GB2140961A - Data store for a microcomputer system - Google Patents

Abstract

A non-volatile, on-line, mass data store is provided that is based on the use of magnetic tape loops 3 housed in magazines 1 readily inserted and removed from an associated tape transport. In order to facilitate the construction of the magazine 1, the latter is provided with only one moving part, namely a backing roller 11 against which the tape is pressed by a rotary drive member 19 of the tape transport during operation of the latter. The rotary drive member 19 is mounted on a solenoid-actuated carriage 21 which is moved between inoperative and operative positions by a solenoid 23. Data is written to, and read from, the tape loop 3 in byte-wide form by a multitrack read/write head 17 that also lays down a taming track on the tape 3. <IMAGE>

Description

SPECIFICATION Data store for a microcomputer system The present invention relates to a non-volatile, on-line, mass data store for a microcomputer system In their basic form, most home microcomputers sold nowadays include anything from 2 to 4 kilobytes of on-board RAM storage in integrated circuit form, and a casette interface enabling data to be transferred for permanent storage from the microcomputer to an ordinary audio cassette tape recorder. (As used herein, the word "data" is to be taken to include programmes as well as raw data upon which a programme may operate).The necessity to provide for data storage on a tape cassette results from the fact that the on-board RAM storage is volatile in nature, that is, the stored data disappears upon power down of the microcomputer system; in contrast data stored in a tape cassette can be re-loaded back into the microcomputer system whenever required since this storage medium is non-volatile.

The storage of data on ordinary audio cassette tapes is effected by using two different audiofrequency tones to represent binary "zero" and "one", each data "bit" being recorded in turn on the tape to produce a serial tone pattern. While this arrangement has the great advantage of being very inexpensive, there are a number of major drawbacks. Thus, for example, the data transfer rate between the microcomputer system and cassette tape is very low (for example, as little as 30 baud) which means that the loading of even a moderatelength programme for a cassette tape into the microcomputer system takes an inordinate length of time. Furthermore, the chances of data corruption occurring are very high.

Due primarily to the very low data transfer rates obtainable using a standard audio cassette as a non-volatile store, the use of such cassettes is almost exclusively confined to the storing of data it is wished to retain on power down of the microcomputer system. Thus, although in theory it is possible, at least with audio cassette recorders provided with remote start-stop controls, to programme the microcomputer system to read and write information from an audio cassette tape during running of a programme (that is, use the cassette tape as an "on-line" data store) this is, in practice, never seriously attempted due to the inordinate delay that would be involved in accessing data stored on the tape.

It can thus be seen that the use of audio cassette recorders as non-volatile data stores for microcomputers systems is solely a result of economics and generally any serious user of a microcomputer system would seek to replace such a storage system at the earliest possible moment, preferably with a data store enabling on-line access to data by the microcomputer system.

Until recently, the least expensive data storage system offering significant improvements over audio cassette tapes as well as on-line storage facilities, was the "mini-floppy" disc system. In this system, a 51/4 inch (133mm) magnetic disc is rotated at high speed while a read/write head is scanned radially. Due to the complex electromechanical arrangement required, in particular in moving the head, and due to the complex disc operating system required to interface the floppy disc storage with a microcomputer system, the cost of such a storage system is at least an order of magnitude greater than that of an audio cassette arrangement. However, the mini-floppy disc system has brought the possibility of mass data storage (upward of 100 k bytes) and rapid on-line data access within the sight, if not the reach, of at least a portion of the domestic microcomputer market.

With a view to narrowing the price differential between audio cassette data storage and the mass on-line non-volatile storage offered by mini-floppy discs, a number of new storage systems have recently appeared. Thus, several manufacturers are presently entering the market with "micro-floppies" in which a 3 inch or 31/2 inch (76 or 89mm) floppy disc is utilised; even so, the cost of such systems is still seven or eight times that of the lowly audio cassette. Another recent storage system is the so-called "stringy floppy" in which a very narrow tape (in the region of 0.062 inches or 1.6mm wide) is driven at high speed and data is serially recorded thereon without error correction.This latter system has not been well received partly because it still suffers from the disadvantages referred to above of an audio-cassette system (low data transfer rate, uncorrected data drop out) though in less exaggerated forms; in addition, the life of the narrow tape is short and the tape is liable to permanently deform and twist in use.

