The protocol is case-insensitive.  Each packet is terminated by a newline
character, and fields are delimited by spaces.  Throwing gratuitous spaces in
might screw things up, though.

Protocol:

### ss-core --> ss-hw #########################################################

RF-FIELD OFF

    Deactivates the RF field.  If an inventory is in progress, field
    deactivation is deferred until after the inventory completes or the
    watchdog timer triggers.  No inventory is attempted while the RF field is
    off.

RF-FIELD ON

    Enables the RF field.  If the inventory delay is positive, then an
    inventory is immediately initiated.

INV-DELAY -x

    Deactivates automatic inventorying.  If an inventory is in progress, it is
    not cancelled, but no more will be scheduled.

INV-DELAY 0

    Deativates automatic inventorying.  Any pending inventory is allowed to
    complete.  If no inventory is in progress (and the RF field is active), one
    is initiated, but no further inventories will be scheduled.  This command
    allows for manual control of inventory timing.

INV-DELAY +x

    Enables automatic inventorying.  x milliseconds after each inventory
    succeeds, a new inventory begins.  Don't set this too low. :^)  If no
    inventory is in progress (perhaps due to INV-DELAY -x) and the RF field is
    on, an inventory is immediately initiated.

### ss-hw --> ss-core #########################################################

DOORS a b

    A door opened or closed.  These update messages are sent at program start
    and when the door state changes.

PRESSURE a b c d

    New values arrived from the pressure sensor.

TAG-ADD uid type

    A new RFID tag has entered the RF field.  uid is a 16-digit hexadecimal
    number, and type is an 8-digit hex number.  At program start, TAG-ADD's are
    sent for all tags initially in the RF field.

TAG-DEL uid

    An RFID tag left the RF field.  uid is a 16-digit hexadecimal number.  This
    will only be sent for tags previously reported with TAG-ADD.

###############################################################################

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