Re: Compters without printer port

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "andrew.mathison" <andrewdavid.mathison@...> wrote:

>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, Andy Wander <awander@> wrote:
> >
> > Hey, Les:
> >
> > I had that same problem with XP(I also had it when I tried Win2K) that the PC would not boot up if the parallel ports were plugged in to the breakout boards.
> >
> > I was beginning to think I was crazy, as I reported it on the Mach forum, and nobody seemed to have ever heard or seen anything like it before.
> > ______________________________________
> > Andy Wander
> Hi Andy,
> with such a problem, the reason may be that the BOB (or equivalent) is drawing far too much current from the individual pins. The boot up check is seeing this and for safety reasons its stopping the boot.
> I know its a fiddle, but you need to measure the current drawn when a pin is activated, not that easy.
> The printer port guarantees only a max of 2.6 ma as a source or 24 ma as a sink (7400 specs). That means that on a BOB with Opto isolation for example that the input resistors need to be around 2K Ohms minimum. More is OK.... I have seen preofessional BOBs with only 370 Ohms...470 Ohms is typical and completely wrong....
> Join DIY-CNC-Interfacing if you wish and read more about this. Also the printer port specs are there to read too....
> If you draw too much current, you could even damage the Printer Port (which is buried in a BIG chip that has many functions other than Printing), or even the mainboard itself. Be carefull.....
> regards
> Andy M.
>

I think there is a misunderstanding here. The parallel port OUTPUTS (meaning the pins that have to source signals like Step & Dir or Relay outputs) would be the ones that would have to have current drive capability. Typically they are not opto isolated from the PC since they normally use the PC ground as common, and use a TTL buss driver chip as the interface. The load the port sees is well within the specs. Pullup on those lines typically is less than 2ma high. You have to provide some level of pullup to lower the noise threshold and line reflection problems. Tne parallel port INPUTS on the other hand are not pulling current and are looking at logic levels. The opto's back to the inputs need a good drive to the LED inputs (thus low values like 470 or 330 ohms) but the output is an NPN transistor that can sink 35 ma. The inputs can be taken directly to +5 or to ground without overloading the pins. The input impedance limits the current the inputs draw.

Most parallel ports OUTPUTS will not self destruct even driving into a short. They just won't give you the right logic levels. One thing they will not tolerate is large voltage spikes coming back on the inputs, thus the protection of the opto's.

The parallel port on PC's are pretty hardy. Over the years the design has improved so simple things like a shorted output pin will not fry the chip. About the only thing a MOS chip can't handle is reverse current flow....where an input voltage is below the chips Vss pins by more than .6 volts.

One big advantage of using opto isolation on all inputs is you break any ground conducted noise and spikes and other ugly stuff from power electronics running on a common ground.

TOM caudle
www.CandCNC.com