Problem with ADS1248 DOUT/DRDY signal

Helo,

I'd like to ask some help for properly setting up the ADS1248. I use a PIC18F4685 micro with Microchip C18 compiler and I'm able to succesfully communice with the ADS via SPI (I can write and read back the registers, retrieve data, etc.)
I'd like to interface only with the 4 SPI lines, thus I tried to use DOUT/DRDY functionality to be able to poll the DRDY signal and to get a signal similar to datasheet figure 68 on page 37 (SBAS426E – AUGUST 2008 – REVISED DECEMBER 2010). I don't want to use continous data output function and interrupts, but I want to be shure that I have valid conversion result after channel and PGA setting.
I tied START pin high & hold CS continously low. I set the DOUT MODE bit in IDAC0 register, and send RDATA and 4 NOP command to force the DOUT/DRDY pin high. Until this point it works. But DOUT/DRDY never goes low however the DRDY pin sends pulses regularly as can be seen on the scope.

See the screenshot, green: DOUT/DRDY, yellow: DRDY. The low pulses on the green trace are the result bits when they are read out.

What am I doing wrong?

Here is my initialization sequence:

//configure SPI module & ADS registers
void ADSopen( void )
{
 unsigned char var;      //putcSPI return value, 0 if no write collision occurred, -1 if a write collision occurred
 
 ADS_CS_TRIS = 0;    //CS is output
 ADS_CS = 1;              //ensure CS is reset
 OpenSPI(SPI_FOSC_64, MODE_01, SMPEND);  //configure SPI module

 ADS_CS = 0;                               //assert chip select
 var = putcSPI( ADS_RESET );    //RESET command holds the registers and thes decimation filter

                                                     // in a reset state for 0.6 ms when the system clock frequency is 4.096MHz,
 ADS_CS = 1;                               //deassert chip select

 Wait(1);      //wait 1 ms
 
 ADS_CS = 0;
 var = putcSPI( ADS_SDATAC ); //SDATAC Stop reading data continously (after reset this is the default state)
 
 //configuration: 
 var = putcSPI( ADS_WREG | MUX0 );                        //start at MUX0 register
 var = putcSPI( 0x03 );                                                  //write 4 bytes (number of bytes – 1)
 var = putcSPI( BCS_off | MUX_SP0 | MUX_SN1 );     //MUX0: BCS_off=0b00000000,

                                                                                      //   MUX_SP0=0b00000000, MUX_SN1=0b00000001
 var = putcSPI( 0x00 );                                                  //VBIAS
 var = putcSPI( VREF_on | REFSELT_int | MUXCAL_off );       //MUX1: VREF_on=0b00100000,

                                                                               //   REFSELT_int=0b00010000, MUXCAL_off=0b00000000
 var = putcSPI( PGA_1 | DOR_2000 );                  //SYS0: PGA_1=0b00000000, DOR_2000=0b00001001

 var = putcSPI( ADS_WREG | IDAC0 );                //start at IDAC0 register
 var = putcSPI( 0x04 );                                          //write 5 bytes (number of bytes – 1)
 var = putcSPI( DRDY_DRDO | IMAG_off );         //IDAC0: DRDY_DRDO=0b00001000, IMAG_off=0b00000000
 var = putcSPI( I1DIR_off | I2DIR_off );                 //IDAC1: I1DIR_off=0b11000000, I2DIR_off=0b00001100
 var = putcSPI( 0b00000011 );       //GPIOCFG
 var = putcSPI( 0b11111100 );       //GPIODIR
 var = putcSPI( 0b00000001 );       //GPIODAT
 var = putcSPI( ADS_NOP );          //a NOP can be sent to force DOUT/DRDY high if no other command is pending
 ADS_CS = 1;
}

If I read back these regiters, I get the values what I wrote in.

Then in a loop I send RDATA command, read the result, send one additional NOP and keep the CS low:

//read conversion result
short long ADSreaddata( void ) 
{
 unsigned char var;
 short long result;
 char res[3];

 ADS_CS = 0;                               //assert chip select
 var = putcSPI( ADS_RDATA );    //Read data once
 res[0] = rwSPI( ADS_NOP );
 res[1] = rwSPI( ADS_NOP );
 res[2] = rwSPI( ADS_NOP );
 var = putcSPI( ADS_NOP );         //a NOP can be sent to force DOUT/DRDY high if no other command is pending
 //ADS_CS = 1;     //deassert chip select ---> THIS IS COMMENTED OUT TO KEEP CS LOW
 
 result = 0x00;
 result = res[0] & 0xFF;    //must & 0xFF otherwise random errors occure
 result = result << 8;
 result = result | ( res[1] & 0xFF );
 result = result << 8;
 result = result | ( res[2] & 0xFF );     
 
 return( result );
}

Any suggestion is very welcome!

Thanks,

Attila