Voltage question

[rodzilla]

I have a PC with a parallel port that is outputting 3.3vdv and i am using a DB25 Interface card between my pc and my drives nothing special just maps pins to wire blocks. Now my question is how can i alter this board to take the 3.3vdc and make it 5vdc without having to buy a breakout board. Being very new to this stuff i don't know how to do this i have read about stuf like pullup resistors and pulldown resistors. and is there a schematic i could use to get this done Please any help is greatly appreciated.

[UKRobotics]

The simplest bet would be to buy an opto isolator parrallel port board. You can pick them up on ebay sometimes for not much money.

If you want to go the DIY route you will need to buy your own opto isolator chips and build a PCB to support them.

Basically each pin from the parrallel port goes through the opto isolator (which is several pairs of LED's and phototransistors in a chip and the output is amplified to 5v.

Somthing along those lines anyway.

[Mariss Freimanis]

Use 74ACT logic for level-shifting. Unless you are an electronics type person or are very young, I'd suggest forget that and buy a breakout board.

Mariss

[Torchhead]

Most opto's are not rated for anything close to high speed. Using microstepping drives you will find common optos fall down at a few kilohertz.

Better to just buffer and use a 5 V chip that will take the 3 volt low level signals and transition them to 5 volts at clean up the signals in the process.

Look at the 74HCT541 chips. 8 wide driver chip. About .50 or less.

[pminmo]

Are you sure you need more than 3.3V for your drivers?

[UKRobotics]

I'm supprised the pc is outputting 3.3v at all. I was under the impressions that only laptops did that and desktops kicked out 5v

[sswitaj]

You have a TTL device driving your parallel port.

There are two basic "families" of 5v logic devices, TTL (also called bipolar) and CMOS.

Because of the way TTL parts are built, it's common for them to not pull "all the way up" to the +5v supply rail. In fact, a TTL logic 1 (a "high") is defined as anything above about 2.4 volts, so what you're seeing out of your parallel port is typical. TTL devices are also not especially good at sourcing current, for instance, for driving an LED connected to ground. You can get only a few milliamps at logic 1.

TTL devices are much better at pulling down to logic 0, which is typically less than .3 volts, and they'll sink much more current, typically 20 or 30mA without complaining (for instance, an LED connected to +5v).

For this reason, many interface signals (like stepper drivers) are structured around getting an active "0" instead of an active "1".

CMOS devices use a different type of transistor, and typically have output levels of .3v and 4.7 v or better, and their structure is such that they pull low or high equally well.

This usually isn't much of an issue, since, in practice, most logic families are interoperable, except for very old (4000 series) CMOS.

If you're coming out of your parallel port to some device that needs a logic 1 that's closer to +5v or a little more current drive, there are two things you can do.

First, you can use a pullup resistor. This is a resistor, typically 510 ohms or so, that goes from the output signal to the +5V rail. When the output goes to 1, the resistor sources additional current to help pull the signal high. (When the output goes to 0, the resistor adds an additional 10mA load, but TTL can sink a lot of current, and this isn't a problem)

Second, you can buffer the signal through a CMOS device, like a 74ACT541. This is an 8-bit buffer that accepts TTL level inputs and provides a 40mA output.

Sadly, there is no +5V pin available on the parallel port (an annoyingly stupid feature, if you ask me) so you'll have to provide that externally.

[pminmo]

A lot of PC's only put out 3.X volts for a HI. The LPT port spec is IEEE-1284 and actually states a 2.4V minimum for a logic HI. That was one of the reasons I asked if there was truely a need for a 5V output.