HP 12920A Multiplexer
                              =====================


Upper Data Card control word format (OTA):

   15 |14  13  12 |11  10   9 | 8   7   6 | 5   4   3 | 2   1   0
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  | - |  channel number   | -   -   -   -   -   -   -   -   -   - |
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


Upper Data Card status word format (LIA):

   15 |14  13  12 |11  10   9 | 8   7   6 | 5   4   3 | 2   1   0
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  | S |  channel number   | -   -   -   -   -   - | D | B | L | R |
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

  Key:

    S = seeking
    D = diagnose
    B = break status
    L = character lost
    R = receive/send (0/1) character interrupt



Lower Data Card control word format (OTA):

   15 |14  13  12 |11  10   9 | 8   7   6 | 5   4   3 | 2   1   0
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  | 1 | R | I | E | D | char size |           baud rate           |
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

  Key:

    R = receive/send (0/1) configuration
    I = enable interrupt
    E = echo (receive)/parity (send)
    D = diagnose

  Character size:
    The three least-significant bits of the sum of the data, parity, and stop
    bits.  For example, 7E1 is 1001, so 001 is coded.

  Baud rate:
    The value (14400 / device bit rate) - 1.  For example, 2400 baud is 005.


Lower Data Card data word format (OTA):

   15 |14  13  12 |11  10   9 | 8   7   6 | 5   4   3 | 2   1   0
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  | 0 | 1 | -   - | S |               transmit data               |
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

  Key:

    S = sync bit
    data = right-justified with leading ones


Lower Data Card status word format (LIA):

   15 |14  13  12 |11  10   9 | 8   7   6 | 5   4   3 | 2   1   0
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  | P |      channel      |             receive data              |
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

  Key:

    P = computed parity
    data = right-justified with leading ones


Lower Data Card flags:

  CLF = acknowledge interrupt
  STC = initiate output of parameters or data to the interface



Control Card control word format (OTA):

   15 |14  13  12 |11  10   9 | 8   7   6 | 5   4   3 | 2   1   0
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  | S | U |channel number | -   - |EC2|EC1|C2 |C1 |ES2|ES1|SS2|SS1|
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


Control Card status word format (LIA):

   15 |14  13  12 |11  10   9 | 8   7   6 | 5   4   3 | 2   1   0
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  | 1   1 |channel number |I2 |I1 | 0   0   0   0 |ES2|ES1|S2 |S1 |
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

  Key:

    S   = scan
    U   = update
    ECx = enable command bit x
    Cx  = command bit x
    ESx = enable status bit x
    Sx  = status bit x
    SSx = stored status bit x
    Ix  = interrupt bit x


Performance:

 - Under simulation, the MUX is the slowest output device.  Running test system
   P and doing a ":DU,'5010P,<termlu>" command, the output times are:

    * BACI =  1.5 seconds
    * TTY  =  2.5 seconds
    * MPX  =  6.5 seconds
    * MUX  = 20.0 seconds