RS 485 communication between two pic micro controllers

[chandrshekar411]

Hi. I am trying rs485 communication between two pic micro controllers 16f876A. I am sending data from master to the slave. I am using rs485.c file provided by the ccs but it is not working. Can anyone provide sample code for building the communication between two micro controllers using rs485 in ccs compiler ?

[Ttelmah]

The example does work.
Problem is probably hardware. Several 'gotchas' on RS485:

1) The termination or the type of buffer used, needs to ensure that when the bus is not driven, the buffer output's a 'high' to the processor. TI and Maxim, both do buffers that guarantee this, or alternatively the termination needs to provide a little bias to the bus to ensure this happens. Most common buffers do not do this, so bus biasing is required.
2) The incoming data line from the buffer, needs a pull up resistor, so when the buffer is turned 'off' there is again a 'high' at this point.
3) Remember RS485, is a 1:1 connection, not a crossover connection like RS232. So the A line on each device needs to connect together.
4) General thing, before starting, check each processor works, and is working at the right clock speed. As with any asynchronous bus, timing is vital.

Best Wishes

[chandrshekar411]

what modifications i have to do for rs385.c sample code for sending "chandu"string as data to the slave how i can retrieve that data in display at slave side.I did not understand how to use the function rs485_send_message(),res485_get_message for sending and reeving.
my code for transmitter.
#include <16f886.h> // transmitter
#fuses HS,WDT,NOPROTECT,BROWNOUT
#use delay(clock=20000000,restart_wdt)
#use rs232(baud=9600, xmit=Pin_b7, rcv=Pin_B6,enable=Pin_B5, bits=8,errors, stream=RS485)
#use rs232(baud=9600, xmit=Pin_b7, rcv=Pin_B6,enable=Pin_B5, bits=8,errors, stream=RS485_CD)

#byte PORTA=0X05
#byte PORTB=0X06
#byte PORTC=0X07

#include<Define.c>
#include<stdio.h>
#include<math.h>
#include<stddef.h>
#include<string.h>
#include<stdlib.h>

#include<LCDLEDOP.c>
#include<rs485.c>

char ch[7]="chandu"
void init_io()

{
set_tris_a(0Xff);
set_tris_b(0x40); //0100 0000
set_tris_c(0x00);
}
VOID SERVOMAX()
{
lcd_gotoxy(1,1);
printf (lcd_putc, "SERVOMAX INDIA LTD.");

delay_ms (1000);
SETUP_WDT(WDT_2304MS);


}

Void main()
{

init_io();
delay_ms(10);
lcd_init();
delay_ms(500);
// SERVOMAX();
while(1)
{
restart_wdt();
rs485_send_message(0x11,3,ch);
lcd_gotoxy(1,2);
printf (lcd_putc, "RS 485 Comm");


}
}

receiver code:
#include <16f876a.h> // reciever
#fuses HS,WDT,NOPROTECT,BROWNOUT
#use delay(clock=20000000,restart_wdt)
#USE RS232(baud=9600,xmit=pin_b7,rcv=pin_b6,ENABLE=pin_b5,bits=8,stop=1,parity=n,ERRORS,STREAM=RS485)
#USE RS232(baud=9600,xmit=pin_b7,rcv=pin_b6,ENABLE=pin_b5,bits=8,stop=1,parity=n,ERRORS,STREAM=RS485_CD)

#byte PORTA=0X05
#byte PORTB=0X06
#byte PORTC=0X07

#include<stdio.h>
#include<math.h>
#include<stddef.h>
#include<string.h>
#include<stdlib.h>
#include<Define.c>
#include<LCDLEDOP.c>
#include<mydriverrx.c>

int8 msg[32];

void init_io()

{
set_tris_a(0Xff);
set_tris_b(0x40);
set_tris_c(0x00);
}
VOID SERVOMAX()
{
// lcd_gotoxy(1,1);
// printf (lcd_putc, "SERVOMAX INDIA LTD.");
lcd_gotoxy(1,2);
printf (lcd_putc, "RS 485 Comm");
delay_ms (1000);
SETUP_WDT(WDT_2304MS);
}

Void main()
{

init_io();
delay_ms(10);
lcd_init();
delay_ms(500);
SERVOMAX();
rs485_init() ;

while(1)
{
restart_wdt();
rs485_get_message(msg);
lcd_gotoxy(1,1);
printf (lcd_putc, "A:%c%c%c%c%c%c", msg[0],msg[1],msg[2],msg[3],msg[4],msg[5]);

}
}

please help me what mistake am doing

[ezflyr]

Hi,

Ttelmah was kind enough to give you a detailed reply, and you essentially ignored 100% of what he said. Basically, he said that you need to validate your hardware before considering any software issues. Because you ignored this advise can we assume this is not a hardware based project, but simulation only, ie. Proteus?

Please post your schematic for the project, and a photo of your hardware so that we know what we are dealing with!

John

PS I know you are new to the forum, but didn't you think it was a little bit strange that the code you posted doesn't look anything like the the code posted by other users? When you post code, you should use the 'Code Tag' feature of the forum message editor. If you do this your code will be much easier to read!

[Ttelmah]

Cart before horse applies here.

Start by just turning the output buffer control on on board one, and use standard RS232 commands to send a repeated message. Have the second device set it's buffer to receive, and have it read the message. Do something simple like turn an LED on/off according to the message.

Once you have the buffer and transmission working in one direction, reverse the buffers, and get it working in the other direction.

For simple one device to another communications, this is really all that is needed.

The point about the RS485 drivers, is they are designed to allow you to have multiple addressed devices on a single bus. If you want this, then move on to trying these, _but get the hardware working first_.

I see ezflyr posted while I was typing.

Best Wishes

[chandrshekar411]

Am sending data using fputs function like below. when am sending single character the data is received at receiver.if i send string in fputs function the data is not received .
transmitter:

[ckielstra]

Computers are stupid, they only do exactly what you tell them to do. When you make only a tiny mistake in telling them what to do, the outcome will be different from what you want it to be.
Your answers are full of the tiny mistakes that make them useless. This is not a language translation problem but is an attitude problem.

[apsua]

Hello,
I would ask, why the #use rs232 directive defined two times? The stream is different yes, but why is it necessary?
Thanks.
_________________
Sara sapsoup.