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An IBM 729 Tape Unit Refresher

Collected by Ed Thelen

1401 Tape-Adapter Unit (TAU) Commands (software)
Operator accessible keys, switches, lights
200 pin signal cable
D.C. Power Requirements
CE adjustments
Possible 729 Tape Test Controller


There is a 729 tape manual at

A24-3069-2 says that the 1401 can use the following 729 Magnetic Tape Units - II, IV, V, VI although the 729 - VI operates at 729 IV speeds only.

A24-3067-2 System Operation Reference Manual
A24-3144-2 Operator's Guide,

page 63 starts the "IBM 729 and IBM 7330 Magnetic Tape Units"
  • Tape Reels - 10.5" in diameter, holds 2400 feet of tape - (a 2400 foot reel can contain about 13 million characters, assuming records 2,000 characters in length, written at 556 characters per inch.)
  • Each reel is equipped with a file-protection ring to safeguard data written on the tape. fits into the groove in the back (machine side) of the reel - no ring - no write
  • Reel-Release Button - below the left-hand tape reel - It must be held pressed to turn either tape reel by hand.
  • REEL DOOR - Interlock prevents any automatic or key-controlled operation when this door is open. This door should never be opened when the READY light is ON or curing load, rewind, or unload operation. With the door open during any of these operations, the tape could become damaged.
  • "File Reel" is on the left, the "take-up" reel is on the right
  • The upper half of the Read/Write Head Assembly pivots
    - up (open) when the UNLOAD is pressed
    or after a REWIND TAPE AND UNLOAD program instruction
    which also unloads the vacuum columns
    and must be open before opening the reel door to make manual adjustments
    - down (normal) after the LOAD REWIND key is pressed
    which also loads the vacuum columns

    The 720 uses a two-gap write/read head to enable on-line check while writing data. There also seems to be erase head logic.

    1401 Tape-Adapter Unit (TAU) Commands (software)

    The Tape-Adapter Unit (TAU)

    Read Tape - The tape unit specified in the A-address is started. The d-character specifies a tape read operation. The B-address specifies the high-order position of the tape read-in area of storage. the machine begins to read magnetic tape, and continues to read until either an inter-record gap in the tape record or a group-mark with a word-mark in core storage is sensed. ...
    Read tape with Word Marks -
    Write Tape -
    Write Tape with Word Marks -
    Backspace Tape Record -
    Skip and Blank Tape - erases about 3.5 inches of tape (bypass defective tape areas)
    Write Tape Mark - causes a tape mark character (8421) to be recorded - when a tape mark is read back from a tape, the end-of-reel indicator is turned on. (may actually be end of file, with more records yet on the tape)
    Diagnostic Read - reposition tape to next Inter-record gap (IRG) with no data transfer.
    Rewind Tape -
    Rewind Tape and Unload -
    Branch if End of Reel -
    Branch if Tape Error -

    Operator accessible keys, switches, lights

    729 Panel
    Address Dial lights->SELECTREADYLOW D | HIGH D?TAPE

    Operator accessible - (can see indicator light, or operate key/switch/dial/... Other switches
    Name Physical Function
    ADDRESS SELECTION DIAL Assigns a number to the tape unit, to associate the unit with the tape-drive number specified in the stored program. Because a maximum of six tape units can be attached to the system, this dial may be set at any number 1-6. If some other number (7,8,9,0, or blank) is selected, the tape unit cannot be used by the stored program. A dial setting of any unit should never be changed while a tape operation is in process.
    Panel Keys and Switches
    This key rewinds tape onto the file reel to its starting point (load point) It is used at two main times in any tape operation.
    1. At the beginning of a job, to position the tape for reading or writing the first record.
    2. At the completion of a job, to rewind all tape onto the file reel ready for unloading.
    Effective only when the reel door is closed and the READY light is OFF.

