SCSI Specs & Comparisons - answers (long) (fwd)

[Mike Bilow]

Martin Mokrejs wrote in a message to Mike Bilow:

 MM> SCSI-2 is a standard that offers two speed choices.  The 
 MM> default speed runs the bus clock at 5 Mhz.  The other, optional 
 MM> speed, runs the bus clock at 10 Mhz.

This is not correct.  The "default" speed is 3.3 Mo/s, which is the maximum
asynchronous operation speed.  If the devices so negotiate, they can operate
synchronously at 5 Mo/s.  Once synchronous operation has been agreed by the
devices, they may elect to operate "Fast" at 10 Mo/s or "Ultra" at 20 Mo/s. 
There is also a coming extension known as "Ultra2" at 40 Mo/s.

(The notation "o/s" is "operations per second.")

 MM> For any given data transfer, the device and host adapter (or 
 MM> two devices) will negociate for the speed they want to use.  
 MM> Cable length plays an important part what speeds are supported. 
 MM> The total cable length, including internal device cables for 
 MM> single-ended SCSI running "Fast" is 3 meters.  Hopefully 
 MM> devices will detect the signal degredation on longer busses and 
 MM> fall back to a safe speed.  If not, you can expect data 
 MM> corruption and errors.  Hopefully, the devices will detect the 
 MM> errors, but it is easy not to.

The devices have no effective way to measure cable length, nor do devices check
the cable length.  The negotiated speed is simply the slower of the two
devices, usually based on their firmware configuration.  If there are devices
which detect errors and slow their bus clocks, I have never seen them.  Some
devices will drop to asynchronous operation if there are errors, but then will
usually just enter a renegotiation phase and try to go synchronous again.

 MM> "Ultra-Wide SCSI" refers to one of:

 MM> o  The Wide32 option of SCSI-2
 MM> o  Ultra-Fast transfers and the Wide16 option of SCSI-2

Devices which transfer 32 bits per operation are extremely rare.  Any "Wide"
devices likely to encountered by the readers of this list are almost certain to
be moving 16 bits (2 bytes) per operation.

Since a "Wide" device moves 2 bytes per operation, a "Fast Wide" device moves 2
B/o at 10 Mo/s for effective throughput of 20 MB/s, and an "Ultra Wide" device
moves 2 B/o at 20 Mo/s for effective throughput of 40 MB/s.

 MM> "Ultra" is the term given to what will be a feature of SCSI-3; 
 MM> 20 Mhz data transfers.  SCSI-3 isn't a standard yet, but as 
 MM> with SCSI-2 vendors are already starting to implement the most 
 MM> useful features.  In general application the maximum length of 
 MM> a single-ended bus running Ultra speeds is 1.5 meters.

Note that "Ultra" is as fast as it goes on single-ended connections, and that
"Ultra2" requires a low-voltage differential connection.  By "single-ended," we
mean that the logic signals are significant relative to ground.  Differential
signals are sent using two wires for each signal, and the relative voltage
between them defines the logic level.  In a differential system, one logic
state is expressed by driving one wire below ground and the other wire above
ground, and the opposite logic state is expressed by reversing them.  Noise
immunity is superior with differential signalling.
 
-- Mike



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