Same with the Atmel, when it sends it will see echos of everything it sends.

The Atmel can easily disable the receiver while it's transmitting (there's a disable pin on the receiver), but the PC can't easily do this, because PCs obviously don't have handy little I/O pins to do this kind of thing.

So the only real problem is on the PC side, I guess I just have to add code to receive and ignore the echo characters of everything I send.

I was hoping that since these units were sold as a set of four just for this purpose, that I could plug them into an existing control system without having to customize the logic on both sides.

Anybody have experience with these units? It seems like there is very little info from people who have actually used them, but they must have sold more that one set of them!

Thanks,
Randy

stamptrol

The echo at the PC is usually controlled by the program that does the sending. In VB for example, there are a set of software controls for the RTS line which typically is used to key the transmitter. Or, the software knows to expect the comm buffer to include the string just sent. It can then flush or ignore the buffer of echoed data.

Its quite common in half-duplex systems for there to be one master broadcasting to everyone who can hear, but including an ID character in the string so only one listener will reply. Thats why many wireless devices come as transceivers which have the transmitter and receiver integrated into one package with an easy way to go from listen to talk. With separate units, you just have to implement the switching in your software.

Regards

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tom Sisk

http://www.siskconsult.com
·

rgamage

By the way, I just finished my initial testing of these devices, and they work great at 4800 baud, but they are totally garbled at 9600 baud or above. I tested them using a simple loop-back check with HyperTerminal, with the tx & rx units 4 inches apart.

On the Parallax website, it clearly says they can go up to 19200 baud.

False advertising is a bummer!

Randy

rgamage

More test results...
Since the transmitters are the same frequency, they interfere with each other. This means only one tx can be on / enabled at any time, so now I realize you MUST have intelligence on both sides of the link to constantly turn the transmitter on only when you are transmitting. This means you either need to add a micro-controller on the PC side, or hack a circuit together that takes the RTS line or some other similar line from the PC's COM port and buffers it and ties it to the TX enable pin on the transmitter. Then your PC's code needs to control this pin during communications. They could have made this so much easier by simply making them different frequencies. What a shame.

I think I'll give up and buy Sparkfun's wireless modem:
http://www.sparkfun.com/commerce/product_info.php?products_id=155

-Randy

John R.

I've also found the Parallax units to be somewhat susceptible to interference from at least some external devices. For example, I can’t work with mine if my wife is on the computer with her wireless keyboard/mouse. I can’t read her signals cleanly, but I do get “garbage” on the receivers.

Now, in my ulitimate application (robot in the woods), this wouldn't be an issue, but it sure makes development a bear...

Also, except for the above mentioned interference, I had no problems at 9600 bd.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
John R.

8 + 8 = 10

Warrlok

what about 1 of these

··············· ·http://www.neteon.net/prod.aspx?clvl=4&c1=1&c2=83&c3=256&p=416



·········· and a cheap bluetooth dongle for the computer end . the interface is uart ?·could this work for short range reprograming ?

······· it says transparent serial cable replacement

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Problems are the "roads" of life,
solutions are only "onramps" to the next problem



············································· "Brad Smith"
·

Steel

As far as the RF Transmitters/recievers...Yes, there are some drawbacks. Unfortunately, they are only single-frequency transmitters, so it is very easy to get cross-traffic. On the webpage, it suggests you use 1 transciever and 1 transmitter or vise-versa...

...and I believe they do have a maximum baudrate of 4800. I had a project in which I had to have the baudrate at 2400 to keep it talking...fairly dismal.

There are other RF Devices that you can use such as XBee / XBee Pro
http://www.maxstream.net/products/xbee/xbee-oem-rf-module-zigbee.php

Nordic:
http://www.sparkfun.com/commerce/product_info.php?products_id=151

or use wifi wireless