    When tape is properly mounted in the tape unit at the beginning of a job, pressing this key

    1. Lowers (loads) tape into the vacuum columns
    2. Closes the Read/Write Head Assembly
    3. Moves the tape to the left (rewind direction) until the load-point reflective marker is sensed. If the tape was mounted with the marker erroneously positioned to the left of the head assembly, the tape will unwind from the take-up reel.
    When a job is completed, pressing this key rewinds tape from the take-up reel to the file reel until the load-point reflective marker is sensed. For this operation a low-speed rewind is initiated if the take-up reel contains less than one-half inch of tape (less than 450 feet). Else, a high-speed rewind (an average of 500 inches per second) is initiated.
    Whenever this key has been pressed, the reel door must remain closed until the rewind operation is completed. If it were opened, the tape could be broken or damaged
    START key This key readies the tape unit for automatic operation, and turn ON the READY light. It is effective only when:
    • The reel door is closed,
    • Tape is loaded in the vacuum columns, and
    • Tape is not in the process of loading or rewinding to the load point If START is pressed during a rewind operation, the READY light turns ON when rewinding is completed.
    Pressing the key makes all other keys, except RESET, inoperative.
    CHANGE DENSITY key Pressing this key changes the density rate of the tape unit from its previous setting (from low to high, or from high to low). the density rate in effect is indicated by the LOW DENSITY or HIGH DENSITY light, and must agree with the density rate of the tape being processed. This key is effective only when the READY light is OFF.
    UNLOAD key
    1. Opens the head assembly.
    2. Unloads the vacuum columns. If the tape has not been rewound to the file reel, the LOAD REWIND key should be pressed before UNLOAD
    3. Turn OFF the TAPE INDICATE light, if it is ON
    It is effective only when the reel door is closed and the READY light is OFF.
    RESET key This key changes the tape unit from automatic to manual control (making the other keys operative< and turn OFF the READY light.

    This key also stops any machine operation in process, except unload or rewind. Once started, an unload operation is always completed. If a high-speed rewind is in process when RESET is pressed, rewinding shifts to low speed. If a low-speed rewind is in process, RESET stops the operation.

    Panel Lights
    SELECT lightWhen the tape unit is under automatic control (READY light ON), the yellow SELECT light turns ON whenever processing selects this unit for a tape operation. This is determined by the execution of a tape program instruction that contains the number of this unit (as set up in the address selection dial).
    READY lightThis light indicates that the tape unit is ready for automatic operation. It is turned ON by pressing the START key.
    When READY is lighted, all keys except RESET are inoperative. This light is turned OFF, and the tape unit is placed under manual control, by pressing the RESET key.
    The reel door should never be opened when the READY light is ON.
    LOW Density /
    HIGH Density
    wire & light These lights indicate the density rate (low or high) in which the tape unit is operating. This must agree with the density rate on the tape being processed.
    HIGH DENSITY turn ON when power is initially turned ON in the system. The density rate can be changed to low by pressing the CHANGE DENSITY key. The red LOW DENSITY light turns ON. Each time this key is pressed (with the READY light OFF) the density rate is switched from its previous setting.
    FILE PROTECT light When this light is ON, i indicates that the tape in this unit can be read but not written on, because a tape reel without a file-protection ring has been mounted. This light also turn ON when no file reel is mounted, when a load, rewind, or unload operation is in process, or when the machine is under manual control (READ light OFF).
    TAPE INDICATE light This light is turned ON by
    • Sensing the end-of-reel reflective marker during a tape-writing operation.
    • Reading a tape mark during a tape-reading operation.
    this light is turned OFF by pressing UNLOAD, or by executing a REWIND TAPE AND UNLOAD or BRANCH IF END OF REEL program instruction.
    FUSE light This red light turns ON whenever a fuse or circuit breaker in the tape unit is open. The IBM customer engineer should be called to correct this condition.
    WRITE ENABLE SWITCH Push switch behind File Reel if depressed by the write Enable Ring on the tape reel, permits write to tape - no ring, no write -
    REEL RELEASE SWITCH Button It must be held pressed to turn either tape reel by hand.
    REEL DOOR Slide door Interlock prevents any automatic or key-controlled operation when this door is open. This door should never be opened when the READY light is ON or curing load, rewind, or unload operation. With the door open during any of these operations, the tape could become damaged.
    METER SWITCH switch? If the use-meter switch is not set to ON or the tape unit is not ready before an instruction calls on this unit, the system interlocks and the processing use-meter continues to run.
    METER TIME meter? Starts when the first tape operation affecting the particular tape unit is issued. The meter stops when the tape unit receives and completes a rewind unload command. The meter can be stopped by manually unloading the unit, or by pressing and releasing the RESET key.
    For "Operations" such as loading tape, see A24-3144-2 "IBM 1401 Operator's Guide" page 70

