A streaming tape drive, the TK50 subsystem, provides fast backup and data transfer for small computers like the Micro VAX II system. A single reel cartridge, using half-inch magnetic tape, stores 100 megabytes of data. A unique tape transport system automatically threads the tape when the cartridge is inserted. The drive reads and writes data in a serpentine manner, going the entire tape length first on one track, then another. For high data integrity, the TK50 subsystem employs a sophisti cated error-recovery algorithm, reading data after writing it and rewrit ing any corrected data farther down on the tape. The Q-bus controller, the TQK50, contains complex firmware conforming to Digital's Storage Architecture and controlling data transfers between the CPU and the
tape.
As the performance of computer syste m s expands whi le their size shrinks, many factors demand special attention. One major factor is storage systems. Over the past few years , disk drives have made dramatic advances, providing storage capacity of hundreds of megabytes i n very small and relatively inexpensive packages. Since the predomi nant technology for today's disk drive is based on the fixed-media concept, some means of providing system backup and data transfer capabi l ities is required . Magnetic tape systems are still the most viable way of providing these capabilities.
Ease-of-use considerations require that a backup/transfer device be matched in capacity to the supported disk systems. It should also be extremely reliable , fast, and very cost effective . This paper describes a peripheral subsystem , t h e TK5 0 m a g n e t i c cartridge tape drive (Figure 1 ) , that meets a l l these requirements. Design Goals of the TK50 Subsystem The TK5 0 cartridge tape subsystem was con ceived to meet the needs of the MicroVAX I I and similar computer systems. A study o f tape products then available indicated that existing quarter-inch cartridge drives did not provide
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either the performance or the capacity required to back up the large capacity disk drives sup ported by these systems. Existing drives also l a c ked t h e re l ia b i l i ty and data i n te g r i ty required to complement the designs of our new microsystems. Therefore , Digital designed the TK5 0 cartridge tape subsystem to meet the needs of the MicroVAX II system and other sma l l to mid-range computers .
A wide variety of factors defined the design goals of the TK5 0 subsystem . It had to fit into a standard 5 l;.:i -inch form factor and provide high capaci ty with high data i ntegrity. The desire for mechanical simplici ty , rel iability, and l ow cost, while maintain i ng good performance, dictated a streaming tape design . The TK50 subsystem had to be compatible with the Q-bus, and the TK50 controller had to support the Tape Mass Storage Control Protocol ( TMSCP) of t he Digital Storage Architecture .
Our i nvestigations led to the concept of an automatic-threading, single-reel cartridge that utilized the established medium of i nstrumen tation tape. This tape supports high bit densi ties and fast tape speeds, allowing great latitude in specifying the performance and capacity of the TK50 subsystem . We also decided to use
Digital Tecbnicaljournal
Figure 1 The TK50 Tape Drive
half-inch tape , rather than quarter-inch, to max imize capacity.
The requirement of the MicroVAX II system, as well as our desire to minimize risks i n a first generat ion product, d ictated that the tape capacity should be 1 00 megabytes (MB) .
System Design
The TK50 cartridge tape subsystem was devel oped with three major components:
• A tape cartridge , cal led the CompacTape Car tridge, that houses 600 feet of half-inch tape and supports the auto-threading feature of the transport mechanism
• A unique streaming tape transport featuring
auto-threadi n g and a microprocessor-con trolled servo-system
• An i ntelligent, microprocessor-based Q-bus
controller that supports TMSCP
Compac Tape Cartridge
The CompacTape Cartridge is unique in many ways. First, it provides a large amount of data
Digital Tecbnical journal No. 2 March 1 986
recording surface for its volume. The cartridge has approximately two hundred and fifty times the recordi ng surface area of a single-sided 5 � -inch floppy disk. Moreover, compared to the only commercial tape product then availa ble to fit the 5 � -inch form factor, the Com pacTape Cartridge is four times as efficient in utilizing tape volume i n relation to cartridge volume . The cartridge is designed to maximize the volume of tape in the standard form factor of the 5 � -inch drive . The cartridge , shown i n Figure 2 , contains a s i ngle reel with the tape occupying forty percent of the cartridge's vol ume . The tape is Y2 inch wide, .00 1 inch thick, and 600 feet long.
Second, the CompacTape Cartridge is a com p letely e nclosed device that never exposes the media to the environment, thus greatly enhanc i ng the data reliability of the entire subsystem .
Third , the CompacTape Cartridge a l lows automatic tape threading once it is inserted i nto the TK50 tape drive . This auto-threading func tion is a key feature of the mechanical design of the tape transport.
The TK50 Cartridge Tape Drive
D R I V E H U B
Figure 2 The TK50 Tape Cartridge
The auto-threading works i n the fol lowing way. When a cartridge is i nserted into the drive, the tape must be threaded around the tape guides, over the readjwri te head, around the take-up reel , and then fastened to the reel hub. Two leaders are used ro accomplish the thread
ing, as shown in Figure 3 . One, made of . 0 0 7 - i nch Mylar, is attached r o the BOT e n d o f the tape in the cartridge ; the second is attached to the hub of the take-up reel i n the drive . This second leader has an arrow-shaped tip that reaches from the reel , through the tape path , and i nto the area that w i l l be occupied by the tip of the first leader when the cartridge is i nserted . During the insertion process , the arrow-shaped tip is moved by a cam into the opening of the cartridge leader. Tension is then a p p l ied to lock the leaders toge t her. This "buckle " is now ready to be pulled t hrough the tape path and wound onto the take-up ree l .
OPENING
0
STEP 1 CARTRIDGE LEADER ARROW SHAPED TIP R E T AINING NOTCH TAPE DRIVE LEADER BUCKLE RELATIVE MOTION STEP 2 STEP 3This buckl i ng process is accompl ished by two l i nks in the drive , in conjunction with a constant tension applied by the motor to the take-up leader. One link uses a cam to move the two leader tips into each other. The other l i nk holds the take-up leader in the correct pos ition and retreats at the right instant, allowing the motor to cinch the buckle . The entire process
Figure 3 Engagement of Drive Leader to Cartridge Leader
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happens du ring the last half-inch of insertion as the cartridge enters the drive . (See Figure 4 . ) This l inking takes place withou t any tape being spool ed out of the cartridge.
When the tape is rewound into the cartridge for removal from the drive, the two ears on the cartridge leader come to rest in a pocket in the cartridge shell . When the cartridge is removed from the drive , two opposing locks hold the reel in this position. The toothed locks engage with rhe teeth on the outer diameter of the reel flange . Thus locked, the tape stays tightly wound and the leader tip is kept in the correct position for a subsequent buckling process.