Protecting my parallel port

Protecting my parallel port

Post by Gregg Rabe » Tue, 12 Jan 1999 04:00:00




> Hi !

> I'd like to use my Pc's parallel port for i/o. I know how to handle
> this, except for a little problem : I don't want to connect an
> extension directly to my PC, so I'd like to use an (fr.)
> "optocoupleur" (a generally 6-pinnel DIL IC composed of a LED and a
> phototransistor, you see what I mean ?).

> I know this is used on lots of cards, but I read that CMOS ans more
> particulary TTL outputs can't properly drive a LED, because of its
> limited output current (2 mA, which I measured on my port). A solution
> is given, using +vcc and the output as a ground, but I can't use it
> (no vcc output on // port !).

> So what ? Does one use special "optocoupleurs", with a limited input
> current (which I suppose are more expensive), or is there a way I can
> use more "classical" optos ?

> Thanks !

Second parrallel port is one Idea that has been mentioned.  The problem
with this is that it has no voltage isolation.  Motors can generate
considerable counter EMF that will intern generate large voltages.  The
best bet is to use optoisolators.  These have the voltage isolation the
additional card doesn't.
 
 
 

Protecting my parallel port

Post by Gregg Rabe » Tue, 12 Jan 1999 04:00:00


If additional current is required use either transistors for current
aplification or Fets has heavy switches.


> : Try to sink the opto LED.
> : Outputs usually can sink more than they can source.
> : I believe you need 10 ma to source the opto LED
> : The port device (74LS374) can't supply that much. It can sink 10 ma max.
> : Short intermittent use should be okay.
> : Note that you will be using negative logic. Turning a
> : bit off at the port will turn on the transistor ( or SCR ) in the opto.
> : Which in turn can either sink or source the next stage. NPN transistors
> : (TIL111 opto) are normally used to sink current. Use PNP for sourcing the
> : current.
> : I have sourced and sinked discrete LED's on
> : my PC at the parallel port for testing purposes..

> : Carl

> Absolutely true ! But I can't use this here, because sinking current
> supposes that I have access to the Vcc line, which isn't possible with
> a PC parallel port.

> Doesn't anybody have the scheme of and extension that use the // port
> and optos ? (This is sold by "Velleman-kit" per example)


 
 
 

Protecting my parallel port

Post by Kyle Rhode » Tue, 12 Jan 1999 04:00:00


just use a simple switching transistor, (2N2222) for the input LED.

Kyle



>> Hi !

>> I'd like to use my Pc's parallel port for i/o. I know how to handle
>> this, except for a little problem : I don't want to connect an
>> extension directly to my PC, so I'd like to use an (fr.)
>> "optocoupleur" (a generally 6-pinnel DIL IC composed of a LED and a
>> phototransistor, you see what I mean ?).

>> I know this is used on lots of cards, but I read that CMOS ans more
>> particulary TTL outputs can't properly drive a LED, because of its
>> limited output current (2 mA, which I measured on my port). A solution
>> is given, using +vcc and the output as a ground, but I can't use it
>> (no vcc output on // port !).

>> So what ? Does one use special "optocoupleurs", with a limited input
>> current (which I suppose are more expensive), or is there a way I can
>> use more "classical" optos ?

>> Thanks !

>Second parrallel port is one Idea that has been mentioned.  The problem
>with this is that it has no voltage isolation.  Motors can generate
>considerable counter EMF that will intern generate large voltages.  The
>best bet is to use optoisolators.  These have the voltage isolation the
>additional card doesn't.

 
 
 

1. Protecting my parallel port

Hi !

I'd like to use my Pc's parallel port for i/o. I know how to handle
this, except for a little problem : I don't want to connect an
extension directly to my PC, so I'd like to use an (fr.)
"optocoupleur" (a generally 6-pinnel DIL IC composed of a LED and a
phototransistor, you see what I mean ?).

I know this is used on lots of cards, but I read that CMOS ans more
particulary TTL outputs can't properly drive a LED, because of its
limited output current (2 mA, which I measured on my port). A solution
is given, using +vcc and the output as a ground, but I can't use it
(no vcc output on // port !).

So what ? Does one use special "optocoupleurs", with a limited input
current (which I suppose are more expensive), or is there a way I can
use more "classical" optos ?

Thanks !

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