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The following article appeared in Computer Design, November 5, 1985.


ADD-ON CONTROLLER PROVIDES THRIFY PC-XT STORAGE


To meet demands for higher capacity and higher performance drives of
disk storage on IBM PC-XT products, Adaptec has introduced the
ACB-2070A, a hard disk controller that uses the 2,7 run-length limited
(RLL) encoding/decoding scheme to increase the capacity of existing
drives by 50 percent. It can transfer data as fast as the PC can handle
it.

Speed, however, is not the only important factor in the PC market.
Adaptec's Vice President of Marketing, Jeff Miller, cites three market
forces as being equally as important as high performance: "price,
price, and price/performance." The ACB-2070A controller attacks the
pricing problem, not by selling for less, but by providing the lowest
cost per megabyte at the subystem level. Achieving a 50-percent
increase in capacity without increasing the cost of the drive cuts the
cost per megabyte of storage by 30 percent.

To gain the increased capacity, Adaptec, according to Miller, uses
IBM's 2,7 RLL encoding scheme. Licensed from IBM, the scheme allows
more data to be stored in the same flux density than is possible using
more common MFM (Medium Frequency Modulation) recording schemes.
Moreover, the RLL scheme is transparent to the IBM PC XT.

By supporting data transfers at a 7.5 Mbit/s rate, the controller not
only provides the fastest possible transfers between disk and system,
it allows disks to be formatted using a 3:1 interleave. While 5 1/4 in.
drives that run at the 5 Mbit/s transfer rate on industry-generic ST506
drives can adopt 2:1 interleaving, according to Miller, the 7.5 Mbit/s
rate requires a 3:1 interleave on the disk. The system can't handle the
data any faster.

Because of the higher interleave factor, the controller's 7.5 Mbit/s
transfer rate supplies the same performance as the slower 5 Mbit/s rate
at 2:1 interleave. This, combined with a 13 usec step rate that speeds
head positioning, give users the fastest possible access to data on the
disk drive.

A plug-in replacement for an existing PC XT controller board, the 2070A
supports all ST506 drives capable of handling 2,7 RLL encoding. This
includes drives from Miniscribe, Microscience, and LaPine that supply
capacities of 20 Mbytes using MFM techniques, as well as drives from
Vertex, Maxtor and Toshiba that provide 40-, 70- and 120-Mbyte
capacities. The controller used with these drives brings capacities to
30, 60, 105 and 180 Mbytes, respectively.

Miller cautions that drives used with the 2070A controller must be
newer versions. Those that supply plated media and have relatively
broad read/write channels incorporate a low-pass filter that goes from
5MHz to 1.8 MHz necessary for RLL operate satisfactorily.

Looking at the price/megabyte that is supplied by the drive/controller
combination, Miller points to a 43-percent savings at the retail level.
A 20-Mbyte drives sells for roughly $75/Mbyte and a 30-Mbyte drive for
about $91/Mbyte. When using the 2070A with the 20-Mbyte drive, however
the cost for 30-Mbytes drips to roughly $50/Mbyte. For the OEM world,
Miller projects that with prices falling over the next year or so, a
40-Mbyte drives will sell for $400; adding the $150 controller will
bring the cost of 60Mbytes down to less than $10/Mbyte.

A controller autoconfigure feature eliminates the need for jumpers by
automatically configuring the drive at power on. This allows system
designers to integrate various drives into their PC XT systems without
changing the system or controller software when a different drive is
used.

Sector-level defect handling provided by the controller allows bad
sections of track to be blocked out on a sector by sector basis,
leaving other sectors on the track available for data storage. The
mapping action presents a zero-defect disk to the DOS operating system,
reducing seek time and increasing disk capacity by allowing all tracks
to be used.

A large disk-partitioning feature circumvents DOS's storage limit (two
32-Mbyte drives). By enabling a single disk to be partitioned into as
many as eight logical disks, each having 32-Mbyte capacity, storage
size of the system is increased with little additional cost or size.

Production quantities of the ACB-2070A controller board are scheduled
for January 1986. Boards are priced at $165 each in 1000 quantities.

Adaptec, Inc.
580 Cottonwood Drive
Milpitas, CA 95035

NOTE: The ACB2070A Product Manual is available for $5.00 from the
following address:


Adaptec, Inc.
Literature Department - TH
580 Cottonwood Drive
Milpitas, CA 95035

11/04/86 (jf)




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