PDP11 PeripheralsHbk 1972 - Trailing-Edge

PART I
Chapter 3
Magnetic Tape Options
Two types of magnetic tape options are available to PDP-ll users:
• Small, duaI3.9-inch reel DECtape systems
• Large single 10 1 !2-inch reel Magtape systems (industry compatible)
Both types of devices offer high performance at low cost. DECtapes are ideal for
applications which do not require writing, reading or storage of large volumes of
data. Magtapes, however, are suitable for handling large volumes of data.
3.1 DECTAPE TCll/TU56
3.1.1 Introduction
The TCll/TU56 is a dual-unit. bidirectional magnetic-tape transport system for
auxiliary data storage. Low cost, low maintenance and high reliability are assured
by:
• simply designed transport mechanisms which have no capstans and no pinch
rollers.
• hydrodynamically lubricated tape guiding (the tape floats on air over the tape
guides while in motion)
• redundant recording
• Manchester recording techniques (virtually eliminate drop outs)
Each transport has a read/write head for information recording and playback on
five channels of tape. The system stores information at fixed positions on magne·
tic tape as in magnetic disk or drum storage devices, rather than'at unknown or
variable positions as in conventional magnetic tape systems. This feature allows
replacement of blocks of data on tape in a random fashion without disturbing
other previously recorded information. In particular, during the writing of information
on tape, the system reads format (mark) and timing .information from the
tape ahd uses this information to determine the exact position at which to record
the information to be written. Similarly, in reading, the same mark and timing information
is used to locate data to be played back from the tape.
The DECtape system consists of the TU56 dual transport, the TCll Control
(which will buffer and control information for up to four dual transports) and
DECtape 3/4-inch magnetic tape on 3.9-inch reels.
3.1.2 Operation
The system utilizes a 10-track read/write head. On a tape the first five tracks include
a timing track, a mark track, and three data tracks. The other five tracks
are identical counterparts and are used for reduRdant recording to increase system
reliability. The redundant recording of each character bit on non-adjacent
tracks materially reduces bit dropout and minimizes the effect of skew. The use of
Manchester phase recording, rather than amplitude sensing techniques, virtually
eliminates dropouts.
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