Re: [CAD_CAM_EDM_DRO] Parallel Port/Relay Problems... more

Hi Matt,

Typical tri-sate output stages of buffers used for parallel ports can
either pull a line up (source) or down (sink). To do either, they
supply or draw current. Sometimes they can sink more then they can
source. So for example, if you are trying to drive a LED (SS switch),
and you try to source it by connecting the LED (and resistor) between
the pin and ground, it may only be able to SOURCE -2mA (74LS365). On
the other hand, if you connect the LED (and resistor) between the pin
and +5V, it can SINK 32mA. A LED and 220 resistor on 5V draws about
20mA. Works one way, not the other!

NOTE: Other parts have different spec's. 74LS241 sources -12mA, sinks
12mA (but not 20mA!). How much current does an optical isolator or
Crydon SS switch need? If it needs more then 12mA, a 2N2222 or similar
transistor is needed to switch the additional current. Tim has a
circuit, as I recall (it's simple).

HTH

Alan KM6VV

Matthew King wrote:

>
> Ok - I'm not an electronics type by any stretch, but I *am* trying to learn.
> I've heard the SOURCE/SINK reference many times. Could someone please
> explain this nice and simple-like? Sorry for the doltish question, just
> trying to understand....
>
> Thanks,
>
> Matt
>
> -----Original Message-----
> From: Alan Marconett KM6VV <KM6VV@...>
> Subject: Re: [CAD_CAM_EDM_DRO] Parallel Port/Relay Problems... more
>
> >Hi Chris,
> >
> >I must have missed the original question. Are you trying to control a
> >solid state relay, or a relay from a pin on a parallel port? And your
> >software has an Mcode to use? If you'll afford me a guess, do you
> >have a 1K pullup to +5v on the pin? and are you trying to SINK
> >current, SOURCE it? a 220 ohm resistor connected to +5 and in series > >with an LED makes a good test probe. You should see the respective
> >pin control the LED of all is working.
> >
> >This will allow you to divide the problem in two! Makes it easier to
> >debug.
> >
> >HTH
> >
> >Alan KM6VV
> >
> >
> >Chris Clough wrote:
> >>
> >> Ok - so I went and got all new components and rebuilt it twice now today
> >> - I'm using the schematic of off Tim's site - I built it, had two EE's
> >> look at it, and said that's the way it is on the page so... what's my
> >> next option?
> >> Any help - PLEASE :(
> >> -Chris Clough
> >> Xtech Engineering