digital equipment corporation · maynard, massachusetts
TU56 DECtape Drive Information

The TU56 DECtape Drive is a dual transport reel to reel tape drive. The DECtape drive served the purpose of a floppy disk drive on modern machines. Even though it was tape it was formatted into fixed size block and could randomly read and write them like the sectors on a floppy. This allowed the tape to have a normal file system on it. The main difference between the DECtape and a floppy is that the DECtape had a much longer seek time since the tape has to be sequentially read. The drive takes about 30 seconds to get from one end of the tape to the other. This drive is compatible with the earlier 555 and TU55 DECtape drives. The DECtape is derived from the LINCtape on the LINC-8

The tape is normally formatted with 1474 block of 129 12 bit words per block. Only 128 words are normally used in each block. This gives a capacity of 184K words per tape. The drive is capable of reading in either direction which is used for locating the correct block but the operating system drivers normally only read the data in the forward direction.

The drive has a 10 track head in which two tracks are used for timing and three for data. The other 5 tracks are redundant copies to reduce the error rate. The data can be recovered if only one of the redundant tracks is good. Each block has several special codes for synchronization plus a block number and checksum in a header area at each end of the block. This allows the tape to be read in either direction.

The operation of this drive is a little different than many other tape drives. It doesn't have the capstans and pinch rollers to drive the tape and vacuum or columns or other devices to control tape tension. It directly drove each reel with an AC induction motor. During full speed operation it would drive one motor with 38 volts in the forward direction and the other reel with 18 volts in the reverse direction to keep tension on the tape. To reverse tape motion it would switch the drive voltages so the motor running in reverse would have full power. To stop the tape it would apply reverse for a adjustable delay and then switch to hold mode. In hold mode it apples 18 volts in opposite directions to each motor. This prevents tape motion but keeps the tape taut.

This method had some disadvantages. The drive can't stop fast enough to halt between tape blocks so if the drive stops when reading sequential data it must back up to the data it overshot. The tape speed also is less well controlled but the manchester data encoding method is insensitive to speed variations of less than 20%.

TU56 Specifications
Start time 150 milliseconds ±15
Stop time 100 milliseconds ±10
Turnaround 200 milliseconds ±50
Tape speed 93 ±12 inches per second
Recording Density 350 ±55 bits per inch
Transfer rate 8325 12 bit words per second
Recording Format 10 Track, Manchester Phase Encoding
Operating Environment 60 to 95 F, 15 to 15 C    20% to 80% Humidity
Power <= 325 watts
Size (HeightxWidthxDepth) 10.5x19x17 3/4 inches
Weight 80 lbs.
Storage Media
Media Mylar sandwich, iron oxide
Width 3/4 inch
Thickness .00125 inches
Length 260 feet
Reel outer diameter 3 7/8 inches
Reel inner diameter 2 3/4 inches
Capacity 2,700,000 bits unformatted, 190,146 12 bit words formatted
Cost Drive and TD8E controller $5500 and $240 installation from DEC in 1974
  $42 per month for maintenance
Dual DECtape drive $5100 and $60 for installation
  $32 per month for maintenance
Single DECtape drive $2350 and $60 for installation
  $21 per month for maintenance

My drive is serial number 5105 and has an inspection sticker dated 9/8/1970

Documents related to TU56 and DECtape

When I first got the drive I needed to clean the bushing behind the hubs, replace some bad components on the cards, and replace the bulbs in the front panel. The entire light assembly is intended to be replaced as a unit but since they are no longer available you can carefully open them up and solder in a new bulb. See the picture below. I used ME353-2182 which is now CM2182 and available from Mouser. All the lights were burnt out in my drive so I didn't have to try to match the intensity of the original bulb. I also haven't seen another drive powered up to know how bright they should be. If you need to match I would buy a couple different bulbs so you can pick the correct brightness.

The following picture links also have descriptions of what is shown in the pictures.

Thumbnail Picture Selector

Current size small. Select picture size for links below   Small  Medium  Large

DECtape Drive ( 33K)  Drive motors ( 56K)  Circuit card area ( 45K)
Circuit cards ( 29K)  Circuit cards back side ( 32K)
DECtapes ( 28K)  Closeup of DECtape ( 13K)
Motor label ( 52K)  Head label ( 45K)

Pictures from recent repair.

First repair was to replace the capacitor in the H725 power supply which had leaked and wires which had the insulation burnt by running too close to a power resistor
H725 Power supply removed ( 60K)  supply internals 1 ( 65K)  supply internals 2 ( 64K)
H725 power supply capacitors ( 48K)  Capacitor bad seal ( 39K)  New capacitors installed ( 63K)
H725 power supply front ( 52K)  supply back ( 40K)
Damaged wires 1 ( 38K)  Damaged wires 2 ( 52K)  Wires repaired ( 51K)

Next I replaced the motor capacitors that were oozing
Oozing capacitors ( 54K)  Motor capacitors ( 52K)  New capacitors installed ( 56K)

Last repair was to reglue the faceplate
Faceplate off drive ( 47K)  Back of faceplate ( 56K)  Hub screws ( 36K)  Tape guide ( 33K)

Pictures of drive after repairs completed
Back view of drive ( 64K)  Closeup of labels ( 48K)  Closeup of cards ( 66K)
Front of drive ( 31K)  Front down view ( 54K)

Replacing bulb again
Bulb repair ( 52K)

Videos of operation of DECtape drive. How to view The large realvideo are the best, the small realvideo picture is smaller and you can't here the drive operation well due to the lower bitrate encoding used. The large MPEG has some artifacts from being recorded with interlaced video from a camcorder.

Running directory of drive 1 from drive 0:
Realvideo: small (191K) large (526K)
MPEG: small (910K) large (2.2M)

Mounting tape on drive:
Realvideo small (192K) large (535K)
MPEG: small (706K) large (2.2M)

Feel free to contact me, David Gesswein djg@pdp8online.com with any questions, comments on the web site, or if you have related equipment, documentation, software etc. you are willing to part with.  I am interested in anything PDP-8 related, computers, peripherals used with them, DEC or third party, or documentation. 

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