Browse Prior Art Database

Use of Ferrite Chip Beads to Condition Bus Signals

IP.com Disclosure Number: IPCOM000120599D
Original Publication Date: 1991-May-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 2 page(s) / 89K

Publishing Venue

IBM

Related People

Eng, RC: AUTHOR [+4]

Abstract

This article describes a circuit arrangement which provides improved signal quality on long bus nets in a computer system.

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

Use of Ferrite Chip Beads to Condition Bus Signals

      This article describes a circuit arrangement which
provides improved signal quality on long bus nets in a computer
system.

      The use of long signal nets can adversely affect signal quality
in transistor-to-transistor logic (TTL) designs. The primary source
of potential problems is signal reflection.  Signal reflections can
cause false triggering at receiving gates.  High-speed TTL drivers
with fast transition times require special consideration in designs
that drive long nets.

      A common expedient for conditioning a signal is to add a series
resistor between the driver output and the net. The value of the
series resistor is based upon the characteristic impedance of the
net.  This improves the circuit by providing an approximate impedance
match between the driver and the net which helps minimize signal
reflections.  The series resistor also slows down the slew rate of
the signal on the net which improves signal quality. The major
disadvantage to a series resistor is that the value may be large
enough such that the DC offset of a net driven to the low state can
be greater than the maximum voltage for an input low (Vil) at the
receiving gate.

      A means to provide some series impedance between the driver and
the signal net is needed.  The fast transition time of the drivers
used is the cause of many problems involving signal quality.  A
driver with a transition time of 1 Volt (V) per Nanosecond (ns) has
an effective switching frequency of 100 MHz.  The addition of series
impedance slows the slew rate and lowers the switching frequency.

      The solution disclosed herein provides series impedance during
the switching interval of the driver output without affecting the DC
levels of the...