RE: opto-isolator pitfalls

These are some really good points that I hadn't considered yet, thanks
Klaus. I had fully intended on reviewing the data sheets for the parts I
was going to select (engineer habits), but I'm thinking now that Doug
Fortunes solution with the NAND gate is probably more prudent than the
optoisolator one.

Interesting thing though, we have a Mini-Mill/2 from Minitech at work and
the literature on their parallel port interface says they use optoisolators
on the inputs. I'll have to check to see if we've been having lost steps or
otherwise with it. BTW, this machine sells for about $5500 and is nowhere
near as rigid or accurate as it should be for that amount of cash. Uses
unipolar drives as well (5804 based).

Gary

> Date: Sat, 03 Nov 2001 20:18:52 -0000
> From: klaus@...
> Subject: opto-isolator pitfalls
>
> Whenever you insert opto-isolators into a signal chain be sure to
> read the data sheet. The main thing to watch out for is propagation
> delay. The turn-on and turn-off times can be substantial. A lot of
> optos have delay times in the order of 100us, which limits them to
> about 5kHz. This is certainly too slow to be used to isolate shaft
> encoder outputs and might even be too slow for step/direction signals
> on some systems. Of course much faster opto-isolators are available.
> The other thing to be aware of is that turn-on and turn-off times are
> usually not identical. If you use optos to isolate step/direction
> signals this will add the requirement of longer setup/hold times for
> the direction signal as the active edge of the step signal gets
> shifted in relation to the direction signal. This could cause
> problems of lost steps in one direction like Boris described in msg
> 32835.
> Another problem associated with opto-isolators is that the output
> rise/fall times can be fairly slow (depending on model). This can
> cause the signal to "linger" around the switching threshold and makes
> it more likely that noise gets picked up causing a double clocking. A
> short connection to the following stage or the use of active output
> opto-isolators will improve things a lot.
> Don't get me wrong, opto-isolators are great tools when used right,
> but like most things in engineering, it's a good idea to look at the
> details. Read the data sheet and know what to make of it!
> Klaus