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Browse Prior Art Database

Automatic Small Computer System Interface Termination Circuit for Narrow/Wide Devices on Wide Bus

IP.com Disclosure Number: IPCOM000118611D
Original Publication Date: 1997-Apr-01
Included in the Prior Art Database: 2005-Apr-01
Document File: 4 page(s) / 180K

Publishing Venue

IBM

Related People

Grasso, LJ: AUTHOR [+3]

Abstract

Small Computer System Interface (SCSI) is a well-known standard peripheral interface used on computers for many years. One of the focus areas for system designers using this interface is providing proper termination for reliable operation.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 33% of the total text.

Automatic Small Computer System Interface Termination Circuit for
Narrow/Wide Devices on Wide Bus

      Small Computer System Interface (SCSI) is a well-known standard
peripheral interface used on computers for many years.  One of the
focus areas for system designers using this interface is providing
proper termination for reliable operation.

      Generally speaking, this involves placing a terminator at each
end of the net or cable.  For systems with internal and possible
external drops, one solution was to provide two terminators inside
the system box,  each at the end of the net.  If an external file was
connected, the internal terminator was automatically disabled and the
customer added an  external terminator at the end of net.  This
method works well when all  the supported devices are the same size
as the bus width.

      In systems that support devices of mixed bus width size (i.e.,
Narrow 8-bit and Wide 16-bit), the problem of providing automatic
termination becomes more difficult because the termination may be
required at different places on the bus.  This is the case when an
8-bit device is connected to the external connector of a 16-bit bus.
The high byte of the but must be terminated at the connector but the
low byte  must be connected at the added external device.  If a
16-bit device is  connected to the external connection, both high and
low byte must be terminated at the added external device.

      This invention provides a means for automatic termination of
SCSI interconnectors without the need for manual intervention when
the host computer is required to support mixed bus size devices.

      Fig. 2 shows the layout of the host computer with the Wide SCSI
controller and this invention circuit built into the motherboard to
support either Wide SCSI devices or Narrow devices or both of them on
the same Wide SCSI bus.

      Referring to Fig. 2, the motherboard contains a single Wide
port SCSI controller - U1, two internal SCSI terminators - T1 and T2,
two internal 68-pin Nonshielded D-type SCSI interconnectors - J1 and
J2, and an external 68-pin shielded D-type SCSI interconnector - J3.
The SCSI interconnector J3 is designed for connection to external
SCSI devices.  There also is an internal ribbon cable - W1 designed
for connection to internal SCSI devices.  The internal SCSI devices
can be  all Wide SCSI devices or all Narrow SCSI devices or both of
them. One  end (P1) of this internal SCSI cable W1 is plugged into
the internal interconnector J1 which is located close to the SCSI
controller and terminated by the terminator T1, and another end (P2)
is plugged into another internal interconnector J2 and terminated by
the other T2 terminator, as shown in Fig. 2.

      Both T1 and T2 terminators are regulated terminators, also
known as Active terminators, and provide termination for both ends of
the internal physical SCSI cable when no external SCSI device is
detected.  The T1 te...