    in T/C connector receptacle

    C.E. Manual has usage on page C77
    Pin Name to/from Comments, Interpretation
    1 Turn on T.I. terminator +1.5v @ 90 ohm
    3 Turn off T.I. terminator -7.5v @ 90 ohm
    5 Go terminator -7.5v @ 90 ohm
    7 BKWD terminator +1.5v @ 90 ohm
    9 Start Rewind terminator +1.5v @ 90 ohm
    11 Set Write Status terminator +1.5v @ 90 ohm
    13 Set Read Status terminator -7.5v @ 90 ohm
    17 Spare terminator +1.5v @ 90 ohm
    19 Spare terminator +1.5v @ 90 ohm
    21 Sel & Rdy Mod IV
    23 Sel & T.I. On
    25 Sel & T.I. Off
    27 Sel & at Ld. Pt.
    29 Sel & not at Ld. Pt.
    31 Sel & Rdy Mod II
    33 Sel Rdy & Read
    35 Sel Rdy & Write
    37 Sel & Rewind
    39 Rewind & Unload terminator -7.5v @ 90 ohm
    41 Set Hi-Density terminator -7.5v @ 90 ohm
    43 Set Lo-Density terminator +1.5v @ 90 ohm
    47 Hi-Density Lo-Density
    77 Sel T.U. 0 terminator -7.5v @ 90 ohm
    79 Sel T.U. 1 terminator -7.5v @ 90 ohm
    82 Read Bus 1
    84 Read Bus 2
    86 Sel T.U. 2 terminator -7.5v @ 90 ohm
    88 Sel T.U. 3 terminator -7.5v @ 90 ohm
    93 Read Bus 4
    95 Read Bus 8
    97 Sel T.U. 4 terminator -7.5v @ 90 ohm
    99 Sel T.U. 5 terminator -7.5v @ 90 ohm
    102 Read Bus A
    104 Read Bus B
    106 Sel T.U. 6 terminator -7.5v @ 90 ohm
    108 Sel T.U. 7 terminator -7.5v @ 90 ohm
    113 Read Bus C
    117 Sel T.U. 8 terminator -7.5v @ 90 ohm
    119 Sel T.U. 9 terminator -7.5v @ 90 ohm
    132 + 6 V probably terminator supply power @ 200 ma
    140 - 6 V probably terminator supply power @ 200 ma
    170 - 12 V probably terminator supply power @ 200 ma
    172 Write Bus 1 terminator +1.5v @ 90 ohm
    174 Write Bus 2 terminator +1.5v @ 90 ohm
    176 Write Bus 4 terminator +1.5v @ 90 ohm
    178 Write Bus 8 terminator +1.5v @ 90 ohm
    180 Write Bus A terminator +1.5v @ 90 ohm
    182 Write Bus B terminator +1.5v @ 90 ohm
    184 Write Bus C terminator +1.5v @ 90 ohm
    192 Write Echo Pulse
    194 Write Check Char terminator +1.5v @ 90 ohm
    196 Write Pulse terminator +1.5v @ 90 ohm

