Browse Prior Art Database

Single-Ended Device to Differential Small Computer System Interface Converter

IP.com Disclosure Number: IPCOM000106784D
Original Publication Date: 1993-Dec-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 2 page(s) / 78K

Publishing Venue

IBM

Related People

Buckland, P: AUTHOR

Abstract

Small Computer System Interface (SCSI) devices and adapters often need to support both single ended (SE) and differential (DE) SCSI buses. However, many devices and adapters support only single ended SCSI buses.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 53% of the total text.

Single-Ended Device to Differential Small Computer System Interface Converter

      Small Computer System Interface (SCSI) devices and adapters
often need to support both single ended (SE) and differential (DE)
SCSI buses.  However, many devices and adapters support only single
ended SCSI buses.

      This article describes the design of a low cost converter that
converts a SE device to a DE device.  This converter takes advantage
of the fact that it knows the SCSI ID of the single ended device and
does not have to drive the single ended SCSI bus with 48ma drivers.
These two restrictions greatly simplify the control logic and allows
the use of standard differential tranceivers with out using high
current open collector drivers on the single ended side.  The single
ended bus uses 1.5K ohm pull up resistors instead of the standard
SCSI terminators.

      The control logic monitors the SCSI states of both the SE SCSI
device and the DE SCSI bus.  With this information and the ID of the
SE SCSI device, it controls the direction and the enables of the
differential tranceivers.

      The converter control logic has a state machine that is based
on order that the BSY and SEL signals are driven as well as the IO
and ATN signals in order to make the following control decisions.  In
this description, SE stands for single ended and DE stands for
differential.

o   Busfree.  (BSY and SEL inactive for 800ns)

       -Set all tranceivers to receive.

       -Clear all state machines.

o   Arbitration

        -If SE BSY low first, drive DE BSY while SE BSY active.

        -If DE BSY low first, drive SE BSY while SE BSY active.

        -Drive data line corresponding to SCSI ID out.

        -If SE BSY inactive and DE BSY inactive go to busfree.

o   SE device has won the bus arbitration by driving SE SEL first and
    is selecting as an initiator or reselecting as a target.

        -Drive DE SEL while SE SEL active.

        -Drive DE BSY while SE BSY active.

        -Delay 800ns and then drive all data and parity lines out.

        -Drive ATN and ACK out if SE IO is high  (initiator)

        -Drive IO, REQ, CMD, and MSG out if SE ATN is high(target)

        -After both SE BSY and DE BSY have gone inactive, drive SE
    BSY

     ...