dsPic33E : RS485 Communication issue - microcontroller

Question

I am facing issue while communicating serially over RS485 port.
I am using dsPic33E microcontroller with Max485.
using breakpoint i analyzed that Whenever I send "0x00" to controller, "0xFF" is received. Then I sent "0x01", "0xFD" is received on controller.and so on.
Also i tried to use loop-back logic, means sending back the received character, but every time I receive "0x00" for any value sent.
I am unable to get the issue. below is the snapshot of the code I am using :
// RS485
TRISBbits.TRISB6 = INPUT_PIN; // RX - RB6/RP38 PIN<42>
TRISBbits.TRISB7 = OUTPUT_PIN; // TX - RB7/RP39 PIN<43>
TRISBbits.TRISB8 = OUTPUT_PIN; // !RE/DE Control Pin RB8/RP40 PIN<44>
// RS485 Config
#define RS485_TX PORTBbits.RB6 // RS485 Transmitter
#define RS485_RX LATBbits.LATB7 // RS485 Reciever
#define RS485_CTRL LATBbits.LATB8 // RS485 Control Pin
void __attribute__((interrupt, no_auto_psv)) _U4RXInterrupt(void)
{
rs485Char = U4RXREG;
RS485_CTRL = 1; // Enable driver
U4TXREG = rs485Char;
RS485_CTRL = 0; // disable driver RE/DO
}
void InitClock( void )
{
PLLFBD = 63; // M=65
CLKDIVbits.PLLPOST = 0; // N2=2
CLKDIVbits.PLLPRE = 0; // N1=2
// Initiate Clock Switch to FRC oscillator with PLL (NOSC=0b001)
__builtin_write_OSCCONH(0x01);
__builtin_write_OSCCONL(OSCCON | 0x01);
// Wait for Clock switch to occur
while (OSCCONbits.COSC!= 0b001);
// Wait for PLL to lock
while (OSCCONbits.LOCK!= 1);
}
void InitRs485(void){
// configure U1MODE
U4MODEbits.UARTEN = 0; // Bit15 TX, RX DISABLED, ENABLE at end of func
U4MODEbits.URXINV = 1; // 1:URXINV Idle state is '0' ; 0=UxRX Idle state is '1';
U4MODEbits.ABAUD = 0; // Bit5 No Auto baud (would require sending '55')
U4MODEbits.BRGH = 0; // Bit3 16 clocks per bit period
U4MODEbits.PDSEL = 0; // 0 : 8 bit,no parity; 1 : 8 bit,even parity; 2 : 8 bit,odd parity; 3 : 9 bit,no Parity
U4MODEbits.STSEL = 1; // 1 : 2 Stop bits; 0 : 1 Stop bits
// Load a value into Baud Rate Generator.
U4BRG = BRGVAL_RS485; // 60Mhz osc, 9600 Baud
// Load all values in for U1STA SFR
U4STAbits.UTXISEL1 = 0; // Bit15 Int when Char is transferred (1/2 config!)
U4STAbits.UTXISEL0 = 0; // Bit13 Other half of Bit15
U4STAbits.UTXINV = 1; // 1:UxTX Idle state is '0' ; 0=UxTX Idle state is '1';
U4STAbits.UTXBRK = 0; // Bit11 Disabled
U4STAbits.UTXEN = 0; // Bit10 TX pins controlled by peripheral
U4STAbits.URXISEL = 0; // Bits6,7 Int. on character received
IPC22bits.U4RXIP = 7;
IPC22bits.U4TXIP = 7;
IFS5bits.U4TXIF = 0; // Clear the Transmit Interrupt Flag
IEC5bits.U4TXIE = 1; // Enable Transmit Interrupts
IFS5bits.U4RXIF = 0; // Clear the Receive Interrupt Flag
IEC5bits.U4RXIE = 1; // Enable Receive Interrupts
RPOR2bits.RP39R = 0x1D; // dsPic33EP512GM604 => RP39 as U4TX PIN<43>
_U4RXR = 38; // dsPic33EP512GM604 => RP38 as U4RX PIN<42>
U4MODEbits.UARTEN = 1; // And turn the peripheral on
U4STAbits.UTXEN = 1;
// Hardware control bits
RS485_CTRL = 0; // disable driver
IEC5bits.U4RXIE = 1;
}
int main(int argc, char** argv) {
InitClock(); // This is the PLL settings
InitPorts(); // Configure all Input/Output Ports
InitUarts(); // Initialize UART1 for 9600,8,N,1 TX/RX
Rs485Initialise( 0x10, 0x10);
while( 1 )
{
}
return (EXIT_SUCCESS);
}
this is a test code. Actual communication will using Modbus Protocol.
Please help me in rectifying the issue.

Answer 1

`Whenever I send "0x00" to controller, "0xFF" is received. Then I sent "0x01", "0xFD" is received on controller.
This seems to indicate that your polarity is reversed. See the U1MODEbits.URXINV and U1STAbits.UTXINV bits in your datasheet or the family reference manual (http://ww1.microchip.com/downloads/en/DeviceDoc/70000582e.pdf).