Not sure I read your post correctly then. What I thought you were meaning was some BOB's couldn't pass the higher frequency, if that was what you meant then the PC then SS then BOB would surely suffer the same way as the PC then PP then BOB?

Exactly. If a particular BOB can only pass say 86K pulse speed, then it would impose that limitation on both a PC, a SS, or anything that was generating a pulse stream.

I did a quick perusal of CNC4PC site and they have made a point of stating that one of their upper end boards has been upgraded to handle a 150KHz pulse stream thru the optos. The obvious conclusion there is that at one time the 'speed limit' was lower.

Of particular interest is that they have a BOB specifically designed to work with the SS via direct plug compatibility with the SS and specifically sporting a 1MHz pulse stream capability. 

So with one board having been ungraded to 150KHz and their flagship board boasting 1MHz, it is obvious that the capabilities vary widely. My CNC4PC product is their Gecko specific model with NO optos on the outputs. This may explain why it can handle a 100KHz stream while my second BOB cannot.

Ok I see where the confusion is now, I was saying the BOB in my case could not have been the problem running the PP at 100KHz as I now use it with the SS and you were thinking I was saying something different.
In any case both my BOBs are quite happy at the 533KHz that I once had the lathe running at, it was me that wasn't happy as it  was frightening

Hood

Just had a look, I knew the PMDX 122 was capable of 1MHz but was unsure of the Acustep, seems it uses 10MHz optos and the manual states >5MHz Opto Isolation speed.
Hood