    D.C. Power Requirements

    From Customer Engineering Manual of Instruction
    Page C30 - from a single phase
    Voltage Amp Use
    +6 1.5 Transistor logic circuits
    -6 1.5 Transistor logic circuits
    +12 1.0 Transistor logic circuits
    -12 1.0 Transistor logic circuits
    -7.5 20.0 Prolay driver circuits
    +73 0.165 Transistor neon indicator circuits
    -48 2.0 Relay circuits
    +140 5.1 Magnetic clutch circuit

    CE adjustments

    First some locations
    Circuit Protector Chart 00.05.1 530039
    Relays 00.06.1, sheet 1 of 1 8021369
    Keys, switches, lamps, pots 00.06.1, sheet 2 of 2 8021593
    Pre-amps, Card Frame, Power transistors 00.07.1 530055
    Neon panel 00.08.1 8015987
    Write amps, Read Pre-amps 00.09.1 530057
    Relay gate & Blowers 00.11.1 8021589
    Power Transistor Panel 00.13.1 8012810
    Voltage Bus +-6, +-12 00.15.1 526208

    in 00.04.2, 8021590 - Component Location
    Pot 1
    Pot 2
    Pot 3
    Pot 14
    (machines with JT. ?2982
    L. STOP
    L. DOWN
    RT. DOWN
    RT. UP
    RT. STOP
    Relay gate & Blowers
    Pot 1
    Pot 2
    Pot 3
    Pot 8
    Clutch & Vacuum Control - 02.02.2 - 8018346
    Pot 13
    250 ohm, 25 watt
    Pot 12
    250 ohm, 25 watt
    Pot 8
    250 ohm, 50 watt
    Pot 11
    250 ohm, 25 watt
    Pot 10
    250 ohm, 25 watt
    Pot 3
    5K ohm, 25 watt
    Pot 9
    250 ohm, 50 watt

    Possible 729 Tape Test Controller.

    I've been doodling with the idea of making a 729 tape
      test controller - to help check out 729s before
      and maybe after the 1401 comes up.
    There is the thought that the extra work in building a
    tape test controller would
       a) aid in bringing up the 729s sooner.
       b) help identify if problems are in
             - the 1401 controller
             - the 729 tape unit
             - cables/plugs
       c) permit parallel bring-up of 729 interface 
          electronics with other components such as the 1401
    (Quite a bit of the 729 can be tested/brought up
         with out any outside simulation equipment
            - power supplies
            - blowers
            - read/write head assembly movement
            - prolay operation
            - tape movement
            - vacuum column operation
            - reel clutches and logic
            - load-point and end-of-tape detection
            - load, unload operations
            - reset/ready/tape density/... logic
    A down side is of course, 
        - development time and resources, 
               including a required T/C plug or socket
        - errors in the test controller :-((
    Presumably the tape test controller could be based on a PC
      except that other aids may be necessary to
         - provide more exact and reliable timing than
            is available in the usual PC
           Ballistic Research Labs mentions 
              tape data transfer rates of 
                  "  15,000 to 62,500 Char/sec "
              which may be within the raw transfer rate,
                but probably not delivered within 
                the timing constraints needed.
         - provide the large number of digital inputs and outputs
               at suitable voltage and strength levels.
        I have done some slower, non-time critical digital interfacing
          but the apparent write data and write clocks
    There is a limited amount of information that I have noticed:
       a) signal names and 3 dc voltages through the "T/C connector receptacle"
            are identified in the 729 schematics on page 530035 - 00.021
           and now at
      b) (Al Kossow's documentation web site
           seems down right  now.)
    There appear to be several challenges, 
    including 729 items
       a) signal sequences and timings to/from the 729 are 
            not obvious from the schematics
       b) expected voltages need to be worked out
       c) direction of signals (or bi-directionality) needs to be worked out.
       d) it would appear that the tape unit has a minimum amount of
           logic, and that the controller in the 1401 provides a large
    new test controller items (
           amount of timing and sets a wide interface of signals

    Updated July 16, 2016