Accordingly, it is an object of the present invention to provide a non-volatile, on-line, mass data store for a micro-computer system which overcomes the disadvantages of an audio cassette system store and yet is less expensive than mini and micro floppy disc storage systems.

According to one aspect of the present invention, there is provided a non-volatile, on-line, mass data store for a microcomputer system, said store comprising a removable tape magazine housing a magnetic tape loop, a tape transport arranged to receive the tape magazine and including tape drive means for circulating the tape loop past a read/write station, a multi-track read/write head carried by the tape transport at the read/write station, and an electronic interface and control unit connected to the read/write head and intended to interface the store with a microcomputer system, the interface and control unit being arranged both to:: (a) receive data from the microcomputer system and write the data in parallel form on the tape loop as the latter is circulated past the head, and (b) read recorded data words and timing information from the circulating tape loop and pass the read-off data to the microcomputer system.

Generally it is convenient to arrange for the reading and writing of data in byte-wide format, that is, as 8 bits in parallel for which purpose the head is designed to read/write nine parallel tracks on the tape loop (that is, eight data tracks and one timing or clock track).

Preferably the tape drive means comprise a rotary drive member mounted on a low-inertia mount, selectively-operable displacement means for effecting rapid displacementofthe mountto movethe rotary drive member into and out of an operative position in which it sandwiches a section of the tape loop between itself and a backing roller rotatably mounted in the magazine, and a continuouslyrotating motor arranged to drivingly rotate the rotary drive member at least when the latter is in its operative position. By virtue of this arrangement, the tape loop can be very rapidly started and stopped as required simply by moving the rotary drive member into and out of its operative position; as a result, it is unnecessary to circulate continuously the tape which would lead to excessive wear of the latter.

In a first embodiment of the aforesaid preferred form of tape drive means, the rotary drive member is constituted by a friction roller which when in its operative position, is arranged not only to sandwich a section of tape between the backing roller and itself, but also to engage a fixed-position capstan which is continuously rotated by the motor. In a second embodiment, the rotary drive member is constituted by a capstan formed by, or directly mounted on, the motor spindle and continuously driven thereby, the motor being carried on the same mount as the rotary drive member.

The displacement means can be arranged to effect either linear movement of the mount carrying the rotary drive member (this being preferred for the first embodiment of the tape drive means discussed above), or pivotal movement of the mount (this being preferred for the second embodiment where the motor can be arranged to pivot either about an axis parallel to that of the backing roller or an axis transverse to that of the backing roller and generally parallel to the direction of movement of the tape therepast).

The tape magazine itself preferably contains no moving parts other than the backing roller and, of course, the tape loop itself; as a result, the cost of the magazine can be kept low.

As is normal, the bulk of the tape loop is stored in random folds in a main reservoir of the magazine with only a short length of tape, extending between a reservoir outlet and inlet, residing externally of the reservoir. As the tape is circulated, it passes out of the reservoir, past the read-write station, between the backing and friction rollers, and back into the reservoir. Tape strippers, integral with the main body of the magazine, ensure the smooth exit and entry of the tape from the main reservoir.

The interface and control unit controls the laying down and recovery of data from the tape loop. Prior to writing data on a new tape loop, the interface and control unit carries out a formatting procedure which involves identifying the tape loop join (by laying down the clock track and searching for an irregularits when the latter is read back), dividing the tape up into blocks of storage space each identified at their beginning by a file number, and using the first several storage blocks as a directory in which each of the remaining storage blocks is identified by its number with space being left for the future addition of a file name identifying the file contents when inserted. Various other operations may also take place during the formatting procedure.

Upon the associated microcomputer system wishing to dump information, identified by a file name, in the store the interface and control unit will refer to the directory to find a vacant block (identified by number) and then write the information in the appropriate blocks, the directory being updated by insertion of the file name underthe block number concerned. To read a named file simply requires the interface and control unit to search the directory to identify the appropriate blocks whereafter this file can be accessed and read off.

