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Ef nriagialins Tapeis

Patricia A. Reslook
Center for Naval Analyses

Alexandria, Virginia

There are many obstacles to success-

fully acquiring machine-readable data.

Difficulties usually occur during the

physical transfer of the data from one

site to another, the most common method

being magnetic tape. Yet, even with
higher quality tape and more efficient

tape drives, tape processing is still

plagued with problems. Parity errors,

incompatible tape labels, multiple
encoding schemes, and poor quality
media have all contributed to tape

users' frustration. In an attempt to

educate the nontechnical professional,
this paper will give an overview of

the technical aspects of magnetic tapes

and their use. Topics include the

physical aspects of magnetic tape, how

data is stored on tape, recording modes,

tape errors, and their prevention.
A data archivist faces two main

tasks: data acquisition and data main-
tenance. To acquire data you must know

how data is stored on tape and what
your installation can handle. The

information necessary to successfully
transfer data between sites will be

covered below, as will housekeeping
measures essential to keeping magnetic
tapes readable.

Data Acquisition

The physical aspects of magnetic
tape. A standard magnetic tape has a

10?^-inch hub filled with '^-inch wide
tape. The tape should be 2400 feet
+50 feet, -0 feet. It is important
to bear this in mind when purchasing
new tapes, as tapes of less than 2400
feet are indicative of poor quality
control

.

A magnetic tape is composed of three

layers. The first layer is made of

oxide; this is where the magnetized data

is stored. This layer must be smooth

and of uniform thickness (0.00045 of an

inch). The second layer (the binder)

is a glue used to make the oxide adhere

to the backing. The binder must be

flexible enough to reduce oxide chipping,

yet, not too sticky, or the tape will

stick to itself. The last layer is the

backing, and it is usually made of

mylar. Creating a magnetic tape is a

complex task; hence it is important to

have rigid quality control standards.

There are three important points to

be aware of when exchanging data on

tape: 1) Is the tape labeled or unla-

beled? 2) How many data files are on

the tape? and 3) How many tape reels

does the data span?
Tapes can come with or without labels,

and files can come as a single file on a

single tape reel, multiple files on a

reel, a single file on multiple reels,

or multiple files on multiple reels

(reference (1),(2)). You must know

what types of tapes your installation

can handle and what types of utilities

are available at your site for process-

ing these types of tapes. Check to see

if one form is easier to handle than

another. If so, see if you can receive

the data from the sending organization

in that form.

Here is a list of the other items

you need to know when receiving a tape:

1. Is the tape 9-track or 7-track

(7-track is fairly old technology,

but does still exist)?



2. How large is the blocksize of the
data? Some systems cannot handle
blocked data.

3. At what density was the data
recorded (6250 BPI , 1600 BPI , 800
BPI)?

4. At what parity was the data
recorded (even or odd)?

5. What is the character set in
which the data was recorded?

Some character set possibilities
include:

ASCII American National Standard
Code for Information Inter-
change. Most commonly found
on Digital Equipment Corpora-
tion systems;

EBCDIC Extended Binary Coded Decimal
Interchange Code. IBM,
Amdahl , Burroughs;

BCD Binary Coded Decimal Charac-
ter Code. CDC6600, Honeywell;

FIELDATA Standarized Military Data
Transmission Code. Univac.

The main point to all this is: Know
what your system has and what it can
handle. Your Computing Services depart-
ment should have a handout telling the
easiest way to receive data. This
information is imperative for a suc-
cessful transfer of data.

I have found the government form
"Transmittal Form for Describing Com-
puter Magnetic Tape File Properties" to
be an invaluable reference when receiv-
ing or sending data (reference (3)).
You can use this form in one of two
ways. When receiving data, fill it out
the way you need to receive the tape,
and send the form along with your data
request. When sending data, send the
form filled out with the specifications
of the way the tape was created.

Tape Maintenance

to contamination of the tape, physical
mishandling, or problems with tape
drives and/or tape cleaning equipment.
Here are some items that you want to

consider to keep your tapes readable
(reference (4)).

Reduce contamination. Most tape
contamination comes from the tapes
themselves. Oxide chips off the sur-

face of the tape each time the tape is

used. One way to reduce this chipping
is to buy high quality tapes made with
good binder. Other contamination comes
from carelessness in handling tapes or

a dirty computer room. Some ways to

reduce contamination are:

1. Reguarly clean tape drives. (At

CNA we clean the drives at the
start of every shift and before tape
intensive jobs.

