Re: [CAD_CAM_EDM_DRO] Parallel port input help needed for limit switches

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
>