The interface and control unit is advantageously arranged to perform error checking and correction by any suitable redundancy coding technique.

The interface and control unit can most readily be implemented by a programmed microprocessor enabling costs to be kept low.

Typically, the tape used will be standard 0.25 inch (6.4mm) tape and the length of tape in a loop will generally range from 6 to 24ft. (1.8 to 7.3m) depending on the required storage capacity (there is, of course, a trade-off between storage capacity and maximum access time). The drive speeds of 10-30 inches per second (0.25 to 0.76 m/s) are obtainable without difficulty and speeds up to 500 inches per second (12.7 m/s) are envisaged. The read/write rate is, for example, 24 kbytes/second; thus, for a 12 ft.

(3.7 m) tape at 24 inches a second (0.6 m/s), each tape magazine would have a storage capacity of 144 kbytes with a circulation time of six seconds. The data store of the invention thus provides for the mass non-volatile storage of data in a manner which makes on-line access viable; due to the simplicity of the electro-mechanical arrangement involved and with the use of a microprocessor for the interface and control unit, the cost of the store can be kept low.

Two on-line data stores each embodying the invention, will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a plan view of a removable tape magazine of a first one of the stores with the top of the magazine removed to reveal internal details thereof; Figure 2 illustrates the position of the front part of the Figure 1 magazine relative to a tape drive mechanism and read/write head when the magazine is inserted into a tape transport unit of the first data store, the tape drive mechanism being shown in an inoperative position; Figure 3 is similar to Figure 2 but shows the tape drive mechanism of the first store in an operative position; Figure 4 is a perspective view, part cut away, of the read/write head shown in Figure 2; Figure 5 is a block diagram of an interface and control unit of the first store; ; Figure 6 is a plan view showing the internal layout of a tape transport unit of the second data store, certain hidden components of the unit being shown in dashed outline; Figure 7 is a part cut-away plan view of a removable tape magazine of the second store; Figure 8 is an enlarged section on line Y-Y of Figure 7; and Figure 9 is a view similar to Figure 6 but showing the Figure 7 magazine inserted in the tape transport unit.

Each of the two low-cost data stores now to be described basically takes the form of a tape transport arranged to receive, one at a time, interchangeable tape magazines each of which houses an endless tape loop. The tape transport is provided with a selectively operable tape drive mechanism to permit the tape loop of a magazine installed on the tape transport to be advanced past a read/write station whereat there is located a multi-track read/write head, the latter being fixedly mounted on the tape transport. The head is connected to an electronic interface and control unit which controls the transfer of data between the magazine tape loop and a microprocessor system.

The first data store will now be described with reference to Figures 1 and 5.

Figure 1 is a plan view of a tape magazine 1 of the first data store, the magazine being shown with its top removed. The major portion of the space enclosed by the outer walls 2 of the magazine constitutes a tape reservoir in which is housed most of an endless tape loop 3, the tape lying in random folds within the reservoir.

The front of the magazine (that is, the lower edge as viewed in Figure 1) is provided with an elongate aperture 4, the tape reservoir being bounded opposite this aperture 4 by an internal wall 5. A length of the tape loop 3 lies outside the tape reservoir between the internal wall 5 and the front aperture 4.

As will become clear hereinafter, in operation of the data store, the tape loop is circulated such that tape passes from left to right in front of the wall 5 as viewed in Figure 1.

Where tape passes out of the tape reservoir, a tape stripper and tensioner 6 is provided to ensure a smooth exit of tape from the reservoir. This device 6 is constituted by two cooperating elements 7 and 8 which together define a curved channel. The element 7 is integral with the internal wall 5. The element 8 is provided with a curved resilient arm 9 integral with the main body of the element 8, the free end of the arm being provided with a pressure pad 10 which serves to press the tape against the element 7 and thereby ensure tensioning of the tape as it is withdrawn from the reservoir.

