R. B. LAWRANCE R. E. WILKINS R. A. PENDLETON
I
N DESCRIBING the tape transport of the DATAmatic 1000, it is perhaps well to begin by reviewing the influential system features and their resulting re-quirements in the following sections:Information Rate
The central processor communicates with the magnetic file units through the input and output buffers at the steady average rate of a quarter of a million bits per second. With any presently prac-ticable tape speed and recording density, this requires a tape width greater than the conventional half inch or so. DATA-matic-lOOO tape has a nominal width of 3 inches (actually 3.056), carries 31 chan-nels of information and 5 chanchan-nels of pre-written magnetic block marks, is trans~
ported in either direction at a speed of 100 inches per second, and utilizes bit densities per channel of 320 and 160 bits per inch.
Organization of Words and Channels
As mentioned earlier by
J.
E. Smithl 48 information bits and 4 checking bits are grouped together in each word. In writing or reading, all the bits of a word are fed sequentially to a single channel so that characters and words are organized completely longitudinally rather than across the tape. In recording, a timing relationship does exist between bits being written in the various information chan-nels, but this is only incidental and a mat-ter of convenience; in playback there need be no interchannel timing relation-ship at all. Transfer of information from a tape into the input buffer or output con-verter is asynchronous. There is no syn-chronous channel and no requirement for simultaneous sensing of bits in the infor-mation channels. The major potential source of trouble from skew is thus elimi-nated.Organization of Words Into Blocks
The locations on tape into which in-formation is written are pre-established and marked by magnetic block marks, which are placed on the tape before it is
magnetically inspected and put in service;
these block marks are never altered there-after. As shown in Figs. 1 and 2, informa-tion space and stop-start space are alter-nated in an interlaced pattern with an over-all length of 1.23 inches. The tape, approximately 2,600 feet long, contains 50,000 blocks, of which 25,000 belong to the so called "first half," normally scanned in the physical forward direction, and 25,000 to the "second half," normally scanned in the physical reverse direction.
On scanning the first half, the interlaced second-half information space serves for stop-start space (somewhat over 0.6 inch) and similarly, for scanning the second-half information space, the first-half information space is available for stopping and starting. Apart from the improved efficiency of tape utilization, this has the considerable advantage that no separate rewind operation is required.
Reading and Writing
The same magnetic head assembly is used for writing and reading. Because of the pre-established information spaces and block mark pattern, and because of the nature of the recording system, it is possible to alter information, when re-quired, simply by writing new informa-tion over the old, in as few or as many channels as desired. It is thus required of the tape mechanism that at the time of rerecording, the relationship of head and tape must be closely controlled both laterally and longitudinally to be essen-tially the same as when the original record-ing was made. Since it is a feature of the DATAmatic-lOOO system that tapes re-corded on any individual tape drive may be played back or rerecorded on any other tape drive, the channel locations on the magnetic heads and the tracking of the tape must be held to within a very few thousands of an inch.
Ability to Scan Information In Either Direction
In the DATAmatic 1000, recording of information is alway~ done with the tape moving in the logic~l forward direction, physical forward for~rst half blocks and
physical reverse for second half blocks.
Playback however, can be done in either direction. The tape mechanism is thus required to scan information in either direction under a variety of continuous.
motion and stop-start-reversal conditions~
hence the stopping, acceleration to nominal speed, accuracy of nominal speed, and tracking all must satisfy certain rather stringent conditions.
In addition to the system requirements outlined briefly in the foregoing, various.
other requirements must be met by the tape mechanism. Of prime importance is the requirement that no normal operation:
or conceivable malfunction of the tape mechanism shall result in deterioration or destruction of the tape or its information content.
MIDDLE OF TAPE
Fig. 1. Alternating first-half and second-half information spaces on D-1000 magnetic tape
The tape employed has a special con-struction shown in Fig. 3. The magnetic head does not come in contact with the magnetic recording oxide but is separated from it by a 0.5-mil layer of Mylar. In manufacturing, the oxide layer, \Yhose thickness is also 0.5 mil, is deposited on this Mylar overlayer so that the oxide surface nearest the head gap has the smoothness characteristic of the :Mylar sheet rather than the less perfect surface characteristics of the air -dried mixture of oxide and binder. A laminating adhesive whose thickness is approximately 1/4 mil is then used to attach the composite oxide and Mylar sheet to a base of 2.0-mil Mylar. The result is a tape which pro-duces much less wear on the magnetic head than do direct contact tapes;
furthermore, the information-carrying oxide is protected from abrasion,
scratch-R. B. LAWRANCE, R. E. WILKINS, and R. A.
