Browse Prior Art Database

Soft-Stop Circuit Permits Hot-Unplugging of Boards

IP.com Disclosure Number: IPCOM000113358D
Original Publication Date: 1994-Aug-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 2 page(s) / 71K

Publishing Venue

IBM

Related People

Cutts, S: AUTHOR [+3]

Abstract

Many computer products now support "hot-plugging" of components such as logic cards and power-supplies which permits "concurrent maintenance" to be carried out on high availability systems without interrupting customer operations.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Soft-Stop Circuit Permits Hot-Unplugging of Boards

      Many computer products now support "hot-plugging" of components
such as logic cards and power-supplies which permits "concurrent
maintenance" to be carried out on high availability systems without
interrupting customer operations.

      The design of a logic card (or other component that consumes
power) often has to take into consideration hot-plugging.  Many
designs use contact systems with pins of varying length.  Because it
is necessary to protect the system by quiescing data traffic during a
hot-plug, various combinations of long, medium, and short pins are
used to generate reset and hold signals.  Typically, the ground pins
are longest to ensure first make and last break.  The power pins are
often shortest, with signal/data pins being of medium length.  Some
designs now include a "soft-start" circuit which avoids a sudden
inrush of current when the component is plugged in.  Soft-start
circuits are usually housed on the hot-pluggable component and make
its initial current consumption ramp up to maximum over a few
milliseconds.

      The problem with this approach is that when a component is
removed with power still applied, the action of using the power pins
to break the circuit (like a switch) can be damaging.  If the
component consumes a lot of power then it will be connected through a
number of pins to share the current.  Because no two pins are exactly
the same length, they will break one at a time until the last pin is
carrying all the current.  When that pin breaks it results in a
flashover.

      This discontinuity of current may cause other components on the
same power bus to experience a "glitch".  The flashover is also
damaging because it causes pitting of the pins which will reduce
their working life.

      This problem is dealt with by expanding the soft-start circuit
function to include a "soft-stop" (Figure).  A short pin is added to
provide a sense signal which tells the electronics on the component
if a hot-unplug is in progress.  The pin will be grounded all the
time that the component is plugged fully home.  If this pin is
disconnected then if floats high, disabling the soft-start circuit.
The component will then reduce its current consumption to almost zero
before the power pins, which are of medium length, break.

      The soft-stop circuit will also activate during a plug-in
operation.  This is a further advantage because the soft-start
circuit operation will be delayed until the short pin makes contact.

      The long/short/medium pins may be on the component (as in the
diagram above) or may be on the motherboard/backplane.  Other
techniques such as variable-length edge-fingers are equally
applicable.

      A reset or hold sign...