At the downstream end of the wall 5, considered relative to the direction of movement of tape in front of this wall during use of the magazine, there is provided a backing roller 11 which is rotatably mounted in the magazine 1. The end 12 of the wall 5 disposed adjacent the roller 11 serves as a tape stripper ensuring that tape is not entrained around the roller 11 in use of the magazine.

Adjacent the exit of the tape stripper and tensioner 6 there are provided two spaced tape guide pins 13.

A resilient pressure pad 14 is mounted on the wall 5 intermediate pins 13; when the magazine 1 is inserted into the tape transport, the read/write head carried by the latter enters the magazine between the pins 13 and lightly presses the tape against the pressure pad 14.

From the foregoing description of the magazine 1, it will be appreciated that its structure is very simple with the only moving parts being the rotatably mounted roller 11 and the tape 3 itself. The main body of the magazine 1, that is, the sidewalls 2 and top and bottom walls (not referenced) are preferably made of a transparent plastics material while the element 8 is made of a suitable resilient plastics material. Due to its simple structure the cost of the magazine 1 can be kept low.

The magazine 1 is designed to be releasably received by a tape transport of the first store. This tape transport includes a deck 15 (Figure 2) with locating bosses 16 and releasable resilient catch means (not shown but of any suitable type) enabling the magazine 1 to be releasably located in a predetermined position on the deck 15.

A single air gap, nine-track, magnetic read/write head 17 is mounted in a fixed position on the deck 15. Upon insertion of the magazine 1 into the tape transport, the front part of the head 17 extends through the front aperture 4 of the magazine 1 to project between the guide pins 13 and lightly press the section of tape extending between these pins against the pressure pad 14. In this manner, close contact between the tape 3 and the operative part of the head 17 is ensured.

The deck 15 also supports a tape drive capstan 18 which is continuously rotated by a motor (not shown), but positioned beneath the deck 15) in an anti-clockwise direction as viewed in Figure 2. The capstan 18 does not directly contact and drive the tape 3 but does so via an intermediate friction roller 19 which can be selectively moved between an operative position in which it serves to transmit drive from the capstan 18 to the tape 3, and an inoperative position where rotation of the capstan 18 does not result in advancement of the tape 3.

The intermediate roller 19 is mounted on a resilient arm 20 of a carriage 21. This carriage 21 is fixed to one end of a plunger 22 of a high speed solenoid 23 fixedly mounted on the deck 15. The plunger 22 is spring biassed into the position shown in Figure 2 in which the roller 19 is in its inoperative position.

Upon energisation of the solenoid 23, the plunger 22, and consequently the carriage 21 and roller 19, is advanced at high speed in the direction of arrow A to move the roller 19 into its operative position (see Figure 3).

In its operative position, the roller 19 is operatively interposed between the capstan 18 and, with the interposition of the tape 3, the backing roller 11 of the magazine 1. As a result, the continuously rotating capstan 18 causes the intermediate roller 19 and the backing roller 11 to rotate in clockwise and anti-clockwise senses respectively as viewed in Figure 3 to thereby draw the tape 3 past the head 17.

In its movement past the head 17, the tape 3 is tensioned by means of the resilient arm 9 and pressure pad 10. After passage between the rollers 11 and 19, the tape 3 is ejected back into the main reservoir of the tape magazine, the tape being prevented from entrainment around the intermediate roller 19 by means of a tape stripper 24 formed integrally with the carriage 21.

In order to facilitate the high speed operation of the solenoid 23, the mass of the carriage 21 and the roller 19 are kept as low as possible. Furthermore, to compensate for any inaccuracies in positioning of the magazine 1 within the tape transport, a degree of lateral play (indicated by arrow L) of the roller 19 is built into the roller advancing mechanism constituted by the solenoid 23 and carriage 21; this play L can be readily provided for by arranging for the solenoid plunger 22 to be a loose fit within the solenoid 23.

The resilient mounting of the roller 19 on the arm 20 provides a measure of cushioning upon the roller 19 being advanced into engagement with the capstan 18 and backing roller 11.

