daisy chaining SCSI devices on a ds3100

[AJ Casamento]

Rob McNicholas writes:

	I have a DECstation 3100 (24M memory, ULTRIX V4.1 (Rev. 55), one
	storage expansion box) with the following SCSI configuration:

SCSI id		device			order in chain
-------		------			--------------
0		rz57			1 ? (internal to storage expansion)
1		rz55			3
2		rz57			1 ? (internal to storage expansion)
3		UNUSED			UNUSED
4		TZK08 (exabyte 8mm)	2
5		TK50			4


	I was wondering if the order the devices are connected together
	affects their performance.




  There are a few items to be aware of:


	* SCSI ID indicates priority, with the highest number receiving the
	  highest priority.

	* Within the 6M limit of electrical cable equivalent, the devices
	  should be impacted by their SCSI ID and not by the order from 
	  the system SCSI port.

	* SCSI Host ID - On the DECstation 2100/3100 systems this will be a
	  default of 6 (the ID 7 was reserved for a Realtime CPU slot). On
	  the DECsystem 5000 series (1XX or 2XX) it will default to ID 7).

	* In the first RISC/Ultrix SCSI driver implementations the following
	  recommendations for ID assignments was made (I haven't seen it
	  contradicted since)


		7 	Realtime CPU
		6	CPU
		5	TK50
		4	RRD40
		3	TZK08
		2	RZ23
		1	RZ55
		0	RZ56


	   Obviously, there isn't a Realtime CPU so that address is useable.
	   The idea is to give the slower SCSI devices (except for CPU) a
	   higher SCSI ID number. In the case of the TK50 this is supposed
	   to be a real help. The RRD40 is somewhat faster. The position of
	   the 8mm is arguable. Although, I prefer to keep the tape media
	   at a higher priority. Most of the disk drives should play well
	   regardless of how you mix them. I will tell you honestly that I
	   remember some flakey things happening when a disk drive was given
	   a higher priority than the CPU, but I don't know if that's still
	   true.

	
	   I hope that this information helps.

				Thanx,
				  AJ


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