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   DUMP:  A diagnostic command used by the PC to obtain the detailed set of
          nutation widths collected by the processor in the most recent test.
   STATS: A command used by the PC to obtain the statistical results of the
          most recent test.

During the normal course of system operation, only the TEST and STATS commands
will ever be used.  The ABORT is used on exceptional occasions such as when a
communication failure takes place, or when a test is aborted by the PC.  The
other two commands are only used by diagnostic programs.

(2) MicroNet
   MicroNet is a relatively simple multi-processor network custom-designed
specifically for use with this test system.  It has a multi-drop topology
in which one master controller (a PC) communicates with several slave
processors (8051FA's).  The network has been designed in several layers,
as is standard practice: a Physical Layer, a Protocol Layer, and a Command
Layer.

   This section has all the details needed for anyone who wants to write client
software at a PC (or elsewhere) to communicate with the microprocessors.

   (a) The Physical Layer
   This layer is concerned primarily with the actual details of the connections
between the processors.  An RS-485 serial communications link is being used,
which is similar to the RS-422 link, except it allows several processors to be
on the line, instead of just two.

   The actual circuit is a loop terminated by a resistor.  Each processor on the
loop receives input from the network by sampling inputs from opposite ends of
the loop across the resistor.
   At any given time, up to one processor can take control of the loop, passing
into a transmission state.  All other processors must be in a reception state
at that time for proper signalling to take place.  For this purpose, each
processor has a transmit/receive control output which allows it to connect or
disconnect its transmitter.

   The microprocessor units use the T0 pin, mentioned briefly in the previous
section for this purpose.  The master PC must likewise use its own control
output for the same function.  Normally, an RS-485 serial communications card
will need to be installed in the PC.

   (b) The Protocol Layer
   The primary purposes of the protocol layer are to:

        (1) Protect the network against multiple access by two or more
            transmitters.
        (2) Ensure proper routing of transmissions.
        (3) Ensure reliable transmission.

This is actually done at two levels: one closely tied to the physical layer
and the other implemented completely in software.

   (i) Low-level, physical, protocol
   For transmission, we use the protocol 9600, N, 9, 1: 9600 baud, no parity,
9 data bits, and one stop bit.  Error-free communication has been proven at baud
rates up to 57600 with the units all within 2 meters of each other.

   The first 8 data bits are used to transfer information or commands.  The 9th
bit is used for protocol purposes in the following way:

     * All transmission to a microprocessors occurs with the 9th bit set to 1.
     * All transmission from a microprocessor occurs with the 9th bit set to 0.
     * All microprocessors ignore transmissions where the 9th bit is 0.

These three conditions make it logically impossible for microprocessors to
communicate with one another, when the underlying Physical Layer is functioning
reliably.  They can only communicate with the master PC.  This is one of the
key conditions that assures the network's protection against simultaneous
access.  Complete assurance is provided by the software portion of the protocol.

   (ii) High-level, software, protocol: Addressing
   Since there are only two micrprocessor units on the network, routing is
greatly simplified.  Call these units A, and B.  All commands to the
microprocessors will have the following format:

                       1   B A   C C C   M M M

The high-order bit is always set to 1, as explained above.  The next two
bits (A, and B) are ADDRESS bits.  They have the following definitions:

              A: Indicates unit A as a recipient of the transmission
                 when set.
              B: Indicates unit B as a recipient when set.

All tranmissions to unit A will therefore have the pattern: 1 x1 xxx xxx; those
to unit B, the pattern 1 1x xxx xxx; and those directed to both units, the
pattern: 1 11 xxx xxx.
   The bit pattern 1 00 xxx xxx is universally ignored by the microprocessors.
It has been used allow communications with other microprocessors that may be
installed in the communications line for diagnostic purposes (for example,
a diagnostic microprocessor that simulates sensor signals).

   The next 3 bits (C C C) are the COMMAND FIELD, and the last 3 (M M M) the
COUNTER/SUBCOMMAND field.  They are used by the Command Layer to implement the
basic commands described in section 1f.

   (iii) Data Transfer Formats
   To provide an added check against transmission errors, two data transmission
formats are defined: one for short transmissions and the other for long
transmissions.  When blocks of data are to be transferred from a microprocessor
one of these formats will be used.

   For short transmissions (in response to the STATS command defined in section
1f), the following bytes are transferred:

                   '#': Indicates start of a short transmission
                  SIZE: Indicates the amount of data being transferred.
                        This can range from 1 to 100 (hexadecimal).  The
                        latter value is indicated when SIZE is set to 0.
                <Data>: A number of bytes indicated by SIZE
             Check Sum: The sum of all the Data bytes, modulo 100 (hex).

   Long transmissions (used for the DUMP command) are formatted the same,
except that the header is ':', instead of '#'.  Also, with long transmissions,
the master PC is expected to send back one of the following responses:

              ACCEPT:  The bit sequence: 1 xx 011 000
              REJECT:  The bit sequence: 1 xx 011 011
              STOP:    The bit sequence: 1 xx 011 111

Any other bit sequence received by the microprocessor is interpreted as a STOP,
for the sake of network security.

   If the microprocessor receives a REJECT, it will resend the data it just got
through sending.  If it receives an ACCEPT, it will send the next block of data.
If it receives a STOP, it will abort the long transmission.  A pre-determined
number of consecutive REJECT's (3) is interpreted as a STOP.

   After receiving an ACCEPT on the last block of data transmitted, the micro
will signal the end of transmission by sending a period: '.'.  This indicates
the successful completion of a long transmission.

   This protocol is a simplification of the XMODEM protocol.

   (c) The Command Layer
   To implement the commands described in section 1f, the following bit
patterns are used:

Commands to the microprocessor:

              STATUS              1  x x  0 1 0  0 0 0
              TEST                1  x x  0 1 1  0 0 0
              ABORT               1  x x  0 1 1  1 1 1
              DUMP from Input m   1  x x  0 0 1  m m m
              STATS from Input m  1  x x  0 0 0  m m m

with the following key
                         mmm          Input
                         000            0
                         001            1
                         010            2
                         011            3
                         100            4
                         101            5

A summary of these commands is given below:

STATUS: Queries the current state of the data collection unit.  The
microprocessor will respond sending one of the following characters:

      '0' = ACTIVE: In active command mode, ready to start a new test.
      '1' = WAITING: In test mode, waiting on the first sensor.
      '2' = TESTING: In test mode, collecting data, waiting on the second
                     sensor.

TEST:  Tells the microprocessor to pass into the waiting mode.
       Ignored unless the micro is in the ACTIVE mode.
ABORT: Forces the processor back into the active mode.  Also used to abort
       long data transfers.
DUMP:  Initiates a transfer by the microprocessor to the master PC of the
       nutation widths for the indicated counter of the most recent test, using
       the long transfer format.  Each nutation width is send as an unsigned
       4-byte integer with the lowest order byte being sent first.
STATS: Initiates a transfer to the PC of the statistical data of the most
       recent test.  The following data is packaged into the short transmission
       format and is then sent:

       Bytes      Name     Meaning
         0       State     Bit 6: set if there was no test since the micro was
                                  reset, or if the last test was aborted.
                           Bits 0-5: Set if the respective input 0-5 never
                                     received any input from a meter.
        1-2      Cycles    The number of complete nutations counted in the test
        3-6      TIME      The total test time
        7-10     First     The time that the first nutation occured
       11-14     Last      The time that the last nutation occured
       15-22     Square    The sum of the squares of all the nutation widths.

Corresponding to the figure shown in section 1c, the last 5 items are,
respectively: N, T - S, B, C, and Q, with the low order byte sent first.