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.
More information about the T10