ulot

sorry i understood the byte and bit stuff, i just made a mistake there. The aim of the project is simply to test the how well data can be transmitted over a power line. the data i send is not the problem. but i think i'll send ASCII characters just to see if i can send an "A" and receive an "A" on the other end. like i said i could also transmit single or byte data also not necessaritly ASCII, is there much of a diffrence so long as it is binary data?

TiagoSilva

Since i'm better on programming than analog electronics, i can give a little tip on this, well by convension, any String sequence is ended with an "invisible" character wich is the '\0', this is the same as the ASCII value 0, in other words, it means nothing LOL, you don't need to detect an String start because you know when it ends !
One last thing, this one last tip is used in a many protocols that use RS-232 as an transmittion channel, since you will send Strings, send all kinds of data as String, convert all numbers to Strings, that way it will be easy to decode and parse the values on the other side

One example of one know protocol that do things this way is the NMEA0183 for GPSs

(don't complicate something as simple as this...)

Have Fun !

__________________
Best Regards,
Tiago Silva.

ericgibbs

hi,
From what you are saying, its just for demonstrating data transimission using the local power lines,
I would suggest ASCII coding would be easier to use as a starting point.
By using ASCII you will be able to use a simple program in your PC, something like the free version of Procomm,
there are lots of free Comms programs on the web.

Terminate the transmitted byte/character with a CRLF code, thats your data, then 0Dh and then 0Ah.
Some PC programs will wait for this CRLf before they display the data/string.

For the Binary data, intially use say 4800,N,8,1 the 8 is the width of the byte [number of bits that make the byte]
The PC program will have to be able to decode the binary data and print the corresponding character.
Note: byte from 0d thru 31d are control characters.

With Binary data transmission you cannot use CRLF as a terminator.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

ulot

The power line modem IC is the ST7540 FSK modem.
It simply receives binary data, modulates it using the FSK modulation scheme and transfers the data over the power line. In receiving mode, the chip demodulates the FSK signal and recovers the digital signal, transmitting it back to a receiving PC or any device.

the problem is that the chip takes some time to analyse the FSK signal before it starts to demodulate it. within this time, some of the data modulates will be lost. this is why a preamble must be added to the actual data so that none of the data is lost. The technical suport from the manufacturer recormends about 2bytes of preamble to be sent with the data

so i need a microcontroller to manage the communication between the PC and the chip, not to mention the fact that it chip has a select pin that changes its mode from transmission to reception and well as a configurable register.

the register selects which kind of transmission mode, (Sync or Async), channel frequency, baud rate with which to expect incoming serial data and many other functions.

i already have a PC program to send ASCII characters over the serial port which i intend to use to test the power line interface.

so i guess thats it about the kind of data i'm using and why the preamble is needed.

ericgibbs

hi,
The preamble for the string is so that the FSK circuit can lock onto to the incoming FSK, you have answered this part.

You could try having a fixed length data string, say 2 bytes of preamble, 12bytes of data and CRLF, a total of 16 bytes.

In the PC program save all the received bytes in a string, get the length of the string, which may vary on how well the FSK locked on during the preamble.
You could use a ASCII Control code for the two pre bytes.

Use the right 14 bytes of data in the string as 'valid' data.

Which part do you require a more detailed explanation.?

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

ulot

2 byte string, 12byte data and 2bytes of CLRF. what is CRLF or did you mean CRC?

my main proble now is how to detect the beginning of my data since i know my first two bytes before the data will not be complete?

What do you mean by save the received data is a string? is that the same as creating a number of registers to store each byte?

Or are you asking that i send all the data received straight to the PC and let it sort out the data itself? Implying that the PLC simply keeps storing the data until the line is empty then transfers it raw to the PC to process. if that's the case, i have to break up the entire data into RS232 data size then have a PC program reassemble the data.

David James

"CRLF" = carriage return, line feed; two characters with the values 0x0A 0x0D.

ericgibbs

hi,
The nature of the transferred data, raw or processed, is a decision you make when you are writing
the programs for the PC and the PIC,
The UART's are not bothered if the data is raw or processed.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/