3.3 Volt Termpwr & Termination, June 2nd Discussion at the fast-20 meeting

Paul D. Aloisi 603-429-8687 aloisi at uicc.com
Sun May 22 14:19:05 PDT 1994

3.3 Volt SCSI Systems

New SCSI 3.3 Volt systems need changes to the SPI termpwr that 
may affect the termination requirements. 

32 bit PCMCIA cards only have 3.3 Volts available with limited 
current. New Laptop systems are working to 3.3 Volt complete 
systems. Even new PCs and workstations are migrating to 3.3 
Volts. These systems will not have 5.0 Volts for a source for 

A meeting will occur with the June 2nd FAST 20 SCSI meeting in 

Newer components, electronic circuit breakers, are available to 
allow fusing and unidirectional current flow eliminating the 
need for the diode and fuse. The parts reduce the voltage drop 
allowing a voltage drop at 1 Amp of 0.2 volts. 

A  3.3 Volt supply with 5% tolerance with the drop for the 
electronic circuit breaker and the cable means the voltage at 
the far end of the cable can drop to 3.0 Volts.

The bus could be connected to 5.25 Volt systems. This means the 
3.3 Volt SCSI termpwr should be specified for 3.0 to 5.25 Volts. 
This means 3.3 Volt terminators must be able to work over the 
full range of 3.0 to 5.25 Volts. SCSI-2 Alternative 2 alternative 
allows a 2.63 Volt regulation point. A terminator with a 0.2 
Volt/Maximum 0.3 Volt low drop out regulator is used the termination 
will work over the full range of termpwr voltages. 

The only difference from SCSI-3 SPI is in 7.3 table 7.   3.3 Volts 
SCSI has an additional 3.3 Volt SCSI Termpwr line with 3.0 to 5.25 
Volts. The single ended ICON should change, adding 3.3V in the 
center of the ICON. This will let users know that lower termpwr 
voltage is all that is supplied by the connector.

PCMCIA and some battery systems limit the  current and can not 
provide power for the terminator at the far end of the cable. This 
requires another device on the cable to provide additional Termpwr. 
A designation or addition to the SCSI ICON should be standardize to 
let the user know that an external device must provide termpwr. 
Suggestion would be a Bar T in the lower right corner.

Low power applications with a very short bus may use low current 
terminators, a bus that is 8 inches or less may have low current 
terminators. If an external cable is attached to the unit the 
termination must switch to the normal current level.

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