IBM Wheelwriter Bus Protocol

Reverse-engineered from two independent projects:

Cross-validated: both sources confirm the same protocol. Applicable to Wheelwriter models 3, 5, and 6 (two-board architecture: Logic Board + Motor Control Board).

Terminology note: Jim Loos calls them “Function Board” and “Printer Board”. John Kua calls them “Logic Board” and “Motor Control Board”. These are the same boards. This document uses both names interchangeably: Logic/Function Board (keyboard controller) and Motor Control/Printer Board (mechanical controller).

1. Physical Layer

Parameter Value
Baud rate 187,500 bps (12 MHz / 64)
Protocol Intel 8051 UART Mode 2 (9-bit multiprocessor)
Frame 1 start + 9 data + 1 stop = 11 bits per word
Bus type Half-duplex, single-wire, shared bus
Electrical Open-collector, idle = HIGH, data pulls LOW
Voltage 5V logic levels
MCU (Logic Board) Intel P8051AH @ 11.9 MHz
MCU (Motor Control Board) Intel P8051AH @ 12 MHz

Connectors

Internal ribbon cable (Logic Board J8F to Motor Control Board J5P, 7-pin):

Pin Signal Description
1 VRM Voltage regulator module, 5.1V
2 PDI Unknown
3 BRQ Bus Request
4 BUS Serial communication bus (187500 bps, 9-N-1, 5V, idle high)
5 GND Ground
6 POR Power-On Reset
7 +5V 5V DC

Option Interface connector (Motor Control Board J1P, 10-pin Molex SL):

Pin Signal Description
1 +5V 5V DC
2 +5V 5V DC
3 GND Ground
4 GND Ground
5 BUS Serial communication bus (same as internal bus)
6 NC Not connected
7 BRQ Bus Request
8 POR Power-On Reset
9 VRM Voltage regulator module
10 PDI Unknown

Mating connector: Molex 50-57-9210 (70066-0079). Contacts for 22-24 AWG: Molex 16-02-0102 (70058-0222).

For basic operation, only +5V (1,2), GND (3,4), and BUS (5) are needed.

Bus Arbitration

  • Before transmitting each word, the sender waits for the bus to go HIGH
  • After transmitting, the sender waits for the bus to return to HIGH
  • Receiver sends ACK by pulling the bus LOW briefly after receiving a word
  • The bus is shared: only one device transmits at a time
  • On a shared bus, the sender sees its own transmission (must be ignored or expected)
  • Alternative to monitoring ACK: insert ~450 us delay between words (less reliable)

Signal Inversion for TX

The bus is idle-HIGH with open-collector drivers. When using an N-channel MOSFET or similar open-collector/open-drain transistor to drive the bus, the TX signal is inverted at the hardware level:

  • MCU output HIGH -> MOSFET ON -> bus pulled LOW
  • MCU output LOW -> MOSFET OFF -> bus idle HIGH

Therefore the MCU TX output must be inverted in software or hardware. The RX input does NOT need inversion (bus LOW = start bit = standard UART).

The wheelwriter-interface project confirms this: it uses an inverted TX PIO program (idle=LOW, start bit=HIGH, data bits bitwise inverted) paired with a non-inverted RX PIO program.

UART Configuration

8051 at 12 MHz (reference):

Baud rate = SYSclk / (65536 - reload_value) / 4
187,500 = 12,000,000 / (65536 - 65520) / 4
reload_value = 65520 (0xFFF0)

2. Word Format

Each word on the bus is a 9-bit value (0x000-0x1FF), transmitted as an 11-bit UART frame:

[START] [D0] [D1] [D2] [D3] [D4] [D5] [D6] [D7] [D8] [STOP]
         \________ 8 lower bits ________/    \9th bit/

The 9th bit (D8) is the address/data indicator (8051 Mode 2):

  • Bit 8 = 1: Address byte (marks the start of a command)
  • Bit 8 = 0: Data byte or ACK

The only address used is 0x21 (Motor Control Board), transmitted as 0x121 (0x21 with 9th bit set).

3. Protocol Overview

Command Structure

All commands begin with the address word 0x121, followed by a command code, then zero or more data words:

0x121  <cmd>  [<data1>]  [<data2>]

Each word is acknowledged individually by the Motor Control Board.