The relative dispositions of capstan 18, backing roller 11, and the intermediate roller 19 when in its operative position, is arranged to be such that an angle of 1200 is included between a line joining the axes of the capstan 18 and roller 19 and a line joining the axes of the rollers 19 and 11. Furthermore, the direction of advance A of the roller 19 is arranged to lie at approximately 500 to the tape advance direction immediately upstream of the rollers 11 and 19. As a result, when in its operative position the roller 19 is, to a degree, drawn into the gap between the capstan 18 and roller 11 by virtue of its interaction with these latter members; this effect enables the holding force required to be exerted by the solenoid 23 to maintain the roller 19 in its operative position to be mini mised.

Upon de-energisation of the solenoid 23 the roller 19 is returned to its inoperative position under the effect of the spring bias provided for the solenoid plunger 22.

The afore-described tape drive mechanism com prising the continuously rotated capstan 18 and the selectively engageable intermediate roller 19 mov able into and out of an operative position by a quick-acting advancement means (the solenoid 23), enables the tape loop 3 to be selectively circulated only when data is to be transferred on and off the tape, the time needed to move the roller 19 between its inoperative and operative positions being minim al. The advantage of this arrangement is, of course, that since the tape loop 3 is not continuously circulating but is only moved when required, the wear on the tape is considerably reduced, the penalty for this (that is, the start up delay in circulating the tape) being negligible in relation to the overall circulation time.

The speed of rotation of the capstan 18 and the diameters of the capstan 18 and rollers 19 are such that the tape circulation speed is preferably in the range of 10-30 inches per second (0.25 to 0.76 m/s); higher speeds are, of course, possible and in certain applications speeds of up to 500 inches per second (12.7 m/s) are envisaged. The actual length of tape in the tape loop 3 will, of course, depend on the amount of data to be held by the tape, but typically the tape length will range from 6 to 24 feet (1.8 to 7.3 m) with the size of magazine shown in Figure 1.

Longer lengths of tape will generally require larger sizes of magazine to that shown.

The magnetic head 17 preferably has a construction similar to that described in U.K. Specification No. 1,290,098 to which reference is directed. However, in order to enable the tracks to be recorded close together while ensuring the had retains its strength and stability, rather than using the core layout illustrated in Figure 2 of the aforesaid specification, a staggered layout is preferred, this layout being shown in Figure 4 of the present specification (the core being referenced 40 and the head casing 41).

The control and interface unit of the first data store is shown in Figure 5. This control and interface unit is arranged to exchange data with a microcomputer system in 8-bit parallel form via a data bus 30 which communicates with a processing unit 31 ofthe interface and control unit. The processing unit 31 also communicates with the microcomputer system via handshake lines 32.

The processing unit 31,which for example is built around a microprocessor, ensures the smooth flow of data between the microcomputer system and the data store and to this end the unit 31 is provided with a limited amount of RAM buffer store.

The processing unit 31 is arranged to control the writing of data onto the tape loop 3 via the head 17 in 8-bit parallel form and to this end the unit 31 is connected to the head 17 via data buses 32, 33 and write amplifiers 34. Similarly, for reading data off the tape 3, the processing unit 31 is connected to the head 17 via the buses 33 and 32 and read amplifiers 35. At the same time as data is written in 8-bit parallel form on the tape, a clock track is also laid down, this clock track being derived from the unit 31.

Thus, nine tracks are simultaneously written on the tape 3 and accordingly nine write amplifiers are provided. Similarly, nine read amplifiers 35 are also provided since the clock track is read from the tape simultaneously with the reading of the eight data tracks. The nine write amplifiers 34 are selectively enabled by the processing unit 31 via a write/read control line 36.

The processing unit 31 also controls energisation of the solenoid 23 via a line 37, the solenoid being energised whenever a request is received from the microcomputer system to read or write information from the store.

Data is arranged to be stored on the tape 3 in blocks of 256 bytes each and to this end, the processing unit 31 is arranged to format blank tape loops by the following procedure. With the solenoid 3 energised and the tape loop 3 circulating, the unit 31 lays down a clock track on the blank tape.

