# Thread: Speed of Optos on BoBs

1. Personally, unless its causing you problems I'd leave it alone.... if you need more speed you might want to consider a different card. When you say a 3.5v swing, between what and what? 3.5v is a good logic one level...

2. Originally Posted by irving2008
. . . . . When you say a 3.5v swing, between what and what? 3.5v is a good logic one level...
Many thanks for sticking with this one. My 'scope is years out of cal' but I'd say the swing on the modified channel is roughly between +1.6v and +4.8v which suggests the ON resistance of the opto is the limitation.

3. Originally Posted by boldford
Many thanks for sticking with this one. My 'scope is years out of cal' but I'd say the swing on the modified channel is roughly between +1.6v and +4.8v which suggests the ON resistance of the opto is the limitation.
Ah, so the transistor is not saturating. with 100R load, the collector current therefore is (5 - 1.6)/100 = 34mA, whereas it needs to be (5-0.5)/100 = 45mA, so the LED needs to be driven harder as previously discussed. However the 74HC14 clamps at 20mA, so there may be a limiting factor here... try reducing R1 from 1k to around 450R or so, theres no point in going lower... if that doesnt improve things then its not going to get better without significant rework

4. Originally Posted by irving2008
Ah, so the transistor is not saturating. with 100R load, the collector current therefore is (5 - 1.6)/100 = 34mA, whereas it needs to be (5-0.5)/100 = 45mA, so the LED needs to be driven harder as previously discussed. However the 74HC14 clamps at 20mA, so there may be a limiting factor here... try reducing R1 from 1k to around 450R or so, theres no point in going lower... if that doesnt improve things then its not going to get better without significant rework
As stated earlier R1 is 330R, so it looks like I need to have a think if I'm going to use this BoB in the way I had intended.
Last edited by boldford; 26-08-2012 at 08:24 AM.

5. A week on and spare time now permits the process of clean-up the O/Ps with a few 74HC14s, It ain't gonna be petty but at least the waveforms, both inbound and outbound, are gonna be sharp. With a bit of care I will retain the complete opto-isolation on the BoB. Elimination of the now un-necessary optos at the front end of the single axis TB6560 will follow blow by blow.
I think it's fair to say "although any CNC electronics system is a series of steps it needs to be integrated as whole to get the performance expected"
The overall plan continues to be to derive the PC side supply from a spare HDD power connector within the PC. The O/P side of the BoB will come from the 5v rail of the homebrew smorgasbord driver PSU. Or have I got it totally wrong?

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