Per-word Handshake

Full bus transaction for each word:

Sender:   [word]  -->
                      <--  [ACK/response]  :Receiver

The sender transmits a word, then the receiver responds with either:

  • ACK (0x000) for address bytes and intermediate data bytes
  • Response data for the last byte of query commands (command-specific)

Command Lengths

Command Length Meaning
0x00 1 (cmd only) Response is the answer
0x01 1 Response is the answer
0x02 3 (cmd + 2 data) ACK after last data
0x03 3 ACK after last data
0x04 3 ACK after last data
0x05 2 (cmd + 1 data) ACK after data
0x06 3 ACK after last data
0x07 1 ACK is the response
0x08 1 Response is the answer
0x09 2 ACK after data
0x0A 2 ACK after data
0x0B 1 Response is status
0x0C 3 Response after last data
0x0D 2 Response after data
0x0E 2 ACK after data

Typing Command Sequence (as seen on the bus)

The Logic Board wraps each keystroke in a status/event sequence:

  1. Query status: 0x0B / response 0x00 (ready)
  2. Keypress start: 0x0C <keycode> 0x46 / response
  3. Type command: 0x03 <wheel_pos> <usteps> / ACK
  4. (optional) Query: 0x0C <keycode> 0x08 / response
  5. Wait for motion: 0x0C <keycode> 0x44 / response (can be slow)
  6. Keypress end: 0x0C <keycode> 0x06 / response

For external devices (our use case): Only the core commands (0x03, 0x05, 0x06, etc.) are needed. The status queries (0x0B, 0x0C) are optional but recommended for reliability.

4. Command Reference

4.0 Query Model: 0x121, 0x000

Queries the typewriter model. No data bytes.

Response values:

Response Model
0x006 Wheelwriter 3
0x025 Wheelwriter 5
0x026 Wheelwriter 6

4.1 Reset: 0x121, 0x001

Power-on reset command. Moves carriage to left stop and returns printwheel pitch.

Response (printwheel type):

Response Printwheel Pitch uSpaces/char uLines/line
0x008 PS (Proportional Spacing) variable 10 16
0x010 15P (Micro Elite) 15 cpi 8 12
0x020 12P (Elite) 12 cpi 10 16
0x040 10P (Pica) 10 cpi 12 16
0x021 No printwheel installed - 10 16

After reset, the Logic Board sends additional initialization:

  • 0x0A 0x00 (unknown, ACK’d)
  • 0x0C 0x00 0x01 (query, response 0x20)
  • 0x06 0x80 0x78 (move carriage right 120 usteps = 1 inch)
  • Further init traffic for ~1 second

4.2 Type Character Without Advance: 0x121, 0x002, <wheel_pos>, <ignored>

Prints a character without moving the carriage. Used for in-place overprinting (bold, underline composites).

Word Description
0x121 Address
0x002 Type-no-advance command
<wheel_pos> Printwheel position (1-96, 0x01-0x60)
<ignored> Second data byte, ignored by controller

4.3 Type Character and Advance: 0x121, 0x003, <wheel_pos>, <usteps>

Prints a character and advances the carriage rightward.

Word Description
0x121 Address
0x003 Type-and-advance command
<wheel_pos> Printwheel position (1-96, 0x01-0x60)
<usteps> Microspaces to advance (max 63)

Microspaces per character by pitch:

  • 10 cpi (Pica): 12 microspaces/character
  • 12 cpi (Elite): 10 microspaces/character
  • 15 cpi (Micro Elite): 8 microspaces/character
  • PS (Proportional): 10 microspaces/character (default)

Note: 1 microspace = 1/120 inch. Maximum advance per command = 63 usteps.

Maximum line width: ~1450 microspaces (~12 inches, ~80 columns at 12cpi).

4.4 Erase Character and Advance: 0x121, 0x004, <wheel_pos>, <usteps>

Prints using the correction (lift-off) tape instead of ink ribbon, then advances.

Word Description
0x121 Address
0x004 Erase command
<wheel_pos> Character to erase (must match printed character)
<usteps> Microspaces to advance rightward

4.5 Vertical Movement (Platen): 0x121, 0x005, <direction_and_usteps>

Moves the paper vertically by rotating the platen.

