TERMPOWER Current Requirements - comments, anyone?

ljr at austin.ibm.com ljr at austin.ibm.com
Tue Nov 7 10:34:51 PST 1995


The SCSI-2 standard requires initiators to be capable of supplying terminator 
power.  For the A cable, the requirement is 900mA minimum current capability; 
for the B cable option an additional 1500mA is required.

The SCSI-3 SPI (rev 15a is the one I'm looking at) also requires initiators to 
supply termpower.  In this case, the specification is for 1500mA minimum with 
no distinction made for A or P cable options.

For adapters that implement multiple SCSI ports, this seems like a significant 
problem.  Consider a three port adapter.  According to these specifications, 
this adapter must be capable of sourcing 4500mA minimum just to the TERMPOWER 
lines of it's three buses.  For a PCI rev2.1 compliant adapter, the maximum 
total power allowed for the entire adapter is 25 watts.  When the three busses 
are drawing current at the SCSI-3 spec'd minimum capability, there is only 
500mA available to drive all the adapter logic, assuming your system can source
25 watts to an adapter continuously.  There are also trace sizing 
considerations on the adapter itself.

Now I realize the typical case is below the 1500mA minimum capability. 
However, if I'm charged with verifying a claim of compliance to SCSI-2 or 
SCSI-3, I would be forced to load termpower to these levels, right?

What ARE typical current levels for TERMPOWER in an 8 and a 16 bit environment,
anyway?

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Les Record
ljr at austin.ibm.com
(512) 838-3622
Austin, Texas
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