Quote Originally Posted by irving2008 View Post
Whats in there already? and what version on the opto is it? and whats the isolated supply Vcc?

The reason being, when you reduce R2 to a lower resistance, you increase the collector current therefore you'll need to increase the drive current (If) to the LED. Current I'd guess R1 is 1K, which gives a forward current of (5 - 1.4)/1000 = 3.6mA. with the lowest spec'd opto the transfer ratio (CTR) is 50 which will give a collector current of 130mA but obviously this is limited by R2 to (assuming Vcc = 5v and R2 = 100R) (5 - 0.4)/100 = 46mA so R1 won't need to be changed. However you may find slew rate improves if you reduce it to increase If to, say, 10mA, which needs R1 = (5 - 1.4)/.01 = 360R, so if R1 = 1k try a 560R in parallel.

The fact that these are both 1k may well be an example of offsetting cost of manufacture v breadth of applicability. by making them all 1k it reduces inventory, handling and piece cost, but means there are performance limits that are probably OK for 95+% of cases... you haven't actually said if this is giving you issues, you only asked about the likely maximum speed.
Hi irvine2008
I overlooked to reply to your question previously about the isolated supply Vcc. It is 5v.
I've now checked the value of R1 on both the inbound at outbound channels. I've found these to both be 330R.
The 74HC14 inputs have 10K pull-ups to stop them floating.

Using ugly construction, I've just tried paralleling the 1K resistor with various values between 100R and 220R. The O/P waveform shape is much improved although reduced in magnitude to circa 3.5v pk-pk.