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%.
|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|
|Media||Mylar sandwich, iron oxide|
|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
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. 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 Replacing bulb again
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.
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)
Bulb repair ( 52K)
Feel free to contact me, David Gesswein firstname.lastname@example.org 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.
PDP-8 Home Page  PDP-8 Site Map  PDP-8 Site Search
The following picture links also have descriptions of what is shown in the pictures.
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
Replacing bulb again
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.