RE: [SI-LIST] : Printer Port

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From: Volk, Andrew M (andrew.m.volk@intel.com)
Date: Mon Apr 10 2000 - 09:15:36 PDT


Peter -

I hope that your are considering the even more common case of the printer
being on and the PC off. These situations are called back-power and will
power the off device to significant levels. I have observed a full PC
powered to 0.7 volts on the 3.3 volt rail - enough to power some CMOS
devices into a obscure level of functionality. This can cause spurious
operation when the device is turned back on. I have even heard of laptop
power modules being damaged by power flowing in the reverse direction.

Many chips meant to drive the printer port have backpower save outputs that
isolate the buffer from the port when there is no power on the chip. I'm
not familiar with Printer designs, but similar circuits are possible.

A simple method of stopping backpower damage is to put a q-switch in series
with the port with the switch enable tied to the 5 volt rail. It will pass
a 3.3 volt signal when powered on and isolate when powered off. It works
for bi-directional ports like 1284 (if you take the 5 ohm switch resistance
into account).

Regards,

Andrew Volk
Intel Corp.

-----Original Message-----
From: S. Weir [mailto:weirsp@a.crl.com]
Sent: Sunday, April 09, 2000 6:12 PM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Printer Port

Peter,

The PC printer port has very weak high current drive capability. If you
want more data, you can either order the IEEE 1284 standard, or either "PC
Printer Complete" by Axelson, or "The Indispensible PC Hardware Book" by
Mesmer.

The weak high drive capability protects against the situation that you
describe. However, there are two other phenomena to watch out for:

1. The weak drive results in rise times, with or without a cable measured
in the 100's of nS.
2. The weak drive is still enough to power many circuits to an uncertain
state, about 1V.

It is wise to design any device that connects to a PC Printer port as
hot-swap to prevent erratic, or even dangerous behavior due to side effects
of the leakage current.

Regards,

Steve.
At 08:23 AM 4/10/2000 +1000, you wrote:
>Hi,
>
>1) I've been curious about the following situation for a while.
>
>A PC is connected to a Printer. The PC is powered on, and the Printer is
>powered off. A fairly normal situation.
>
>Assume that signals on the PC's printer port are at high levels (ACK_,
>STB_ ... any of them). The possibility therefore exists, for "large"
>currents to flow out of the PC's printer port, along the printer cable
>and into the Printer's IC. Inside the Printer's IC, it will forward bias
>the ESD protection diodes and try to power up the Printer IC.
>
>What prevents the IC's at either end from being damaged? Are IC's
>current limiting/short protection circuits after the ESD protection
>network or has that nothing to do with it?
>
>Do they rely on the "SELECT" line to high impedance all outputs? Do they
>rely on current limit resistors in series with the line?
>
>I realise that HOT Socket ICs prevent this, by having a different "ESD
>network", but not all IC's are Hot Socket-able. I've come acroos a lot
>of situations where-by low current "Auxilary" circuits which were
>thought to be un-powered, were actually being power-up via the IC's ESD
>diode networks.
>
>
>
>2) As this is not SI reated, it's probably better to respond off-line.
>
>Many Si-List people live outside USA, so we don't always know what
>opportunities are available to us. I've noticed that "IPO's" etc, are
>the big thing in USA these days.
>
>Are there still companies out there who specifically going looking for
>electronic products (not wanting to do an IPO) to put on world markets?
>I'd be interested in being made aware of any.
>
>Best regards,
>
>Peter Baxter
>
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