Word Description
0x121 Address
0x005 Platen movement command
<direction_and_usteps> Direction + distance

Bit fields:

Bit 7:    Direction -- 1 = paper UP (toward printhead), 0 = paper DOWN
Bits 6:0: Number of microlines to move (7-bit value, max 127)

Microlines per line by pitch:

  • 10 cpi, 12 cpi, PS: 16 microlines = 1 full line
  • 15 cpi: 12 microlines = 1 full line

Common vertical movements:

Movement 10/12cpi, PS 15 cpi Value (10/12cpi)
Line feed (up 1 line) 16 12 0x90
Reverse line feed (down 1 line) 16 12 0x10
Half line up 8 6 0x88
Half line down 8 6 0x08
Micro up (1/8 line = 1/48”) 2 1-2 0x82
Micro down (1/8 line = 1/48”) 2 1-2 0x02

4.6 Horizontal Movement (Carriage): 0x121, 0x006, <dir_hi>, <usteps_lo>

Moves the carrier horizontally.

Word Description
0x121 Address
0x006 Carriage movement command
<dir_hi> Direction + upper bits of distance
<usteps_lo> Lower 8 bits of distance

dir_hi bit fields:

Bit 7:    Direction -- 1 = RIGHT (toward right margin), 0 = LEFT (toward left margin)
Bits 6:0: Upper 7 bits of the microspace count

The total distance is an 11-bit value (max 2047) split across two words:

total_microspaces = ((dir_hi & 0x07) << 8) | usteps_lo

Note: in practice the upper bits beyond bit 2 are rarely used (max ~1450 usteps for a full line = 11 bits).

Examples:

Operation dir_hi usteps_lo
Space right (12cpi) 0x80 0x0A (10)
Space right (10cpi) 0x80 0x0C (12)
Backspace (12cpi) 0x00 0x0A (10)
Tab right 5 chars (12cpi) 0x80 0x32 (50)
Carriage return from col 80 (12cpi) (n>>8)&0x07 n&0xFF where n=800

4.7 Spin Printwheel: 0x121, 0x007

Spins the daisy wheel as a visual/audible indication. No data bytes. Used as a “bell” or mode-change feedback.

4.8 Query Printwheel: 0x121, 0x008

Queries the printwheel pitch without resetting. Returns the same values as the reset command (Section 4.1).

4.9 Set Impression Control: 0x121, 0x009, <level>

Sets the hammer strike power. On Wheelwriter 3: 0x00 or 0x01. Later models may have more levels. Also triggered by Code+Q on the keyboard.

Additional modes observed in the enum:

  • 0x00 – Repeat off
  • 0x02 – Shift right half space
  • 0x04 – Underline
  • 0x08 – Repeat character
  • 0x80 – Center

4.10 Unknown Init Command: 0x121, 0x00A, 0x00

Emitted during power-up sequence after the reset command. Purpose unknown.

4.11 Query Status: 0x121, 0x00B

Queries the Motor Control Board status. The Logic Board sends this before every typing/motion command.

Response values:

Response Meaning
0x00 Ready
0x04 Carriage moving (or motion complete)
0x07 Carriage move complete
0x10 Platen moving
0x14 Carriage + platen moving
0x40 Printwheel changed or left limit switch pressed

4.12 Keypress Event Query: 0x121, 0x00C, <keycode>, <sub_cmd>

Complex query used by the Logic Board to wrap keystroke events. Sends keycode and sub-command, receives status response.

Sub-commands:

Sub-cmd Purpose Typical response
0x46 Start keypress event 0x04, 0x10
0x08 Backspace/space event 0x10, 0xF0 at left margin
0x44 Wait for motion complete 0x40, 0x80 (can be slow)
0x06 End keypress event 0x04, 0x10, 0x39
0x01 Wait for platen/carriage move variable

4.13 Code Key Combination: 0x121, 0x00E, <key_code>

Sent by the Logic Board when a Code+key combination is pressed.

Note: On Wheelwriter 3, bit 7 of <key_code> is cleared. On Wheelwriter 6, bit 7 is set. The Shift key sets bit 7 as well. Use key_code & 0x7F for model-independent decoding.

Code key release: 0x067 (or 0x0E7 with shift/WW6) indicates the Code key was released.

