On Thursday 08 October 2009, Andy Pugh wrote: >2009/10/8 Kirk Wallace <[email protected]>: >> That seems risky to me because generally the LED in the opto-coupler >> needs 10 to 20 milliamps to turn on well enough to switch the coupler's >> output. You have three LED's to sink current for, which is 30 to 60 mA. >> Generally, a parallel port pin is designed to sink or source about 3mA >> (some can do much more, but which ones?). I would prefer to see the >> parallel port pin connected to three inputs of a driver chip like a >> 74LS244. > >I did these sums last night and came to much the same conclusion. I >will see what I have in the way of buffers (I definitely have some >Op-Amps I can configure in voltage-follower mode, but I will see what >the 74LS255 is) > >When I wired up the drives last year I was thinking very much in terms >of TTL logic, +5 =1 0V = 0, not in terms of opto-isolators, source and >drain currents etc. In the meantime I have built a few opto-isolated >interface boards and have a better feel for the subject, but hadn't >really realised until last night that I need to revisit my original >wiring scheme. It was working, but on reflection I have no idea how. > I was gonna be helpful if I could, but failed to locate a data sheet for the 74LS255. Are you sure that is the right number?
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