Thereafter, this clock track is read back and the clock signal monitored by the unit 31 until an interruption is observed; this interruption corresponds to the join in the tape loop and is, for convenience, taken as a reference point on the loop. Once this reference has been identified, the processing unit 31 marks off block spaces along the tape by writing a block number onto the tape every 256 bytes (as deter mined by the clock signal which, of course, controls the rate at which data is written onto the tape; - a typical rate being 24 kbytes per second). The first few blocks are used as a directory in which block numbers are stored together with the file name associated with data held in that block as and when such data is entered.

After a tape has been formated, whenever a request is received from the microcomputer system to store information under a particularfile name, the processing unit 31, after initiating tape circulation, will first read data from the directory in order to identify a vacant block (that is, a block number in the directory for which no corresponding file name is recorded). Thereafter, the processing unit 31 will wait for the appropriate block number to come up as the tape is circulated and then write into that block the data to be stored. At the same time as this data is written in in 8-bit parallel form, the clock track is also refreshed.

Due to the provision of a buffer store in the processing unit 31,the operation of writing data onto the tape 3 does not have to be precisely synchronised with the receipt of this data from the microcomputer system.

When the microcomputer system requests access to data under a particularfile name, the processing unit 31 effects the same initial operations as when writing information, that is, energisation of the solenoid 23 and reading of the directory. This time, however, the directory is read to find the block number or numbers corresponding to a particular file name. Once the appropriate block number(s) is identified, the processing unit 31 will monitor the tape until the required block number(s) is identified and then read off the data requested into the buffer store of the processing unit 31 from where it is passed on to the microcomputer system.

The processing unit 31 is arranged to effect error checking and correction by a redundancy coding technique, such techniques being well understood to persons skilled in the art.

The tape magazine 1 can, of course, be readily interchanged with other magazines of the same form to enable the non-volatile storage of very large amounts of data.

The second form of data store shown in Figures 6 to 8 is similar to the first store in many respects and only the main differences between the first and second stores will be described below. These differences all relate to the mechanical features of the tape magazine 1 and tape transport, the interface and control unit being the same for both stores.

The first important difference to note between the first and second stores is that in the tape transport of the second store (see Figure 6), the solenoid 23 is arranged to activate the tape transport by bodily moving the drive motor 40 such as to engage the motor capstan 18 with the tape 3, sandwiching the latter between the backing roller 11 of the tape magazine 1 and the capstan, and thereby causing displacement of the tape 3. The motor 40 is mounted on a carriage 41 that is pivotally supported for rotation about an axis 42 (extending perpendicular to the plane of Figure 6). The motor 40 is disposed beneath the deck 15 of the tape transport, the motor capstan 18 projecting through an aperture 43 to the upper side of the deck 15.The diameter of the aperture 43 is such as to allow several millimeters of movement of the capstan 18 in its rotation with the carriage 41 about the axis 42. A bias spring (not shown) exerts a clockwise biasing force on the carriage 41.

The plunger 22 of the solenoid 23 is connected to the carriage 41 via a damping spring 44 and an adjuster rod 45 which can be screwed into the plunger 22 to a greater or less extent as may be required. Energisation of the solenoid 23 results in the plunger 22 being drawn into the solenoid causing anticlockwise movement of the carriage 41 against its bias spring.

The deck 15 also mounts the read/write head 17 and a locating post 46. Sides 47 of the deck 15 are turned upwards to define a channel for receiving a tape magazine 1 inserted through an aperture 48 in the front of a casing 49 of the tape transport. The shaped ends of spring fingers 50 project through the deck sides 47 to engage and retain a magazine 1 inserted in the tape transport.

The tape magazine 1 shown in Figure 7 has three apertures 51,52,53 in its front wall rather than a single elongate aperture 4 as is provided in the Figure 1 magazine. The aperture 51 permits the engagement of the motor capstan 18 with the backing roller 11 of magazine 1 with the interposition of the tape loop 3. The aperture 52 forms a seat for the locating post 46 while the aperture 53 provides access for the read/write head 17.

