XTR-434 transceiver

[dhouston]

Page 3 of the manual says...

• Bit ON/Bit OFF relation: the Data Slicer is optimized for a 50:50 duty cycle. It will continue to operate,
  even with bigger distortion and less tolerance to interferences, till a 30:70 or 70:30 duty-cycle.
  Therefore there is no possibility to directly transmit an RS232 sequence if no action is done to balance the 1
  and 0 bits ratio, as the duty-cycle could reach up to 90% preventing the proper performance of receiver. It
  will continue to operate, even with bigger distortion and less tolerance to interferences, till a 30:70 or 70:30
  duty cycle.

• XTR-434
  While modulation is applied to TX, conditioning input pin [pin 14] with a logic signal, it is reccomended not
  to exceed 200 μs continuous time with no transition from ON to OFF or OFF to ON. This not to downgrade
  the RX sensitivity. It is requested that modultation is carried on with techniques that allow a low duty-cycle,
  such as Manchester coding, 8 to 12 bit coding or other available technique. If no bit balancing technique is
  used, if it is requested to work with RS232 protocol, a minimum speed of 57.600 bps is required to assure
  maximum performance, transmitting, for example, one byte followed by the complementary byte (byte
  balancing).

The Data Slicer is a comparator with a capacitor on one input. The data goes to both inputs but the capacitor charges to an average of the data. When the data is higher than the average level, the output is high and when the data is lower than the average level, the output is low. The higher baud rate allows less time for the capacitor to discharge which may explain why it appears to work at higher rates but you should not count on this being reliable unless you use byte balancing and rates of 57.600 bps or higher.

Follow the manufacturer's recommendation and use manchester coding (or byte balancing) or find another transceiver. You can find thousands of web pages that describe manchester coding and a search of the forums will turn up Melanie's example code.

Byte balancing means you send each byte twice - once normally but followed immediately by its bitwise complement (i.e. each 1 becomes 0 and vice versa). The sequence is Byte1 ~Byte1 Byte2 ~Byte2...ByteN ~ByteN. On the receiving end, each pair must sum to $FF. If not, there was an error in transmission or reception.

You can see some oscilloscope screenshots showing the effects of the data slicer here...

• http://davehouston.org/rf-noise.htm

[Ioannis]

What Dave is trying to tell you is that you cannot raise the transmit line to 1 and expect that at the receivers output you will get a steady 1 for long time. This is NOT possible with the XTR module. It is possible with other tranceivers that are capable of direct RS-232 transmission.

RF is very different than direct cable link.

As for Manchester do a search and you shall find. Alot of infos. With 2 words works like this:

to send 1 0 0 0 0 0 0 1 the PIC should send the 1 0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 0, that is the 1 is translated to 1 0 and the 0 to 0 1. So even if you send a steady 1 or 0 the XTR wil get a changing 1 0 or 0 1 at the input, cancelling out the DC component that a steady 1 or 0 carries.

Hope it makes some sense now.


Ioannis

[Toufic]

thank you guys...
i think now i am going to change the transmitter...
but the problem is that we are a small country here and cann't find all kind of transceivers here..so if u know a transceiver that could work in my situation please send its number to me and i will check it out..
thank you again

[Ioannis]

If the encoding can and it sure can be done in software there is no need to change the tranceiver. It is no difficult to do it. But then, it is up to you...

Ioannis

[dhouston]

Byte balancing is super simple. I've provided an example in the Code Examples forum. So, it's not absolutely necessary that you change the hardware.

However, there may be other factors (e.g. cost) that would make a change more attractive but, without more detailed knowledge of the application, it's impossible to suggest other hardware. Is this a one time project or do you plan to manufacture a product? If it's a product, you'll need to meet FCC and/or CE requirements, depending on where you sell. What is the application? What range is needed between transmitter and receiver? Is a transceiver necessary or can you use separate transmitters and receivers? ASK modules are usually 20-30% as costly as FSK. There are some very good RF modules (made in China) available through an EBay store for extremely attractive prices (cheaper than dirt - even with collapsing real estate prices).