Code+Key mapping (key_code & 0x7F):

Code Key Code Key
0x00 Code++/- 0x001 Code+1
0x002 Code+Q 0x004 Code+A
0x006 Code+Z 0x009 Code+2
0x00A Code+W 0x00C Code+S
0x00E Code+X 0x011 Code+3
0x012 Code+E 0x014 Code+D
0x016 Code+C 0x018 Code+5
0x019 Code+4 0x01A Code+R
0x01B Code+T 0x01C Code+F
0x01D Code+G 0x01E Code+V
0x01F Code+B 0x020 Code+6
0x021 Code+7 0x022 Code+U
0x023 Code+Y 0x024 Code+J
0x025 Code+H 0x026 Code+M
0x028 Code+= 0x029 Code+8
0x02A Code+I 0x02B Code+]
0x02C Code+K 0x02E Code+,
0x031 Code+9 0x032 Code+O
0x034 Code+L 0x036 Code+.
0x038 Code+- 0x039 Code+0
0x03A Code+P 0x03B Code+1/2
0x03C Code+; 0x03D Code+’
0x03F Code+/ 0x042 Code+L Mar
0x045 Code+T Clr 0x046 Code+Micro Dn
0x047 Code+Space 0x048 Code+Mar Rel
0x049 Paper Feed Lever 0x04A Code+Tab
0x04B Code+R Mar 0x04C Code+T Set
0x04F Code+Erase 0x050 Code+Backspace
0x051 Code+Paper Up 0x052 Code+Paper Dn
0x054 Code+Micro Up 0x056 Code+C Rtn
0x057 Code+Line Space 0x067 Code key released
0x072 Code+N (Caps) 0x076 Code+N

5. Initialization Sequence

Power-up sequence as observed on the bus:

  1. Hardware reset (POR line pulsed)
  2. 0x121, 0x001 – Reset command, response = printwheel type
  3. 0x121, 0x00A, 0x00 – Unknown init, ACK
  4. 0x121, 0x00C, 0x00, 0x01 – Init query, response 0x20
  5. 0x121, 0x006, 0x80, 0x78 – Move carriage right 120 usteps (1 inch)
  6. Further initialization traffic for ~1 second
  7. Bus becomes idle; ready for normal operation

Timing: Printwheel detection may take up to 6 seconds. Total initialization including post-reset traffic ~7 seconds.

6. Printwheel Translation Tables

The daisy wheel has 96 character positions (0x01-0x60) arranged in a circle, plus position 0x00 = space (no print). Printwheel positions do NOT follow ASCII order.

Physical Arrangement of the Printwheel

Starting at the 12 o’clock position, going counter-clockwise:

Position 01: 'a'    Position 02: 'n'    Position 03: 'r'    Position 04: 'm'
Position 05: 'c'    Position 06: 's'    Position 07: 'd'    Position 08: 'h'
Position 09: 'l'    Position 10: 'f'    Position 11: 'k'    Position 12: ','
...

Printwheel Variants

Multiple printwheel types exist with different character mappings at positions 0x25-0x48:

Keyboard ID Type Notes
001 (US) Standard US < > mapped to non-ASCII symbols
103 (ASCII) Full ASCII All standard ASCII characters available
026, 029 German umlauts, eszett
067, 253 UK pound sign
070 Spanish n-tilde, inverted punctuation
231 USSR Cyrillic characters

Positions 0x01-0x24 and 0x49-0x60 are identical across all printwheel variants (common letters, digits, punctuation).

ASCII to Printwheel Code (for printing)

Table for ASCII printwheel (keyboard 103). Index = ASCII code - 0x20. Space (0x20) maps to code 0x00.

