How do SCSI drives in the 3/160 tower communicate?

[Christopher J. Pikus]

>From article <1990Sep12.175832.11438 at rice.edu>, by AKONSTAM at TRINITY.BITNET (AARON KONSTAM):
> I have a question about disk connections to theSCSI bus in a 3/160, and I
> assume other Suns with towers. Our 3/160 has two Micropolis 1355 internal
> disks in its tower together with an Emulex controller to control them. We
> also have a WREN VI drive in a shoebox connected directly to the SCSI
> board in the tower with a ribbon cable.
> 
> According to the block diagrams that I have seen for the 3/160 the SCSI
> board should be connected to the VME bus (and it is through three edge
> connectors) and the Emulex controller in the tower should be connected
> between the SCSI board and the disk drives. I can find no conections on
> the SCSI bus except the edge connectors to the VME bus and the ribbon
> cable to the shoebox.
> 
> How are the the Micropolis 1355 which are supposed to be SCSI disk drives
> getting control signals from the SCSI bus? I will have to admit the
> machine works with out my understanding this but if someone could clear up
> this mystery I would appreciate it.
> 

This is a very common question asked in this group. (perhaps someone
should collect all these answers and post them monthly. :-)

The SCSI cable connection to the "VME bus" is not really directly to the
bus. On slot 7 of the sun motherboard, the "P2" connector on the back has
the extended pins for a connector. In the VME spec, the outer 2 rows of
the P2 conector is left for OEM defined functions. On the Sun they are
left unconnected on slots 7, 8, and 9.

If you look in your 3/160 card cage, you will see a card in slot 7. THis
is the SCSI controller. It outputs its SCSI control signals on the outer 2
rows of the P2 connector which go to the Emulex ESDI bridge controller
(and the external shoebos). To prove this hypothesis, move the SCSI
controller from slot 7 to another slot and watch it not work. :-)

Be Seeing You ...
Christopher J. Pikus,               Megatek Corp.
INTERNET:     cjp at megatek.uucp      San Diego, CA
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