SPI-2 driver test conditions

Kevin Gingerich S=Kevin_Gingerich%S=Gingerich%G=Kevin%TI at mcimail.com
Tue Apr 30 14:47:00 PDT 1996

```* From the SCSI Reflector, posted by:
* Kevin Gingerich <S=Kevin_Gingerich%S=Gingerich%G=Kevin%TI at mcimail.com>
*

To  SCSI Reflector   X400

>From  Kevin Gingerich  GING

Subj  SPI-2 driver test conditions

* From the SCSI Reflector, posted by:

Form: Memo
Text: (32 lines follow)
Following up my proposal at the Milpitas SPI-2 meeting:

I recalculated the test conditions under the new assumptions for terminator
characteristics, i.e., Rt ranges from 100 to 110 ohms and Vb from 100 to 125
mV.

I also added another limit as Bill Ham suggested to enforce asymmetric
driver strengths, by limiting In to 11 mA.

The new test conditions I am proposing for Section 10.1.1, Table 10, are as
follows. Replace the first condition:
416 mV <= Va <= 706 mV
with:
249 mV + 0.3 * |Vn| <= Va <= 706 mV

> I calculate that this to be 170 mV + 0.47 * |Vn| <= Va <= 650 mV
>
>My derivation is rather lengthy so, I'll just give the result here as the
>constraints on the assertion current as a function of the negation current
>(In), cable impedance (Zo), the reflection coefficient at the load (pL),
>attenuation factor (A), termination impedance (Zt), and the bias voltage
>(Vos). The basis is to achieve at least 25% of the steady-state voltage on the
>first transition to the opposite state at any point on the bus.
>
> (In(1.25 Zt/2 - (1+pL)AZo/2) + 1.25Vos)/((1+pL)AZo/2) <= Ia
> Ia <= (In(1+pL)AZo/2 + 1.25Vos)/(1.25 Zt/2 - (1+pL)AZo/2)
>
>This can be converted to the form you give by inserting the test circuit
>values, rearranging terms, and multiplying through by 55 ohms to get voltages.
>(If you try it, watch your sign convention.)

Replace the second condition:
381 mV <= |Vn| <= 977 mV
with:
255 mV + 0.3 * Va <= |Vn| <= 714 mV

> and this should be 170 mV + 0.47 * Va < |Vn| <= 700 mV.
>
>I got the new maximums from trial and error and the spreadsheet model. The
>constraint was to keep the differential voltages less than +/-1 V including
>reflected waves.

Remove the third condition:
-277 mV < |Va| - |Vn| < 181 mV
It turns out that this condition is not necessary because restricting the
maximum In to 11 mA (or |Vn| <= 714 mV) keeps the voltage ratio at the
receiver within the 4:1 we are looking for.

I'd like to point out that my derivation assumes current mode drivers. Are
these test conditions valid for voltage mode? Unfortunately I haven't had a
chance to study that question.

>Either voltage- or current-mode drivers must deliver the first-step voltages
>required. The difference occurs in the polarity and magnitude of subsequent
>reflected waves. Within the defined bus parameters, the reflections should not
>cause problems.
>
>/Kevin/

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