Re: Parallel port input help needed for limit switches

Jeff:

1) in your application - yes.

2) the suggestions place the chip in between the switches and the
parallel port as a buffer. The same chip can be used to buffer
inputs to, or outputs from the computer. It just depends on which
end is connected to the parallel port.

I'd tend to use the 4049/4050 devices in close proximity to the
computer to provide extra noise immunity. One volt induced onto a 5
Volt line is 20%. On a 12V line this drops to 8.3%. Also consider
using at least a twisted pair between the switch and your circuitry.
The same amount of noise is induced on both wires of the pair and the
noise is cancelled out... or so says the theory.

Tony

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "washcomp" <jeff@w...> wrote:
> Harvey:
>
> Thanks for the advice, but I'm still a little hazy on a couple of
> points:
>
> 1) Does the 7805 convert the 12 volts to 5 volts?
>
> 2) I need inputs and I think your explanation is from the
> perspective of attaching outputs.
>
> I have no problem with logic diagrams, what I don't understand is
> TTL design.
>
> Any help would be appreciated.
>
> Regards,
> Jeff
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, Harvey White <madyn@i...>
> wrote:
> > On Sat, 20 Sep 2003 17:00:28 -0000, you wrote:
> >
> > >I'm trying to build an interface to limit switches to the inputs
> of
> > >a parallel port. I have 12 volts DC available and don't want to
> use
> > >a breakout board for the purpose (I'm modifying something which
> > >already exists, but does not currently have an input function).
> > >I know I asked a similar question about a week ago, but as I've
> > >compared the answers, first I thought I got it, then it became
> clear
> > >as mud.
> > >
> > >Please give me a sanity check on the following as I have about
> zero
> > >experience in these things. If anyone can provide me with a
> > >schematic I'd appreciate it.
> > >
> > >1) To convert 12 volts D.C. to 5 volts D.C.: a circuit I found
> used
> > >a 7805 voltage regulator with a .1 micro-farad capacitor between
> Vin
> > >and, ground and Vout and ground. Is this kosher? If this
> doesn't
> > >fly, how would I easily convert the 12v to 5 v (or can I run
this
> > >circuit on 12v)? Else, I guess I could get a 5 volt wallwort.
> >
> > circuit is good. Heat sink might be needed for the 7805, good
> > practice says to do so regardless. Probably not needed for the
> amount
> > of chips you will run, though. Capacitors are used for stability
> of
> > the regulator.
> >
> > >
> > >2) A number of interface chips have been suggested:
> > >
> > >74HC541
> > >74LS373
> > >74LS244 (2 of them)
> > >CD4069 (or Motorola MC14069UB)
> > >
> > >Some guidance on their tradeoffs and suggestions of how to use
> them
> > >would be appreciated. I'm also unclear where to use resistors
> and
> > >capacitors in this context (or what appropriate values would be.
> >
> > Some of this depends on what you want to do.
> >
> > 5 volt logic does not like to see a 12 volt signal on the input.
> > There are various schemes for dropping the 12 volts to 5 volts,
> > however, the CD4049 and the CD4050 are designed to accept a 12
volt
> > signal on the inputs, and then translate to a 5 volt TTL type
logic
> > signal. You run the 4049/4050 off the + 5 volts.
> >
> > If your signals are just logic level, then you can buffer them by
> > adding any of the gates above. Some invert, some do not. (logic
> high
> > to logic low at output is inverting). (logic high to logic high at
> > output is not).
> >
> > You'd take the output pins from your printer port, run each one
to
> an
> > input on the chip (making sure that the outputs are enabled by
> > grounding the appropriate pin on the chip (4049 will not need
> this),
> > and then take the outputs from the chip outputs as if they came
> from
> > your printer port.
> >
> > Most likely, you'd want the chips not to invert... it makes life
> > easier.
> >
> > See if you can find some basic tutorials on TTL logic on the web,
> > those might be helpful.
> >
> > Harvey
> >
> >
> > >
> > >Please help!
> > >
> > >Regards,
> > >Jeff
> > >