How do I create a set of quadrature encoder pulse from a pair of digital output

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MFC

VisualC++

Hello Experts,
I'm trying to produce a set of quadrature output which is shifted at 90deg by manipulating a pair of 0-5 volt signals from an SDS device as below:

// Channel A ON
canL2->SDSWriteAO(0x27C, 1, 0x13, 5); // 5 = 5V output			
// Channel A OFF
canL2->SDSWriteAO(0x27C, 1, 0x13, 0); // 0 = 0V output

// Channel B ON
canL2->SDSWriteAO(0x27C, 2, 0x13, 5);					
// Channel B OFF
canL2->SDSWriteAO(0x27C, 2, 0x13, 0);

I need to achieve maximum speed 200 ft/m with above resources.
Any feedback is highly appreciated.

Thanks in advance,
mraaf

What I have tried:

// Speed control by manipulating OnTimer() timout
int nTimout = static_cast<unsigned int>(1000/m_nSliderCtrl); // 0-200 ft/m
SetTimer(1, nTimout, NULL); // Timer 1

DWORD dwInterval = static_cast<unsigned long>(nTimout/3);

// calling OnTimer() for Timer 1 
if(nIDEvent == 1)
{
// Encoder rotation Clockwise
// at 0deg			
canL2->SDSWriteAO(0x27C, 1, 0x13, 5); // Channel A ON				
canL2->SDSWriteAO(0x27C, 2, 0x13, 0); // Channel B OFF
Sleep(dwInterval); // shift duration

// at 90deg
canL2->SDSWriteAO(0x27C, 1, 0x13, 5); // Channel A ON				
canL2->SDSWriteAO(0x27C, 2, 0x13, 5); // Channel B ON
Sleep(dwInterval);

// at 180deg
canL2->SDSWriteAO(0x27C, 1, 0x13, 0); // Channel A OFF				
canL2->SDSWriteAO(0x27C, 2, 0x13, 5); // Channel B ON
Sleep(dwInterval);

// at 270deg
canL2->SDSWriteAO(0x27C, 1, 0x13, 0); // Channel A OFF				
canL2->SDSWriteAO(0x27C, 2, 0x13, 0); // Channel B OFF
Sleep(dwInterval);
}

Output:
Output volt = 1.25V
Stable output 0-25Hz = 0-10 ft/m
Unstable output 25-50Hz = 10-20 ft/m
Max out at 50Hz (20ms)

What I have discovered:
12.5Hz = 5 ft/m --> Stable
25.0Hz = 10 ft/m --> Stable
50.0Hz = 20 ft/m --> Unstable, channel B occasionally shifted ahead channel A (resulting anti-clockwise rotation)
100.0Hz = 40 ft/m --> Unachievable