RS232 problems

[sandy wilson]

Hello

Can anybody help me with this.

I am using the following code for transmitting a packet on the RS232 line using UART1

In transmit function:-

disable_interrupts(INT_RDA);
output_high(RS485_DRIVER);

while(i < Lengths)
{
TxtmpHead = (USART_TxHead + 1) & USART_TX_BUFFER_MASK;

while(TxtmpHead == USART_TxTail);

USART_TxBuffer[TxtmpHead] = REPLY_MESSAGE[i];

USART_TxHead = TxtmpHead;

i++;

enable_interrupts(INT_TBE);
}

output_low(RS485_DRIVER);
enable_interrupts(INT_RDA);

In TX interrupt:-

#int_TBE
void USART_TX_interrupt(void)
{
byte tmpTail;

if(USART_TxHead != USART_TxTail)
{
tmpTail = (USART_TxTail + 1) & USART_TX_BUFFER_MASK;

USART_TxTail = tmpTail;

TXREG = USART_TxBuffer[tmpTail];
}
else
{
disable_interrupts(INT_TBE);
}
}

RS485_DRIVER is defined as pin c4.
TXREG is defined as #byte TXREG = 0xFAD
PIC type is 18F8722
Compiler version is 4.114


What is meant to happen is the the packet to be sent is transfered from array REPLY_MESSAGE into array USART_TxBuffer[tmpTail] and then the TBE interrupt is enabled. When there is still data in the USART_TxBuffer to be sent it is shipped out otherwise the interrup is disabled.

Whats happening is that the RS485_DRIVER pin goes high as reguired but as soon as the first bit of data is sent from the UART it is set to low again. It happens before the command output_low(RS485_DRIVER);
is reached and is occuring somewhere in the while loop or in the interrupt.

Has anybody got any idea why the pin c4 should be set to low when transmitting on the uart?


Thanks for any help

SandyW

[Ttelmah]

1) Learn to use the 'code' button. Makes reading your code 100* easier......

2) Of course it does. You are buffering the data to send, then dropping the RS485_DRIVER line in the main, as sooon as you have stored the data, while the TX interrupt is still sending. You need to drop this line _at the same point where you disable the TX interrupt - where the buffer is empty_. Or, don't use TXREG directly, use the CCS putc function, and add the enable line to the #USE RS232 setup.

3) Don't bother disabling the RS232 receive. You are presumably doing this because your RS485 hardware will receive the bytes being sent?. Turning off the interrupt will still leave a couple of characters in the hardware receive buffer. Have the RS232 receive routine, read characters and throw them away (rather than storing them), if RS485_DRIVER is low. Leave the interrupt enabled, and the characters transmitted will then automatically be read and the buffer will remain clear.

Best Wishes