PENDLETON are with the DATAmatic Corporation, Newton Highlands, Mass.
84
Lawr(1nce, Wilkins, Pendleton-Magnetic Storage on Wide TapesMOTION OF TAPE: Fig. 2 (left).
Organization of information spaces and block mark
channels
RECORDING SIDE
/
ON SCANNING FIRST HALF, THE INTERLACED SECOND HALF 'INFORMATION SPACE SERVES FOR STOP-START SPACE.
ON SCANNING SECOND HALF, ViCE VERSA.
Fig. 3 (right).
Enlarged section D-1000 magnetic
tape
ing, flaking, embedment of dust, mois-ture, and other forms of deterioration.
Fig. 4 shows two magnetic head as-semblies, one of which is shown un-mounted and the other un-mounted in its cylindrical cartridge. The heads are of staggered gap construction and have the following significant dimensions and specifications:
Number of channels: 36
Offset of alternate channels (stagger):
0.20 inch on centers Gap width: 1 mil (.001 inch) Gap shim: 1 mil beryllium copper Channel width: 60 mils
Gap height: 20 mils
Spacing of channels on centers: 82 mils Tolerance on channel locations: ± 1,5 mils Radius of curvature of head and cartridge:
2 inches
Magnetic material: 3 mil mu-metallamina-tions
Number of turns: 100
Voltage output on normal playback; 10 millivolts peak-to-peak
Tape Transport Mechanism
The appearance of information re-corded on tape is shown in Fig. 5. For clarity, only the locations of information pertaining to first-half record spaces are shown; those pertaining to second-half information would be interlaced in ac-cordance with Fig. 2. The alternating pattern due to the staggered gap construc-tion is evident from Fig. 5, although for simplicity it was not shown in Figs. 1 and 2. In the enlarged portion of the figure is shown the appearance of a portion of a typical word as it would be made visible by magnetic development with colloidal Fe304. The vertical lines correspond to pole concentrations, alternately north and south, produced by _ reversal of the head current. It will be noticed that the spac-ings of pole concentrations have two characteristic values, approximately 3.0 and 6.0 mils (30 and 60 J,Lsec) as written on tape. A 30 J,Lsec interval between
head current reversals represents a zero bit and a 60 J,Lsec interval represents a one bit. It is possible with a little prac-tice to read visually the bits of a word when the magnetic image is carefully de-veloped and viewed under a medium power microscope.
The attainment of the high bit densi-ties quoted above, with the relatively wide head gap and the unusually large 0.5-mil separation between the pole face and the magnetic oxide, is due to the characteris-tics of the recording and playback system, which represents a radical departure from present practice. Space does not permit further description of these features in the present paper.
As indicated in the opening section of this paper a considerable part of the flexi-bility of the DATAmatic-lOOO system results from the use of unnumbered but accurately located information spaces -specified by the permanently placed
mag-netic block marks. The action of the tape handling equipment in decelerating, re-versing, and accelerating tape must ac-cordingly be fast and reproducible under all conditions of motion. In one
im-portant mode of motion the tape moves continuoesly; this mode is so simple that it does not require further discussion.
In stop-start motion, when the tape is in-structed to stop after scanning a block it must come to rest well within the allotted space before the next information space.
Upon restarting in either direction it must reach full speed and be in steady motion by the time the next information space is entered, as indicated by sensing the be-ginning block mark. In order to fulfill these requirements, considerable care has gone into the evolution of the as-sembly shown in Fig. 6, which shows a closeup of the capstan, brake, head mounting, valve, and actuator assembly, with the tape draped in position. The contour of the tape and the relative lo-cations of the surfaces with which it comes in contact are shown in more detail in Fig. 7.