)

2. Maintain the proper recommended
temperature and humidity--this
helps to reduce the oxide chipping.

3. When cleaning the floors around
tapes, clean the entire floor with
a damp mop--DO NOT sweep, dry mop,
or dust.

4. Minimize floor waxing— if you
must wax, machine buff to remove
the excess wax, damp mop with cold
water to harden the surface, and

buff again when dry. NEVER use

steel wool or other metal abrasives
for buffing.

Another way to reduce contamination
is to regularly clean tapes (once every
eight uses). Be very careful if you
decide to do this, as some tape clean-

ers do more harm than good. I would
not recommend using a tape cleaner on

a tape that is error free.
Tape drives can also produce tape

errors. Your organization should have

a regular tape-drive maintenance pro-

gram where a field engineer does rou-

tine cleaning and alignment of the

drives. This preventive maintenance
is invaluable.

How to minimize, correct, and pre-
vent errors. Most tape errors are due

fTF



Long-term storage. Once a tape has

been hanging on a rack for about six

months, it starts to deteriorate.

Pieces of dirt and chipped oxide start

to cause dents in the backing of the

tape. The tape may become unreadable
because the dent is causing the tape

to be too far away from the read head

of the tape drive. The best way to

prevent this type of problem is to spin

tapes every six months. The preferred
method is to have a program that scans

the data on the tape to be sure the

tape can be read. If there are problems,

you will know it early and will be able

to copy the data to another reel. If

you wait for a long period of time to

spin a tape, the damage could be perma-

nent. The concept is the same as

walking with a rock in your shoe. If

you walk a block and take it out, you
probably will not suffer any long-term
damage. If you walk a mile with a rock

in your shoe, you will have a hole in

your foot.

Tape handling. Tips for tape
handling include:

1. Keep tapes in the computer room.

2. Tapes should not be laid on top

of the tape unit.

3. External tape labels should be

sticky labels that peel off and
leave no residue.

4. Never allow the beginning of the
tape to trail on the floor.

5. Smoking should not be permitted
around tapes.

Tape storage. Tapes should be

stored in an upright position in a cabi-
net or shelf elevated from the floor,
and as far away from sources of paper
and card dust (line printers and card
reader/punch) as possible.

Backups of valuable data sets should
be kept off site. To save money, look
for a sister institution to swap tapes
with instad of paying for vault storage.

Tape Transmittal. Very often tapes
are damaged in the mail or while being
hand carried. Tapes sent through the

mail should be clearly marked
MAGNETIC TAPE--KEEP AWAY FROM ELECTRIC

MOTORS, SCANNING DEVICES AND MAGNETS--
DO NOT X-RAY. Be sure to give the

above adivce to the courier for tapes

being hand carried.

Conclusions and Recommendations

To ease data transfer, know the

answers to all of the questions on a

form like "Transmittal Form for Des-

cribing Computer Magnetic Tape File

Properties" (reference (3)). Be sure

to ask for some type of a dump or map
of the tape being created. This makes

the sending installation look at the

tape after it is written--that is, it

forces them to be sure something was

written to the tape. Finally, be sure

to get documentation on the data.

These items include record layout,
descriptions of all the variables and

how they were derived, and the count
of the total number of records in each

file.

To insure in-house data reliability,

scan data tapes every six months to a

year to be sure they are readable. At

first sign of trouble, recopy the data.

Recopy vauable data sets to NEW tapes
every three or four years. Try to

maintain cleanliness at your sit. DO

NOT use a cheap tape cleaner. It will

do more damage than good. Most impor-

tant of all: buy the best quality tapes

you can afford.

Referenaes

(1) VAX/VMS Magnetic Taoe User's Guide,
VAX/VMS Version 3.0. Digital

Equipment Corp, Maynard, MA; Chap-
ter 2. May, 1982.

(2) U.S. Department of Commerce.
National Bureau of Standards. Mag-

netic Tape Labels and File Struc-
ture for Information Interchange.
Federal Information Processing
Standards Publication 79. 1980.

(3) U.S. Department of Commerce.
National Bureau of Standards.
Transmittal Form for Describing

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