       0x_0 0x_1 0x_2 0x_3 0x_4 0x_5 0x_6 0x_7 0x_8 0x_9 0x_A 0x_B 0x_C 0x_D 0x_E 0x_F
0x2_:   00   49   4B   38   37   39   3F   4C   23   16   36   3B   0C   0E   57   28
        sp    !    "    #    $    %    &    '    (    )    *    +    ,    -    .    /
0x3_:   30   2E   2F   2C   32   31   33   35   34   2A   4E   50   45   4D   46   4A
         0    1    2    3    4    5    6    7    8    9    :    ;    <    =    >    ?
0x4_:   3D   20   12   1B   1D   1E   11   0F   14   1F   21   2B   18   24   1A   22
         @    A    B    C    D    E    F    G    H    I    J    K    L    M    N    O
0x5_:   15   3E   17   19   1C   10   0D   29   2D   26   13   41   42   40   3A   4F
         P    Q    R    S    T    U    V    W    X    Y    Z    [    \    ]    ^    _
0x6_:   3C   01   59   05   07   60   0A   5A   08   5D   56   0B   09   04   02   5F
         `    a    b    c    d    e    f    g    h    i    j    k    l    m    n    o
0x7_:   5C   52   03   06   5E   5B   53   55   51   58   54   48   43   47   44   00
         p    q    r    s    t    u    v    w    x    y    z    {    |    }    ~   DEL

US printwheel (001) differences from ASCII (103):

  • < (0x3C): US = 0x00 (unavailable), ASCII = 0x45
  • > (0x3E): US = 0x00, ASCII = 0x46
  • \ (0x5C): US = 0x00, ASCII = 0x42
  • ^ (0x5E): US = 0xA2 (cent sign), ASCII = 0x3A
  • ` (0x60): US = 0x00, ASCII = 0x3C
  • { (0x7B): US = 0x00, ASCII = 0x48
  • | (0x7C): US = 0x00, ASCII = 0x43
  • } (0x7D): US = 0x00, ASCII = 0x47
  • ~ (0x7E): US = 0x00, ASCII = 0x44

Printwheel Code to ASCII (for reading keystrokes)

Index = printwheel_code (code 0x00 = space 0x20). Table for US printwheel:

Idx  Char | Idx  Char | Idx  Char | Idx  Char | Idx  Char | Idx  Char
0x00  SP  | 0x01  a   | 0x02  n   | 0x03  r   | 0x04  m   | 0x05  c
0x06  s   | 0x07  d   | 0x08  h   | 0x09  l   | 0x0A  f   | 0x0B  k
0x0C  ,   | 0x0D  V   | 0x0E  -   | 0x0F  G   | 0x10  U   | 0x11  F
0x12  B   | 0x13  Z   | 0x14  H   | 0x15  P   | 0x16  )   | 0x17  R
0x18  L   | 0x19  S   | 0x1A  N   | 0x1B  C   | 0x1C  T   | 0x1D  D
0x1E  E   | 0x1F  I   | 0x20  A   | 0x21  J   | 0x22  O   | 0x23  (
0x24  M   | 0x25  .   | 0x26  Y   | 0x27  ,   | 0x28  /   | 0x29  W
0x2A  9   | 0x2B  K   | 0x2C  3   | 0x2D  X   | 0x2E  1   | 0x2F  2
0x30  0   | 0x31  5   | 0x32  4   | 0x33  6   | 0x34  8   | 0x35  7
0x36  *   | 0x37  $   | 0x38  #   | 0x39  %   | 0x3A  cent| 0x3B  +
0x3C  +-  | 0x3D  @   | 0x3E  Q   | 0x3F  &   | 0x40  ]   | 0x41  [
0x42  sup3| 0x43  sup2| 0x44  deg | 0x45  sect| 0x46  para| 0x47  1/2
0x48  1/4 | 0x49  !   | 0x4A  ?   | 0x4B  "   | 0x4C  '   | 0x4D  =
0x4E  :   | 0x4F  _   | 0x50  ;   | 0x51  x   | 0x52  q   | 0x53  v
0x54  z   | 0x55  w   | 0x56  j   | 0x57  .   | 0x58  y   | 0x59  b
0x5A  g   | 0x5B  u   | 0x5C  p   | 0x5D  i   | 0x5E  t   | 0x5F  o
0x60  e   |

7. Composite Operations

Some typewriter actions require multiple bus commands in sequence.

