RE: (slightly OT:) Parallel port question

From: Davison, Lee (
Date: 2003-10-09 00:26:47

'Lo all,

> I'm just forwarding this question to the cbm-hackers mailing list, as
> are some people who might know more about this.

>> There must be a way to protect the parallel port using simple components.

There is, simply connect a resistor in series with each bi-directional line
you wish to protect. A value of 180 to 220 ohms should work.

One thing to note is that this may delay the signals if you're trying to
high speed transfers, it may also reduce the noise immunity on longer

It may help to put the resistors all at the mostly receiving end of the
or, if the data goes equally both ways, split the resistance between both
of the cable. At worst this will nearly double the signal speed compared to
single resistor solution.  

> I think the maximum sourcing und sinking current of the parallel port is
> really defined

There is more than one "maximum current" limit. The lowest one is usually
maximum that can be sourced or sunk and the output still be within the
voltage limits for the logic levels for the device. This is usually only a
mA at best.

The next is the short circuit current that can be sourced or sunk by one
this can be a couple of hundred mA.

Lastly is the maximum current per package before you let the magic smoke
this can be half an amp or so but isn't recommended as common practice.


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