Using EZ915 RF modules

[circuitspecialists]

I need help with a simple algorithm or flow chart to help me implement a simple remote data link using the EZ series modules. I would like to use RS-232 protocol but know that I need a preamble and possibly a post-amble string before and after my RS-232 data. Any suggestions would be appreciated. The EZ series of RF modules are Data In/Data Out which means whatever digital data is fed into the transmitter unit will be available as output data from the receiver unit. The problem is that with nothing being fed into the transmitter, the receiver will just pick up random noise which will be output as random data.
Thanks

 

[MikeMl]

...The problem is that with nothing being fed into the transmitter, the receiver will just pick up random noise which will be output as random data.

Isn't that why pull-up or pull-down resistors were invented?

 

[Mike odom]

Or have the transmitter transmit nulls, but that's a waste of transmitter power and it doesn't take care of a turned off transmitter. And I've never heard of pulling down an rf receiver input (or up, for that matter).

I had the same problem when I did an rf control link years ago. It worked fine, until my transmitter turned off then all kinds of digits came pouring out. I used a simple code a mentor of mine had invented for a totally different type of project years before. Basically, my data is broken down into nibbles (4 bits). I transmit a preamble, which tells what version of coded data is being sent. Then, I put a '1' in the high nibble with the first nibble of data in the lower nibble, followed by '2x' '3x' etc. If I need more than 16 nibbles of data, I just repeat the '1x' '2x' etc data stream. My receiver is able to lock on the data stream and decode it with perfection, knowing it is receiving the correct data in the correct format, with a check sum at the end, so it even knows the data it received is correct. Then it knows the preamble isn't some fluke of random patterns. And with the preamble, different systems or parts of a network can ignore data not meant for it and take what is meant, and use the same frequency. Since this system was used to remotely open and close gates at a zoo, you certainly don't want to open the lion's gate when you meant to open the elephant's... etc!!!

 

[circuitspecialists]

Unfortunately a simple resistor doesn't work....the problem is that the noise is real and is seen as random data bits by the circuitry connected to the receiver.

 

[circuitspecialists]

Thanks Mike odom, that is what I need to get started.

 

[Mike odom]

also, using a module like you are, you bypass the FCC requirement of licensing your own by buying into their license. However, the FCC does have rules about encrypting data. You can encode (as in example above), but you can't encrypt. For a given set of conditions, your transmission must be the same.

 

[Mr RB]

Please post some info on that "encrypting" rule Mike, or a link or something.

I think was mentioned before and does not apply to this use. Almost everything sent using these free band transmitters ie encrypted, like garage door opener codes etc. From memory the rule against "encryption" applies only to radio amatuer licenses and the type of transmitting that they do, not to general digital transmitting devices on unlicensed bands.

 

[Mike odom]

no... it implies directly to digital transmitting devices, and there are no 'unlicensed' bands.... all bands are controlled by FCC and have to be licensed. Certain bands, like SW etc are set aside but are still strictly controlled (ie, you must broadcast your call sign every ?). Garage door openers operate on very specific bands set aside for their use, and you are not allowed to send digital information like this on them. Since you are using a 'prefab' device, you are transmitting and receiving on it's licensed band, one that falls in the "one station mobile, one fixed" category. I am away right now, but will post these rules when I get a chance...

BTW, build a transmitter and start transmitting on an 'unlicensed' band, and see how long before someone comes knocking at your door...

 

[Mike odom]

this is from the LINX Technology website:



https://www.linxtechnologies.com/en/support/faq

For applications like amateur HAM radio, the FCC requires that the end user have a license to operate the radio equipment. For the portions of the spectrum governed by Part 15, the end user does not need a license. Instead, the product manufacturer must have the equipment tested in an FCC authorized facility and receive certification and an FCC ID number for the transmitter and a Declaration of Conformity (DOC) for the receiver before the product can be legally sold in the US. In this sense, the equipment must get the license so that the end user does not need one.
▾ Where can I obtain a copy of the FCC rules and regulations?

The FCC maintains an electronic copy of the rules on their website. Refer to the FCC resource document for a copy of the sections applicable to Linx products. For a printed copy, you should contact the Government Printing Office in Washington, D.C., and indicate that you need a copy of Title 47 of the Code of Federal Regulations (47CFR). If your need is for equipment authorization, you will require Volume 1 which contains Parts 0-19. Their telephone number is (202) 512-0132. You can also contact the Government Printing Office in your local area for a copy of the rules. The telephone number for the GPO in your area can be obtained from your telephone directory or operator. It would be listed under the federal government.

 

[Mike odom]

it's not letting me follow the link into the rules section, or maybe the wireless here is so slow it's timing out. Go to the Linx link, click on the "where to find out the rules" link, and go in there, if you can...

BTW, there used to be a PRB, personal radio band, where you could transmit/receive freely, but with the advent of personal computers making it possible that you could make your own cell phone, the fledgling cell phone industry paid congress off to remove it in the 80's.. just another case that shows we have the best congress money can buy...

 

[Mr RB]

Sorry, I'll apologise for using the term "unlicensed" as that is not technically correct as you pointed out, although "unlicensed" and "license free" are terms that are very commonly used to describe these little modules in the 433Mhz bands etc.

The OP's module is a 915 MHz band device so according to your Lynxtechnologies link;

"The FCC per Part 15, section 231 (a-d) does not allow the continuous transmission of real time variable data in the 260 – 470MHz band. It is restricted to control signals, such as for garage doors, remote keyless entry, and security systems. Per section 231 paragraph e, you can send variable data as long as it is at half the power, the transmission does not last more than 1 second, and there is at least a 10 second gap between transmissions. At 900MHz, there are no restrictions on the type of data to be sent. Please see Application Notes AN-00125, AN-00126, and the FCC Resource Document for more details.

Unfortunately the FCC rules link; https://www.fcc.gov/oet/info/rules/ does not seem to open on my PC either, it just 100% hangs. Trust a govt department to be 100% useless.

I still have seen nothing officially stating the data sent using these 315/433/915 MHz digital RF modules "cannot be encrypted". They are used everywhere for security alarms and door opener type apps and it would be vital, even normal, for the data to be sent encrypted.

 

[4pyros]

I still have seen nothing officially stating the data sent using these 355/433/915 MHz digital RF modules "cannot be encrypted". They are used everywhere for security alarms and door opener type apps and it would be vital, even normal, for the data to be sent encrypted.

I never heard of the FCC regulating data encryption.