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Device Circuit Protection for Low Powered Personal Computers

IP.com Disclosure Number: IPCOM000113363D
Original Publication Date: 1994-Aug-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 2 page(s) / 58K

Publishing Venue

IBM

Related People

Franko, O: AUTHOR [+5]

Abstract

Described is a hardware circuit implementation to provide device port back bias protection for low powered Personal Computers (PCs). The circuit prevents a device attached to the parallel port of a battery powered PC from back biasing the port and adversely affecting operation.

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Device Circuit Protection for Low Powered Personal Computers

      Described is a hardware circuit implementation to provide
device port back bias protection for low powered Personal Computers
(PCs).  The circuit prevents a device attached to the parallel port
of a battery powered PC from back biasing the port and adversely
affecting operation.

      Typically, low powered PCs, such as battery operated units,
utilize Application Specific Integrated Circuits (ASICs) because of
their small size and power requirements.  Fig. 1 shows a typical CMOS
ASIC structure at an output circuit of a device port.  Fig. 2 shows
the same structure but with two diodes positioned between Vcc and
ground and the output pin.  Normally, when power is on at the PC, Vcc
is high and the diode is reverse biased.  However, when power at the
PC is off, Vcc is low and the diode is forward biased whenever the
voltage at the output pin exceeds the diode forward voltage drop.  In
the event an external device, such as a printer, is attached to the
PC and the PC is off, but the external device is on, the external
device will attempt to power up the PC.  Due to current limiting
criteria, this cannot fully occur, however, in the event many output
lines and devices are connected and power is applied at the same
time, a quasi-operational state can exist.  This can cause an
increase in current drain at the battery back-up unit of the PC as
well as potential problems when attempting to power the PC...