Semi-circular recesses 58 are formed in opposite side walls of the magazine, for engagement by the shaped ends of the spring arms 50 of the tape transport.

Opposite the aperture 53 and behind the tape loop 3, is located a felt pressure pad 14, this pad being mounted on a resilient plate 54 fixed at its ends in corresponding slots formed in internal walls of the magazine.

The internal walls of the Figure 7 magazine provide a first tape stripper 55 to ensure smooth exit of tape from the main tape reservoir of the magazine, and second and third tape strippers 56,57 for preventing entrainment of the tape around the capstan 18 and the roller 11 respectively.

An internal wall 60 extends into the central zone of the magazine from the first tape stripper 55. Adjacent its feed end, the wall is provided with an upstanding pillar 61 which engages with the top cover 62 of the magazine. The pillar 61 ensures that the cover 62 remains spaced from the base 63 of the magazine by a distance slightly greater than the width of the tape 3 whereby to facilitate the free movement of the latter within the magazine. The pillar 61 is particularly useful in preventing crushing of the magazine and thus of the tape. Generally, the magazine will be fabricated from top and bottom moulded halves joined together in any suitable manner adjacent the corners of the magazine; in this case, the pillar 61 is preferably slightly higher than the side walls of the magazine so as to positively engage and slightly push out the central zone of the magazine. The pillar 61 could be formed independently of the wall 60 but the integral formation of the pillar and wall does ensure that there is no possibility of tape loops becoming caught up around the pillar.

Downstream of the second tape stripper 56 (considered in the direction of tape circulation), tape re-enters the tape reservoir through a continuouslydivergent entry passage defined between the backing roller 11 and an internal wall 64 of the magazine that curves away from the second tape stripper 56 around to a side wall of the reservoir; typically, the radii of curvature of the roller and wall 64 are respectively 15.5 and 40.5mm. The wall 64 is of stepped form as can best be seen in Figure 8; indeed, all the walls defining the tape reservoir may be of this stepped form. The purpose of the stepped wall form is to reduce the contact area between the tape and the wall whereby to minimise electrostatic adhesion forces therebetween.Such forces may occur at the tape splice due to the build up of electric charge in the adhesive used to join the tape into a closed loop (it should be noted that build up of charge on the tape itself is generally prevented by using a conductive plastics material or coating for the magazine). By reducing the contact area between the tape splice and magazine walls, the problem of adhesion is avoided. Itwill, of course, be appreciated that other wall forms in addition to the illustrated stepped wall form, could be used to reduce the tape/wall contact area; for example, a ribbed wall form would serve the same purpose.

Upon insertion of the Figure 7 magazine into the Figure 6 tape transport (see Figure 9) the magazine is located in its required operating position by engagement of the seat defined by the aperture 52 about the locating post 46, the deck side walls 47 providing sideways constraint of the magazine. In its required operating position, the shaped ends of the spring arms 50 engage in the recesses 58 in such a manner as to provide a residual force holding the magazine against the post 46.

With the magazine located in its operative position, the read/write head 17 projects into the magazine to contact the tape 3, pressing the latter lightly against the pressure pad 14.

When the solenoid 23 is de-energised the capstan 18 is spaced by a small amount (for example, 1 mm) from the backing roller 11 so that no drive is transmitted to the tape 3. However, upon energisation of the solenoid 23, the capstan 18 presses the tape against the roller 11 to advance the tape. The motor 40 is preferably continuously energised whenever a magazine is present in the tape trans port, this condition being readily sensed by any suitable means, such as a microswitch, not shown; it is also possible simply to arrangeforthe motorto be energised whenever data is to be written to or read from the tape.

From the foregoing, it will be appreciated that both of the described data stores are mechanically simple and require few components. Furthermore, by building the interface and control unit of each store around a microprocessor, it is possible to implement the complex interface and control requirements at low cost. As a result, the overall cost of the data store can be kept very low thereby providing for on-line non-volatile data storage at not much more than the cost of an audio cassette recorder data storage arrangement.

