Annex B (informative) Fast SCSI skew time

This annex is included to explain the skew budget for the fast SCSI option which is defined in Clause 4.

Synchronous transfer rates using a transfer period between 100 ns and 200 ns are known as the fast SCSI option. Fast data transfer times have been tested using the following skew budget (see table B.1) with the differential alternative using transceivers with 25 m of 0,080 42 mm2 (28 AWG) twisted pair cable as specified in 5.2.3. The transceivers were subjected to a maximum temperature difference of 25 degrees C and a maximum of 200 mV of VCC difference.

Table B.1 - Fast SCSI jitter budget

+================-============================================-===============+
|       Parameter|                                            | Jitter budget |
|           label|           Description                      |    +/- ns     |
|----------------+--------------------------------------------+---------------|
|              a | clock offset                               |       5       |
|              b | transmitting logic skew                    |       3       |
|Transmitting  c | foil delay                                 |       1       |
|  Device      d | transmitter propogation delay skew         |       6       |
|              e | foil delay                                 |       1       |
|              f | drop cable propogation delay               |       1       |
|-- Connector  --|--------------------------------------------|---------------|
|              g | external cable - skew between pairs        |       5       |
|   Cable      h | distortion due to cable impedance          |       1       |
|              i | distortion due to intersymbol interferance |       2       |
|              j | bias distortion                            |       2       |
|-- Connector  --|--------------------------------------------|---------------|
|              k | drop cable propogation delay               |       1       |
|Receiving     l | foil delay                                 |       1       |
| Device       m | receiver propogation delay skew            |       9       |
|              n | foil delay                                 |       1       |
|              o | logic setup/hold                           |       5       |
|----------------+--------------------------------------------+---------------|
|                | total jitter budget                        |      44       |
+=============================================================================+

Mapping the above jitter or skew budget to the SCSI format in 5.7 and 5.8 is done in Table B.2.

Table B.2 - Mapping of jitter to SCSI

+=======================-=============-===============+
| SCSI timing parameter | Calculation | Value (ns)    |
|-----------------------+-------------+---------------|
| Fast cable skew delay |      g      |   5           |
| Fast deskew delay     |  SUM(h..n)  |  20 (note 1)  |
| Fast hold time        |      o      |  10 (note 1)  |
| Fast assertion period |   (note 2)  |  30           |
| Fast negation period  |   (note 2)  |  30           |
|-----------------------------------------------------|
| Notes                                               |
|  1) Value is rounded up from the jitter budget.     |
|  2) The fast assertion period and fast negation     |
|     period are derived from isolated pulse          |
|     mesurements and represent a minimum pulse width |
|     with a satisfactory margin.                     |
+=====================================================+

The maximum driver skew allowed was 6 ns (tPLH min. - tPHL max.) and the maximum receiver skew tested was 9 ns (tPLH min. - tPHL max.). Values greater than these could be used if other numbers could be reduced -- the sum is what is important.

Fast data transfer timing parameters were not tested for the single- ended transceiver option prior to publication of this International Standard.