Re: [SI-LIST] : Printer Port

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From: S. Weir (weirsp@a.crl.com)
Date: Sun Apr 09 2000 - 18:11:37 PDT


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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