7.1 Bold Printing

Print character in place, shift 1 ustep, re-print in place, then advance:

0x121, 0x002, <pw_code>, 0x00           -- print without advance
0x121, 0x006, 0x80, 0x01                -- move right 1 ustep
0x121, 0x002, <pw_code>, 0x00           -- re-print (offset by 1 ustep)
0x121, 0x006, 0x80, <uSpaces - 1>       -- advance remaining

Alternative (from Jim Loos, using cmd 0x03):

0x121, 0x003, <pw_code>, 0x001          -- print + advance 1 ustep
0x121, 0x003, <pw_code>, (uSpaces - 1)  -- re-print + advance remaining

7.2 Underline Printing

Print character, then overprint underscore:

0x121, 0x003, <pw_code>, 0x000          -- print + don't advance
0x121, 0x003, 0x04F, <uSpacesPerChar>   -- print '_' + advance

7.3 Erase Character

Move carrier left, then print character on correction tape:

0x121, 0x006, 0x00, <uSpacesPerChar>                 -- move left 1 char width
0x121, 0x004, <pw_code>, <uSpacesPerChar>             -- erase + advance right

Net result: character erased, carrier back at original position.

7.4 Carriage Return

Move carrier left by the total accumulated horizontal microspaces:

0x121, 0x006, (count >> 8) & 0x07, count & 0xFF      -- move left by count

Bit 7 of third word = 0 (left direction).

8. Keystroke Decoding State Machine

When monitoring the bus, the Logic Board sends multi-word command sequences for each keystroke. A state machine decodes them:

State IDLE:
  Receive 0x121 --> GOT_ADDR

State GOT_ADDR:
  0x003 --> CHAR_1 (character sequence)
  0x005 --> VERT (vertical movement)
  0x006 --> HORIZ_1 (horizontal movement)
  0x00E --> CODE_KEY (code key combo)
  other --> IDLE

State CHAR_1:
  Save printwheel code --> CHAR_2

State CHAR_2:
  Microspaces received, decode: ASCII = printwheel2ASCII[saved_code]
  --> IDLE

State VERT:
  Receive dir|usteps byte
  If usteps == uLinesPerLine AND dir == UP --> Carriage Return key detected
  --> IDLE

State HORIZ_1:
  If bit 7 set --> HORIZ_RIGHT
  Else --> HORIZ_LEFT

State HORIZ_RIGHT:
  usteps == uSpacesPerChar --> Space key
  usteps > uSpacesPerChar AND < uSpacesPerChar*10 --> Tab key
  --> IDLE

State HORIZ_LEFT:
  usteps == uSpacesPerChar --> Backspace key
  --> IDLE

State CODE_KEY:
  Decode key_code & 0x7F (see Section 4.13)
  --> IDLE

9. Application to TKB Project (Teensy 4.0)

Connection via Option Interface

Both reference projects confirm the Option Interface port uses the same bus protocol as the internal ribbon cable. Our Teensy connects to the 10-pin Option connector.

Minimum wiring: +5V (pins 1-2), GND (pins 3-4), BUS (pin 5).

Teensy UART Configuration 

Serial2.begin(187500, SERIAL_9N1);  // 9 data bits, no parity, 1 stop bit (TX=pin 8, RX=pin 7)

The 9th bit distinguishes address words (bit8=1, e.g., 0x121) from data/ACK words (bit8=0).

Hardware Requirements

  1. Level shifter: 3.3V (Teensy) <-> 5V (Wheelwriter bus), open-drain compatible
  2. TX inversion: Required because the open-collector driver inverts the signal. Options:
    • Hardware: N-channel MOSFET naturally inverts (MCU HIGH -> bus LOW)
    • Software: Teensy 4.0 UART supports SERIAL_9N1_RXINV_TXINV flag
    • If using MOSFET for bus drive, MCU TX must be inverted so that MCU HIGH -> MOSFET ON -> bus LOW = correct data
  3. RX does NOT need inversion when reading bus directly (bus LOW = UART start bit)

Bus Timing

  • At 187,500 bps with 11-bit frames: ~58.7 us per word
  • Expect ACK within ~450 us after each transmitted word
  • On shared bus, sender sees its own transmission (ignore the echo)
  • Between commands, no strict timing (bus idles HIGH)

Based on both reference projects:

  1. Send 0x121, 0x00B (query status) before each command to ensure Motor Control Board is ready
  2. After sending each word, read back the echo (own transmission) and the ACK/response
  3. Check ACK = 0x00 for non-query commands; check response for queries
  4. Track horizontal position (microspaces) for carriage return calculations

10. References

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