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  1. #11
    The particular BoB I'm playing with doesn't seem to be defective as such. (It's almost too simple not to work). I'm commenting on the difference between I/P and O/P 'scope wave forms. To test it I'm feeding it with a 555 astable on a breadboard producing close to a 50/50 square wave. I'm varying the frequency with a simple change of C. What comes out is 'orrible!
    What I find odd is the channels outbound from the PC pass through 74HC14s before they get to the optos. No such buffering/inverting on the inbound direction.

  2. not odd at all... those optos need driving and the parallel port on some PCs isnt up to the job. The question I would ask is where are the LEDs and the 74HC14 being powered from?

    Also what are the pull up resistors either side of the opto?

  3. #13
    Quote Originally Posted by irving2008 View Post
    ... those optos need driving and the parallel port on some PCs isnt up to the job.
    Of course! I was forgetting some crappy PCs only output 3.3V
    Quote Originally Posted by irving2008
    The question I would ask is where are the LEDs and the 74HC14 being powered from?

    Also what are the pull up resistors either side of the opto?
    At the present they are being powered from the 5V rail of a smorgasbord bench PSU. I'll confirm the R values tomorrow.

  4. #14
    Quote Originally Posted by irving2008 View Post
    .........Also what are the pull up resistors either side of the opto?
    The channels outbound from the PC have 1K pull-up resistors.

  5. on which side of the opto? R1 or R2 below? I'm guessing you meant R2?

    To get any reasonable slew rate R2 needs to be closer to 100ohm not 1k.

    Click image for larger version. 

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  6. #16
    Quote Originally Posted by irving2008 View Post
    on which side of the opto? R1 or R2 below? I'm guessing you meant R2?

    To get any reasonable slew rate R2 needs to be closer to 100ohm not 1k.

    Click image for larger version. 

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    I did mean R2. I'm sorry for not being clearer. Many thanks for the information. I'll try with a 120R in parallel using ugly construction and report my findings. If that works I might try find out if I can still solder SMT devices.

    What would you recommend for the I/P side? I.e R1.

  7. Quote Originally Posted by boldford View Post
    I did mean R2. I'm sorry for not being clearer. Many thanks for the information. I'll try with a 120R in parallel using ugly construction and report my findings. If that works I might try find out if I can still solder SMT devices.

    What would you recommend for the I/P side? I.e R1.
    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.

  8. #18
    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.
    Once again, many thanks for your information and explanation. I will try and check the I/P R values later today.
    At present I'm not having issues with this kit as I'm presently trying to get acceptable (in my book "best possible") performance out of this equipment.

    This takes us back to my original question where I sought the collective wisdom about the maximum clock frequency I could reasonably expect with the active devices fitted.

  9. #19
    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.

  10. #20
    Click image for larger version. 

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ID:	6656This is the BoB I've been referring to. (I said it was a simple design).

    The plan now is to either obtain some 150R 1206 SMT resistors to mount on top of the existing 1K ones or simply replace them with 120R. With this mod applied tests indicate the O/P voltage swing will now only be approximately 3.5V. This suggests the next step will require a veroboard hack to introduce 74LS14s on all channels ensuring the I/P and O/P isolation isn't compromised.
    Last edited by boldford; 24-08-2012 at 08:55 PM.

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