Two continuously counter-rotating cap-stans of approximately lO-inch circum-ference are used, and these are driven at approximately 10 revolutions per second by a synchronous motor. To move the tape in a desired dire,ction, the slotted
Fig. 4. Two magnetic head assemblies, right: head mounted in cylindrical cartridge; left:
head unmounted
Lawrance, Wilkins, Pendleton-Magnetic Storage on Wide Tapes 85
BLOCK MARKS:
\
SECOND HALF
FIRST HAL~
15 INFORMATIO~
CHANNELS CARRY A COMPLETE INSURANCE POLICY RECORD AS AN EXAMPLE
I
"'iIdMiIIMfjiiijijiijijjij!i!!jjj1i1
.",iriJ::::1r;:::::::;:;::::,;,!riliUM
Mi"iiJ:::1::::rJ::r::::::::iMftM
ildNiliiiiibllUM!ii'i!diM iijlUMriM!ii!iiMMmM!ffl
"w':m:::;:oo-_JiIIN',,::l"!!.
iMrili
, i
i
i r:=~
i
[
MAGNETIC HEAD COUNTER-ROTATINGCARTRIDGE CAPSTANS: SURFACE
SPEED
)
100 INCHES SEC.FIXED BRAKE
I
[
TAPEWORKING AIR PASSAGES DYNAMIC
STIFFE.NING
~
Fig. 5. Information on tape, showing channel stagger TO LOOP CHAMBERS
surface of the appropriate capstan is communicated (through an internal com-mutator, a hollow shaft, and a fast-acting electro-pneumatic valve) to a reservoir containing air at reduced pressure. At-mospheric pressure then presses the tape into contact with a portion of the surface of the chosen capstan.
For arresting the motion of the tape a stationary brake surface of similar slotted
Fig. 7. Capstan, brake, and head assembly
construction is used. The brake is lo-cated closely adjacent to tbe magnetic head assembly and surrounds it on botb sides. This has the desirable effect that wben tape motion is arrested by engaging the tape to the brake surface tbere is a minimum of lateral or longitudinal dis-placement relative to the head. Upon
subsequent resumption of tape motion in eitber direction, the tape in contact with tbe head needs to make essentially no tracking adjustment before it is again running true.
The contour of tbe brake and its loca.
tion relative to .the magnetic head are such that the tape has a slight wrap
Fig. 8. Capstan assembly, with valve and actuator assembly Fig. 6. Capstan assembly, showing tape in position removed. Note slotted surFaces of capstan
86
\&.I
a: ~en
en \&.I
en a:
\&.1\&.1
a::c
CLCL
\&.len .... 0
~~
~~ 1 aaen Z
cr-PRESSURE AT CAPSTAN SURFACE
COMPRESSED AIR RESERVOIR PRE SSURE
r,/
I ' I '
\ ,
...._-PRESSURE AT
~BRAKE SURFACE
SUCTION RESERVOIR PRESSURE
o
~---r~---+~---=T~IM~E~DRIVING
DRIVING---"BRAKE"/
~IIDRIVE
FORWARD"COMMAND COMMAND VALVE
VALV E OPERATION OPERATION EFFEC TED
EFFECTED
Fig. 9. Capstan, showing internal commutator Fig. 10. Approximate pressure wave Forms during a drive-brake-drive cycle
around the head, thus assuring good con-tact for reading and writing. The spacing between brake and head is several times as wide as the thickness of the tape how-ever, so that edgewise insertion of the tape is very easy. Edge guides mounted on the brake provide accurate tracking of the tape in the vicinity of the head.
Adjacent to and beneath each capstan there is, mounted a long flat inclined plate which is also equipped with edge guides.
The top end of the plate is adjacent to the capstan and assists in separating the tape from the capstan. Since the plate is closely adjacent to the normal path of the tape it serves as a dynamic stiffener and oscillation damper for the tape in the vicinity of the capstan assembly.
Fig. 8 is a closeup view similar to Fig. 6 but with the valve and actuator assembly removed, and with the tape removed from the capstans so as to show the slotted surfaces which connect internally to the working air passages. The entrances to the two capstan air passages are visible as 1/4 inch diameter a-ring sealed apertures on the front of the valve mounting plate.
A similar aperture leading to the brake is midway between the two capstan aper-tures. The medium size aperture exposed at the bottom is the line carrying com-pressed air to the valve and actuator as-sembly, and the larger hole immediately above is the connection to the vacuum reservoir.
A closeup view of a capstan and its in-ternal commutator is shown in Fig. 9.
The purpose of the commutator is to com-municate vacuum to only that portion of the capstan about which the tape is wrapped. The circumference of the cap-stan is accordingly divided into segments each of which is connected to one passage on the commutator, and the number of
commutator segments connected to the vacuum line at anyone time varies be-tween two and three as the capstan ro-tates.
