Single-ended termination current

Louis Grantham lgdatcom at ix.netcom.com
Wed Oct 14 11:35:01 PDT 1998


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* "Louis Grantham" <lgdatcom at ix.netcom.com>
*
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At the SPI-3 working group on 9/15/98, I brought up the following =
(98-220r0):
(X3T10/96-222r1) identified the single ended termination =
incompatibilities between SCSI-2 and SPI. The current each terminator =
can source according to SCSI-2 is 22.4 mA at 0.5 V and above and SPI is =
24 mA at 0.2 V and above. This can make SCSI-2 terminators not SPI =
compliant. The notch in the terminator IV curve document was intended to =
reconcile this difference. The original proposal (X3T10/96-245r2) =
accomplished this with 25.4 mA at 0.2 V and 24 mA at 0.5 V. This was =
subsequently changed to 22.4 mA at 0.5 V and 25.4 mA at 0.2 V. This =
change meant that one could still have an incompatible terminator if at =
0.5V the terminator sourced between 22.4 mA and 24 mA which defeats the =
original intent.

I want the spec changed back to 24 mA at 0.5 V in 7.1.1 b). =20

The only technical reason that I can find of why the current at 0.5 V is =
spec=92d at 22.4 mA is for FAST-5 leakage current (Table 25 FAST-5 c))  =
ASSUMING the potential leakage from receivers or open drain drivers to =
be 0.4 mA at 0.5 V and given the driver sink current of 48 mA, then the =
terminator source current should be limited to 22.4 mA (max) at 0.5 V. =
This number is driven entirely by assuming the 0.4 mA worst case leakage =
of legacy FAST-5 SCSI drivers and receivers.
i.e. (22.4 mA)(2 terminators) + (0.4 mA leakage)(8 devices) =3D 44.8 mA =
+ 3.2 mA =3D 48 mA.

Millions of terminators have been shipped to the SPI spec. of 24 mA at =
0.2 V (and above) with no adverse consequences for end users. This tells =
me that in the real world:

    1.. leakage currents will not approach the 0.4 mA @ 0.5 V =
number...especially with today=92s receivers and drivers=20
    2.. drivers have excess margin beyond 48 mA
QUESTIONS FOR REFLECTOR:

    1.. Was SPI wrong?=20
    2.. Is there really 0.4 mA leakage with FAST-5 devices?

Regards,
Louis


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 At the SPI-3 working group on 9/15/98, I brought up the following=20 (98-220r0):
(X3T10/96-222r1) identified the single ended termination=20 incompatibilities between SCSI-2 and SPI. The current each terminator = can source=20 according to SCSI-2 is 22.4 mA at 0.5 V and above and SPI is 24 mA at = 0.2 V and=20 above. This can make SCSI-2 terminators not SPI compliant. The notch in = the=20 terminator IV curve document was intended to reconcile this difference. = The=20 original proposal (X3T10/96-245r2) accomplished this with 25.4 mA at 0.2 = V and=20 24 mA at 0.5 V. This was subsequently changed to 22.4 mA at 0.5 V and = 25.4 mA at=20 0.2 V. This change meant that one could still have an incompatible = terminator if=20 at 0.5V the terminator sourced between 22.4 mA and 24 mA which defeats = the=20 original intent. I want the spec changed back to 24 mA at 0.5 V in 7.1.1 b).  = The only technical reason that I can find of why the current at 0.5 V = is=20 spec;d at 22.4 mA is for FAST-5 leakage current (Table 25 FAST-5 = c)) =20 ASSUMING the potential leakage from receivers or open drain drivers to = be 0.4 mA=20 at 0.5 V and given the driver sink current of 48 mA, then the terminator = source=20 current should be limited to 22.4 mA (max) at 0.5 V. This number is = driven=20 entirely by assuming the 0.4 mA worst case leakage of legacy FAST-5 SCSI = drivers=20 and receivers.
i.e. (22.4 mA)(2 terminators) + (0.4 mA leakage)(8 = devices) =3D=20 44.8 mA + 3.2 mA =3D 48 mA. Millions of terminators have been shipped to the SPI spec. of 24 mA = at 0.2 V=20 (and above) with no adverse consequences for end users. This tells me = that in=20 the real world: leakage currents will not approach the 0.4 mA @ 0.5 V=20 number...especially with today;s receivers and drivers=20 drivers have excess margin beyond 48 mA QUESTIONS FOR REFLECTOR: Was SPI wrong?=20 Is there really 0.4 mA leakage with FAST-5 = devices?
 
Regards,
Louis



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