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

On Sat, 20 Sep 2003 20:26:41 -0000, you 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?

Yes. It's a one chip regulator... 8 to 35 volts or so input, 5 volts
output at up to an amp and a half, depending on package.

>
>2) I need inputs and I think your explanation is from the
>perspective of attaching outputs.

Not exactly... the inverters can be used going the other way, from a
switch to the parallel input port. I would recommend using opto
isolators, because once wired up, just about anything can happen to
the machine, and the computer will be safe.


The questions I have are as follows:

1) how are you sensing the end of travel/home position? What voltages
are the chips going to see (that's why the 4050 and the 4049). If 5
volts only, then you don't need the 4050/4049. If 12 volts (as
possibly in existing equipment), you will need to go from 12 to logic
levels. Running 12 volts into a TTL chip WILL blow the inputs.
(Don't ask how I know this...).

>
>I have no problem with logic diagrams, what I don't understand is
>TTL design.

Ok... I can deal with that...

On a parallel port, some pins are dedicated inputs, some are not.
Assuming that you use only the dedicated inputs, you'd wire up the
opto-isolators so that the transistor (or gate) output goes to the
input pin. Power for the transistor (run with emitter grounded,
collector pulled up to 5 volts with a 2.2K resistor) comes from a
separate power supply or the computer.

That leaves you with an LED to drive, which is the diode part of the
opto isolator. Figure running 10 ma through it from the power supply
common to your board/driver/stepper etc.... if you have 5 volts
available, that's good to use. You want to avoid a common ground
between the stepper system and the driving computer... Let the ground
wires in the power supplies do their thing.

I did miss the type of sensor you want for home and limit.

You want a current transfer ratio (ma into the LED vs ma that the
output of the isolator can sink) of about 100% or so. 4N35 chips are
good for this, and relatively inexpensive.

Does this help a bit more?

Harvey


NOTE TO LIST MOM: this is likely to wander off topic, possibly, if
you feel that it is useful, I can stay here, or it can go off list.

>
>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
>> >
>