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Date:	Tue, 18 Jun 2002 08:50:09 PST7
From:	"mrakijas" <mrakijas@oco.net >
Reply-To:	scoug-help@scoug.com
To:	<scoug-help@scoug.com >
Subject:	SCOUG-Help: after meeting idea

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---------- Original Message ----------------------------------
From: Peter Skye
Reply-To: scoug-help@scoug.com
Date: Mon, 17 Jun 2002 21:36:19 PST7

>But you're not supposed to switch between the printers while they're
>running because then the setup codes go to one printer and the data goes
>to the other. I don't remember if any of the data lines on a parallel
>port are kept high (non-zero volts) when the port isn't being used.
>I'll have to get out my breakout box and check.

The only thing I wanted to say here is that this isn't the only time not to switch printers (if you have to avoid it and they are sensitive to this kind of thing) because it can be very difficult to tell when the computer and printers with bidirectional communications are talking to one another. Depending on the driver, there can be continuous status checking going on (e.g., drivers for some HP inkjets on Win type machines know when the printer is disconnected - this implies regular non-printing related communications). In any case, it is conceivable that at the time you choose to switch, it can be at the wrong time for your printer.

Don't get me wrong, though. I use mechanical switches all the time. I've never damaged a printer though I have gotten randomly printed characters and some blank page feeds.

>Hmm. The bouncing should only occur during the time the switch is
>actually being thrown, and should have an effect (it should "bounce") in
>both directions.

True. It's only how it is interpreted at each end. At the printer, strange control or data characters could be realized. At the computer, control that is unlikely to be understandable may result. Again, not likely to damage but not certain.

>I used three Transtar 130's (Silver Reed daisy wheels) and a DeskJet 500
>ink jet printer for a while but don't remember if they did any return
>signaling.

I would guess not since older printers (certainly, pre-Windows) hadn't yet incorporated these concepts. I think the 500 series do not have much bi-di comms incorporated but the 600's (that were released right after) do.

>Right now I'm running five LaserJet II's and, again, don't
>know if they do any return signaling.

I think LaserJet II's are too old, too.

>> HP for a while threatened to void warranties of those who
>> used mechanical switches

They also tended to insist that one use IEEE-1284 certified printer cables (i.e. those that can handle bidirectional communications).

>If you're going to throw the switch while it's carrying data

Again, it may be hard to determine when it is safe and when it is not.

> then you
>ought to at least solder some zeners onto the data lines.

That ought to screw up the bidirectional comms pretty well. :-)

> Some of the
>mechanical switch boxes might already have them (zeners are cheap).
>There aren't any in the spare printer switch box I just opened up.

For what some would think are obvious reasons. :-)

>> If you want certainty (and network printing is out), go
>> with the electronic switch. The switches there are
>> debounced to avoid this problem.
>
>Good advice, thanks Rocky. (The debouncing is typically done with a
>"one-shot" circuit on each data line which makes the switch look like it
>is electronically closed even while it's still bouncing.)

I would have guessed some sort of electronic latch but you're probably right.

>> > - Electron Pete
>>
>> -Electric Rocky
>
>- Electron, The Duck :>>

-Shockin Rock

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