LPT port conection

[patroclus]

Sometimes, when attaching circuits to a LPT port, 4,7ko resistors are use as pull-up. Fine, I understand more or less why. But I've seen some other perfect-working schematics that don't use them, and instead they use a very small capacitor between a certain LPT pin and ground. And for some other pins, no resistor and no capacitor are used.

Could someone explain me why (the theory) ??
Thanks

 

[Nigel Goodwin]

Sometimes, when attaching circuits to a LPT port, 4,7ko resistors are use as pull-up. Fine, I understand more or less why. But I've seen some other perfect-working schematics that don't use them, and instead they use a very small capacitor between a certain LPT pin and ground. And for some other pins, no resistor and no capacitor are used.

Could someone explain me why (the theory) ??
Thanks

You need to study the origins of the parallel port, it was intended simply to feed a printer - and was designed accordingly.

Because of this a standard port only has a very limited number of input lines (5 handshake lines if I recall correctly?), and some of these are inverted as well. I also seem to remember that there's one output pin which is open-collector, and would require a pull-up resistor.

If you search on the net there's a great deal of information about them!.

If you are planning experimenting with your parallel port you need to bear in mind that they are now usually part of the motherboard - killing your port will require the motherboard replacing.

 

[patroclus]

well, I'm not going to actually "experiment", but build some project already built by others. I just wanted to know why some people just don't use the resistors, when they're supose to be necesary for pull-up.

(by the way, I want to build an EEPROM programmer, maybe you know of any I can build?).

 

[patroclus]

This is an example of a LPT conection that does not use pull-up resistors, just some capacitors, and not always. Why exactly is this??

 

[patroclus]

nobody knows why it does not use pull-up resistors?
those capacitors at IC Clock inputs won't smooth the raising and falling signal edge?? That would be bad, not good, as far as I know..

 

[Nigel Goodwin]

nobody knows why it does not use pull-up resistors?
those capacitors at IC Clock inputs won't smooth the raising and falling signal edge?? That would be bad, not good, as far as I know..

As I mentioned previously, only open collector outputs require pull-ups, there are only four pins that are open collector.

STB - pin 1.
AUTOLF - pin 14.
INIT - pin 16.
SELIN - pin 17.

Small capacitors are often used to help prevent noise.

 

[patroclus]

I know. the problem is that I've senn many projects that use a 8xSIP resistor in data lines (2 to 9), as pulldown resistors "required to cope with the vagaries of some parallel ports", and also says "If you have problems, try connecting the common line to +5V rather than ground.".

And yes, I also saw one project that uses these resistors as pullup resistors. Really, my only question is "why?"

 

[Nigel Goodwin]

I know. the problem is that I've senn many projects that use a 8xSIP resistor in data lines (2 to 9), as pulldown resistors "required to cope with the vagaries of some parallel ports", and also says "If you have problems, try connecting the common line to +5V rather than ground.".

And yes, I also saw one project that uses these resistors as pullup resistors. Really, my only question is "why?"

You've already answered your own question above - "required to cope with the vagaries of some parallel ports". Parallel ports can vary considerably, even though they are supposed to conform to a standard. Laptops in particular tend to be non-standard.