Re: Parallel port input help needed for limit switches

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
> >