Signal naming conventions

[John Lohmeyer]

* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* John Lohmeyer <lohmeyer at ix.netcom.com>
*
I think Mark has found something that T10 and T13 agree on:

>1)	asserted:  a signal is driven by an active circuit to its logical
>true state (above ViH if active high and below ViL if active low)
>2)	negated:  a signal is driven by an active circuit to its logical
>false state (below ViH if active high and above ViL if active low)
>3)	released:  a signal is not being driven.  For tri-state drivers,
>this means that the driver is in the high impedance state.  For
>open-collector drivers, the driver is not asserted.

The only subtle difference is that SCSI terminators bias undriven signals
to the false state so a released SCSI signal will go false (if no other
device is driving it).  For single-ended drivers and receivers, all SCSI
signals are active low, so asserting a SCSI signal takes it low (and true).

Asserted, negated, and released are actions taken by drivers, while true
and false are states observed on the bus.  These five terms were defined in
the original SCSI standard; they seem to have passed the test of time.

John


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John Lohmeyer                  Email: lohmeyer at ix.netcom.com
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