Arduino not sensing Optocoupler Signal

[dm41nes]

So post a wiring diagram of your hardware setup!

This is just for one IO pair. Essentially it would be reiterated for 'n' number of wires I want to test.

For debugging purposes with an oscilloscope I ran the code below to see what was going on. So I believe my issue is that after the optocoupler implementation, the signal is not being sent quick enough for the arduino to read it in time before checking the next set of IOs

void setup()
{
	pinMode(28,OUTPUT);
}
void loop() 
{
	digitalWrite(28,HIGH);
	delay(500);
	digitalWrite(28,LOW);
	delay(500);
}