Date   : Tue, 17 Apr 2007 23:10:29 +0100
From   : adsb@... (Andrew Benham)
Subject: Acorn SCSI board arbitration

On Tue, 17 Apr 2007 09:05:33 -0500
Jules Richardson <julesrichardsonuk@...> wrote:

> * Possible. I can't for the life of me find a SCSI-1 spec anywhere,
> so I'm going from bits of info gleaned from the SASI spec and various
> SCSI target devices (which don't participate in arbitration anyway).

If you go to:
http://www.t10.org/x3t9_2.htm

Then about halfway down is a link entitled

""""
Small Computer System Interface (SCSI)
{Status: Superseded, Project: 0375-M; Designation: X3.131:1986}
This was the original SCSI standard (X3.131:1986).
"""

which is what I've always used as the first spec.

Link is to:    http://www.t10.org/ftp/t10/drafts/s1/s1-r17b.txt


This does specify the negative logic of the signals, and that
output signals should be open-collector or tri-state:

"""
4.4 Electrical Description

NOTE:  For these measurements, SCSI bus termination is assumed to be
external to the SCSI device.  An SCSI device may have the provision for
allowing optional internal termination.

  4.4.1 Single-Ended Alternative.  All assigned signals shall be
terminated with 220 ohms to +5 volts (nominal) and 330 ohms to ground
at each end of the cable.  (See Figure 4-5.)  All signals shall use
open-collector or three-state drivers.

     4.4.1.1 Output Characteristics.  Each signal driven by an SCSI
device shall have the following output characteristics when measured at
the SCSI device's connector:

  Signal assertion = 0.0 volts dc to 0.4 volts dc 
  Minimum driver output capability = 48 milliamps (sinking) at 0.5
volts dc Signal negation = 2.5 volts dc to 5.25 volts dc
"""

-- 
Andrew Benham         adsb@...       
Southgate, London N14, United Kingdom

The gates in my computer are AND OR and NOT, not "Bill"