[SI-LIST] : Printer Port

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From: Peter Baxter (peter.baxter@tantau.com.au)
Date: Sun Apr 09 2000 - 15:23:25 PDT


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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