While the described data stores embodies a number of specific innovations of substantial inventive merit in their own right, it is considered that one of the most important inventive concepts herein presented is the realisation that a low cost on-line non-volatile mass data store can be provided by the writing of data in parallel form on a tape housed in a tape magazine. Of course, the writing of data in parallel form onto magnetic tapes is well known but no system exists which provides the low-cost on-line data storage facilities presented by the present data store. Although the present data stores have been described as for use with a microcomputer system, and indeed, although this will be its primary application, it will be understood that the store could also be used for data provided from another source.

Claims (14)

1. A non-volatile, on-line, mass data store for a microcomputer system, said store comprising a removable tape magazine housing a magnetic tape loop, a tape transport arranged to receive the tape magazine and including tape drive means for circulating the tape loop past a read/write station, a multi-track read/write head carried by the tape transport at the read/write station, and an electronic interface and control unit connected to the read/ write head and intended to interface the store with a microcomputer system, the interface and control unit being arranged both to: (a) receive data from the microcomputer system and write the data in parallel form on the tape loop as the latter is circulated past the head, and (b) read recorded data words and timing information from the circulating tape loop and pass the read-off data to the microcomputer system.

2. A data store according to claim 1, wherein the head is designed to read/write nine parallel tracks on the tape loop, eight tracks being data tracks and one track a timing or clock track.

3. A data store according to claim 1 or claim 2, wherein the tape drive means comprise a rotary drive member mounted on a low-inertia mount, selectively-operable displacement means for effecting rapid displacement of the mountto move the rotary drive member into and out of an operative position in which it sandwiches a section of the tape loop between itself and a backing roller rotatably mounted in the magazine, and a motor arranged to drivingly rotate the rotary drive member at least when the latter is in its operative position.

4. A data store according to claim 3, wherein the motor is continuously energised whenever a magazine is present in the tape transport.

5. A data store according to claim 3 or claim 4, wherein said rotary drive member is constituted by a friction roller which when in its operative position, is arranged both to sandwich a section of tape between the backing roller and itself, and to engage a fixed-position capstan which is directly rotated by the motor.

6. A data store according to claim 5, wherein the said displacement means is arranged to effect linear movement of the mount carrying the rotary drive member.

7. A data store according to claim 3 or claim 4, wherein said rotary drive member is constituted by a capstan formed by, or directly mounted on, the motor spindle, the motor being carried on the same mount as the rotary drive member.

8. A data store according to claim 7, wherein the said displacement means is arranged to effect pivotal movement of the mount carrying the motor and rotary drive member.

9. A data store according to any one of the preceding claims, wherein the length of tape in a loop is in the range of 1.8 to 7.3m and the tape advance speed is in the range of 0.25 to 0.76 m/s.

10. A data store according to claim 1, wherein said magazine includes a rotatably-mounted backing roller, a tape reservoir having a tape exit and tape entry between which a length of said tape extends externally of the reservoir past the backing roller; the magazine having an apertured front wall through which both a drive roller and said read/write head can project respectively to drivingly engage the tape on the backing roller and to read/write data to/from the tape as it passes between the tape reservoir exit and entry, and the magazine being provided in the region of the backing roller with a tape stripper arranged to prevent entrainment of the tape around said drive roller when the latter is in driving engagement with said tape, and with a guide wall curving from the tape stripper around towards a side wall of the tape reservoir, the guide wall and backing roller together defining a continuously divergent entry passage for tape entering the reservoir.

11. A magazine according to claim 9, wherein when considered in cross section part of the surface of the guide wall is recessed to reduce the contact area between the guide wall and tape ejected from between the backing and drive rollers into said entry passage.

12. A magazine according to claim 9 or 10, wherein a support pillar is provided centrally in the tape reservoir to prevent deformation of the magazine in this region.

13. A magazine according to claim 11, wherein the magazine is provided with an internal wall extending from the boundary of the tape reservoir to said pillar.

14. A non-volatile, on-line, mass data store substantially as hereinbefore described with reference to Figures 1 to 3 or Figures 6 to 9 of the accompanying drawings.