Since a vacuum system is used for con-trolling tape motion and since com-pressed air is used in the valving arrange-ment, it is a simple matter to provide air lubrication for those capstan surfaces not in engagement with the tape. By this means friction, tape wear, and the genera-tion of Mylar dust are greatly reduced.
There is still another way in which con-trolled selective injection of compressed air into the capstan and brake is of great benefit. Consider the vacuum to be ap-plied to the right-hand (clock-wise ro-tating) capstan so that the tape is moving to the right, in the forward direction. If it is now desired to stop, the vacuum is shifted to the brake member and discon-nected from the right-hand capstan. In order to disengage the tape rapidly and affirmatively from the capstan a short puff of medium pressure air is blown into the right-hand capstan immediately fol-lowing disconnection of the vacuum sup-ply. This puff reaches the capstan and disengages the tape at essentially the same time as the vacuum newly applied to the brake causes the tape to be attracted to the brake. Transfer of the tape from engagement with the moving capstan to engagement with the stationary brake is thereby quickly accomplished without subjecting the tape to a tug-of-war be-tween these two surfaces. Tape abrasion and the maximum stresses in the tape are accordingly greatly reduced.
Fig. 10 shows a typical cycle of operat-ing pressures in a drive capstan and in the brake, as a braking operation and a sub-sequent start in the same direction are performed. Similar considerations apply Lawrance, Wilkins, Pendleton-Magnetic Storage on Wide Tapes
to a braking operation followed by ac-celeration in the opposite direction.
Air/vacuum connections to the two capstans and the brake are controlled by three individual identical assemblies, each of which consists of two electrical actua-tors and a control valve. A schematic of one of these electro-pneumatic valves is shown in Fig. 11. Compressed air at ap-proximately 35 pounds per square inch (gauge) is contained in a chamber with two compressed air exit ports, each of which is normally closed by the armature of an electromagnet. On the control valve sides these two apertures are ad-jacent to the two faces of a control vane, which teeters about a resilent fulcrum which also acts as a pressure sealing 'bar-rier. On the other side of the control vane a large passage equipped with a seal com-municates to the vacuum reservoir, and another passage equipped with a seal at one end bypasses the sealing fulcrum barrier. A little consideration shows that two stable positions of the control vane are possible, with the holding force being supplied by the vacuum reservoir in both instances. Since the two positions of the control vane are stable, the valve is a mechanical non-binary flip-flop, and only a short puff of high-pressure air on the proper surface is required to effect a transition. These puffs of air are initiated by a short burst of current in the ap-propriate electromagnet, whose arma-ture uncovers the aperarma-ture and allows air to escape from the compressed air cham-ber and impinge on the control vane.
Immediately after the vacuum is dis-connected from a passage by driving actuator A' the partially spent com-pressed air is directed into the working air passage and thence to the capstan (or brake) providing the pressure blow off 87
Fig. 11. Schematic of electro-pneumatic valve
Fig. 12. Valve and actuator mechanism
described above. Spent air from actuator B' is conserved by leading it to the reser-voir of low pressure air used for lubrica-tion; the pressure of air in this chamber is regulated by a feather vane which con-trols the exhaust to atmosphere.
The principal components of the valve and actuator mechanism are shown in Fig. 12. The control vane is made of surface-hardened aluminum and fits into the cavity with a clearance of approxi-mately 2 mils. Both in the mounting of the control vane and in the mounting of the relay armatures use is made of re-silient pivots held in compression. Valve seats are of silicone rubber to avoid stick-ing when fast operation is required after a long interval of tight closure. The en-tire valve and actuator assembly can be removed from the capstan by unscrewing four easily accessible machine screws, and the chamber containing the actuators is similarly easily disassembled. Passages
from the vacuum reservoir and the com-pressed air line are cast into the heavy aluminum housing, which serves both to contain the medium pressure compressed air and to shield the nearby magnetic heads and tape from the magnetic fields generated by the actuators. Since the actuators are driven in pairs the actuator connections are arranged ~o that the ex-ternal fields in the vicinity of the head substantially cancel each other and cause no difficulty.
The power handling capacity of the ac-tuator coils in this application is greater
The power handling capacity of the ac-tuator coils in this application is greater