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Easiest way to setup communications? RS232?

 
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DarkKobold



Joined: 21 Jan 2004
Posts: 4

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Easiest way to setup communications? RS232?
PostPosted: Mon Jan 26, 2004 10:01 pm     Reply with quote
I've not yet done any communication between 2 pics. From what I understand, it isn't extremely difficult, but I am hesitant to proceed on my own. I am guessing that the built in RS232 communication is the best, but I have a few questions before using it.

First - here is my setup -
One PIC is acting as a Timer and score collector from the judges watching the match, taking care of the main controls.

The second PIC is running the score board - telling it what to update the scores and times to be.

I'd like it if I could alway send the information in a constant manner - I.E.

Time - Round - Score A - Score B - Extra Command (Ring Buzzer, etc)

I am hoping to avoid using the TBE interrupt...

So, where do I start?
Neutone



Joined: 08 Sep 2003
Posts: 839
Location: Houston

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PostPosted: Tue Jan 27, 2004 8:19 am     Reply with quote
You need a comunication protocal that comunicates in packets of data. I suggest you design your own because your needs are very simple and consistant. You can download MODBUS protocal specifications and get some ideas from it. It is availabe from www.modbus.org
DarkKobold



Joined: 21 Jan 2004
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PostPosted: Tue Jan 27, 2004 10:45 pm     Reply with quote
That modbus site confused me greatly...

Is RS232 communication too difficult for a small usage? What circuitry does it require?
Jannie Hamman



Joined: 16 Sep 2003
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Location: South Africa

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PIC to PIC comms
PostPosted: Wed Jan 28, 2004 12:15 am     Reply with quote
Laughing Look at the help file of CCS PCW! All is explained there...
_________________
Cheers,

Jannie
Ttelmah
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PostPosted: Wed Jan 28, 2004 3:27 am     Reply with quote
DarkKobold wrote:
That modbus site confused me greatly...

Is RS232 communication too difficult for a small usage? What circuitry does it require?

Understand, 'RS232', 'RS485' etc., are descriptions of 'signalling standards'. They describe the voltages to be used to send a signal, together with other features (how many receivers can exist, and the 'range' for given data rates etc.). Then seperately, there is the 'protocol'. This describes the actual 'format' the data is sent, with 'headers', the data itself, and methods to verify that the data is intact (checksums, parity etc.).
Now the PIC chip, generates a standard asynchronous serial stream, using TTL levels, directly from the chip. To turn this into 'RS232', simply requires a 'transceiver' chip (this inverts the signals, and increases the voltages to the levels required by 'RS232'). Depending on the enviroment involved, such a transceiver can be as simple as a DS275, which is a single 8pin chip, giving 'pseudo RS232' levels, with one transmit buffer, and one receive buffer. Now I say 'pseudo RS232', this chip 'steals' voltage from the incoming receive line to power the transmit line. The next step 'up', are chips like the MAX232. This has two receive buffers and two transmit buffers (allowing a 'handshake' line to also be generated), and a small power supply circuit to generate the +/-12v required for RS232 transmission. The original version, needed capacitors externally for this power supply, but latter variants reduced the size of these (MAX232A only needs 0.1uF), and the MAX233, runs without any external capacitors, making the circuit very simple indeed.
So you need two seperate things. The hardware transceiver, that then allows you to 'signal' at RS232 levels, and then the protocol, to determine how the data is sent. The 'simplest', would just involve you sending something like a 'description' character, and a block of text, with the actual 'values' involved, with no checksum etc.. Proper 'protocols' (like Modbus), contain all the extra 'goodies' to go round the outside.
Think of 'RS232', a bit like the 'tones' used for morse. Then the serial data, is the 'code' itself, with the 'protocol', being the language/keywords used to send the message.

Best Wishes
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