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Path: utzoo!utgpu!water!watmath!clyde!cbatt!ihnp4!ptsfa!maxepr!ken
From: ken@maxepr.UUCP
Newsgroups: comp.sys.att
Subject: Re: wanted info on 7300 disk drives
Message-ID: <180@maxepr.UUCP>
Date: Thu, 14-May-87 13:31:54 EDT
Article-I.D.: maxepr.180
Posted: Thu May 14 13:31:54 1987
Date-Received: Sat, 16-May-87 09:50:43 EDT
References: <6485@mhuxu.UUCP> <5970@steinmetz.steinmetz.UUCP> <817@rtech.UUCP>
Reply-To: ken@maxepr.UUCP (Ken Brassler)
Distribution: world
Organization: Brassler Engineering Co., Mill Valley, CA
Lines: 32
Summary: A 7300 can access 8 heads, 1024 cyls

In article <817@rtech.UUCP> daveb@rtech.UUCP (Dave Brower) writes:
>
>I am using a Priam (nee Vertex) 72M.  You can use practically any ST506
>drive and roll your own format.  BUT -- you can only get at 7 heads worth
>of drive because of the number of head selects brought out to the drive
>connector.

Although iv(1m) describes the number of heads as "This must be a
positive number not greater than 7", it will actually address 8
heads. This is the old problem of 0 thru 7 = 8. There are 3 head
select lines, which in binary notation, 111, equals the decimal
number 7, but including 000, describes 8 states.

Consider this;

The current hard disk driver on the 7300/3B1 is limited to 8 heads,
1024 cylinders.

Number of tracks = heads x cylinders. Each track has 17 sectors,
each sector is 512 bytes. The 7300 reserves the last sector of each
track as an alternate bad block, leaving 16 available sectors. The
maximum hard disk capacity on a 7300 is then:

8 x 1024 x 16 x 512 = 67,108,864 bytes.

Which is exactly the maximum capacity advertized for a 3B1, and also
what I achieved by adding an ST4096, which has 9 heads, 1024
cylinders.
-- 

Ken Brassler {ihnp4|qantel|pyramid|lll-crg}!ptsfa!maxepr!ken
			   ....ethos!gladys!ptsfa!maxepr!ken