Re: ParPort output low - hi-Z or ground? opto to parallel

--- In CAD_CAM_EDM_DRO@yahoogroups.com, JanRwl@A... wrote:

> In a message dated 4/8/2004 8:35:53 PM Central Standard Time,
> davemucha@j... writes:
> Would a pull-up or pull-down on the parallel port side offer any
> benifits
> No. And there is a good 5 VDC supply available on the USB ports.

Or

> Joy-Stick port (but I don't know which pin(s))
>
>

My breakout board has 4 12VDC relays and 5 opto inputs.

the optos are powered from any exteranal power and with selecting the
proper resistors, one selected the voltage to get the 10-15mA on the
LED side. complete isolation as there is no other connection (ie
ground or otherwise) to the parallel port.

I bring 5, gnd and 12 from the PC to power the relays (12V) and
Gecko's (5v).

The 8 outputs (2-9) are direct to the steppers with no buffer or
other chip.

The relays are driven by simple PN2222 transistors and are only about
3 amps. enough to control small external things, but spindles and
pumps should be controlled by external relays.

When I was looking at the optos. I didn't expect any problems with
the external power, and the data sheets I tried to read seemed to
indicate that the current was 3:1 on the opto, they make pooto
darlingtons with a greater ratio.

http://www.cel.com/pdf/datasheets/ps2501.pdf

The voltage was not as clearly expressed. But. they show a pull down
resistor on the first page of the data sheet but I was not connecting
it in a similar way.

I am using the parallel port (9-13) to the collector and then
connecting the emitter to ground.

The part that is confusing to me is that I have seen references to
using resistors on the collector/emitter side but have not found the
reason why we need them on a layout like this.

It does sound like there is a desire to use a opto to feed a buffer
(74HC541) or Schmidt trigger (74HCT14) and that would enhance the
signal for low voltage parallel ports.

As I understand, the parallel port spec is that 2.5volts with a 2.5mA
load is needed to meet the spec. And that switching is done below
the 2.5 volt level. That would tend to make any switcing to be done
below 2.5 volts....

As I said, the data sheets are a little confusing. Am I missing or
overlooking something ?

Dave