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Browse Prior Art Database

Selectable Capacitor Circuitry for Personal Computers

IP.com Disclosure Number: IPCOM000110827D
Original Publication Date: 1994-Jan-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 2 page(s) / 80K

Publishing Venue

IBM

Related People

Heinzmann, AS: AUTHOR [+2]

Abstract

Described is a hardware implementation to provide selectable capacitor circuitry (SCC) for personal computers (PCs) designed with expansion capabilities. The circuitry is designed to minimize capacitance load variations in signal traces that go to expansion slots so as to optimize network drivers for maximum performance while maintaining signal quality throughout the loading range.

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

Selectable Capacitor Circuitry for Personal Computers

      Described is a hardware implementation to provide selectable
capacitor circuitry (SCC) for personal computers (PCs) designed with
expansion capabilities.  The circuitry is designed to minimize
capacitance load variations in signal traces that go to expansion
slots so as to optimize network drivers for maximum performance while
maintaining signal quality throughout the loading range.

      Typically, the capacitive load on a signal trace at an
expansion slot, such as a memory system interface memory module
(SIMM) slot, is a function of the planar trace capacitance and the
loads installed in a system.  The installed loads for a given signal
trace can vary from one to N, where N is the maximum available slots.
The  capacitive load on the trace will have some minimum value
determined by the planar characteristics.  When expansion cards are
installed, the line capacitance increases.  The new capacitive load
on the trace when expansion cards are added is the sum of the trace
capacitance and the capacitive loads of the expansion cards and can
be expressed in the following algorithm:

Let:

     Co    - represent the planar trace capacitance

     Cload - represent the capacitive load of a memory SIMM

     N     - represent the maximum available expansion slots

     n     - represent the number of installed loads (memory SIMMs)

Using a typical PC memory bus network as an example, for a twelve
inch trace, Co can vary from 25pf (2.1pf/in.x12in.) to 35 pf
(2.9pf/in.x12in.).  Cload can vary from 10pf to 60pf, and n can vary
from one to four.

The maximum effective capacitance can be expressed as in the
following equation:

                 Ceff = Co + n * Cload

                      = 35pf + (4 * 60pf) = 275pf

The minimum effective capacitance can be expressed as in the
following equation:

                 Ceff = Co + n * Cload

                      = 25pf + (1 * 10pf) = 35pf

      This large capacitance variation in possible loading makes it
difficult to optimize the net...