Sun, SPARCs and SCSI:
Ever feeled confused about SCSI and Sun boxes? Here is a small brief.
SCSI Types:
This might cause you some problem, if for example you get +2 volt (might occur in case of an error somewhere, or if its a long cable) , is that a binary 0 or a 1?
The maximum guaranteed lenght for a single ended SCSI cable to work is 3 metres.
The maximum guaranteed lenght for a differential SCSI cable is 25 metres.
A rather common way is to combine singleended with differential, first you have
a single ended controller, then you passes it thru a converter and a diff scsi cable, followed by another controller and a singleended disk.
(most disks are single ended).
This way you can have longer scsi cables.
Now adays its common to combine SCSI with opticalcables, since optical cables can transfer more data than SCSI cables, and they can be much longer.
Sun/SCSI standards:
These are the SCSI standards you might find on a Sun box:
| Standard: | Clock rate: | Bus width: | Data rate:*1 | Command rate:*2 | Max #of devices:*3 |
| SCSI-1(very very old) | 5 MHz | 8 bit | 5 MB/sec | 5 MB/sec | 7 |
| SCSI-2(fast SCSI) | 10 MHz | 8 bit | 10 MB/sec | 5 MB/sec | 7 |
| SCSI-2(fast/wide SCSI) | 10 MHz | 16 bit | 20 MB/sec | 5 MB/sec | 15 |
| SCSI-2(ultra) | 20 MHz | 16 bit | 40 MB/sec | 5 MB/sec | 15 |
| Optical*4 | 25 MHz | - | 25 MB/sec Duplex | - | |
| Optical*5 | 100 MHz | - | 100 MB/sec Duplex | - | |
Explainations:
*1: The Data rate can be calculated by looking at how much the device can send at each cycle. SCSI-1 for example, sends 8 bits/cycle and makes 5 million cycles/second, therefor the data rate is 5 MB/sec.(8 bits = 1 byte)
*2: As you can see, the command rate is always set to 5 MHz, this is due to compability issues. All commands, for example "read" is sent at 5 Mhz, but before the device starts sending its data, there is a negotiation between the controller and the device.
*3: Equal to the bus width, minus 1 for the controller
*4: This old type of optical connection was used on the old arrays such as SSA1xx and SSA2xx.
*5: This is the current optical one, it can be found for example in the A3500.
