Marheine                Expires February 3, 2011                [Page 2]

Internet-Draft                   TI-SDIP                     August 2010


1.  Introduction

   While the existing link protocol for TI graphing calculators is
   suitable for general usage, it is needlessly complex for most
   purposes as well as being implemented in an incredubly arcane manner.
   For the sanity of developers targeting such platforms, this document
   specifies a data interchange protocol suitable for use on all TI
   calculating devices and associated peripherals.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  Objectives

   The designer has sought to meet certain objectives in the design of
   the protocol, as outlined in this section.

   Ease of implementation
         The protocol seeks to replace a proprietary and difficult-to-
         implement protocol, so ease of implementation is very much
         desirable.

   Robustness
         Linking between devices is usually done across noisy links and
         in the presence of potentially careless users, so the protocol
         should be able to handle data errors and unexpected loss of
         connection.

   Speed
         Data transfers are typically done in the presence of users and
         in an interactive manner, so a reasonably high effective data
         rate is desired in order to impose less upon the user's time.
         As this document makes no claims about the link layer in use,
         the specification should seek only to minimize protocol
         overhead.


2.  Data formatting

   The TI-SDIP is byte-oriented, and expects to receive data from the
   link layer in 8-bit bytes.  This section outlines the terminology
   used to refer to bytes and subdivisions thereof.






Marheine                Expires February 3, 2011                [Page 3]

Internet-Draft                   TI-SDIP                     August 2010


2.1.  Figures

   In illustrations, a '+' is used to delimit individual bits of a
   field, while '|' delimits fields, and '++' denotes a byte boundary.

2.2.  Byte and bit ordering

   Byte values are represented in this specification as groupings of 8
   bits, numbered right to left from 0 to 7.  When bit groupings are
   used to represent integers, bit 0 is the least significant bit, with
   significance increasing in order with bit number.

                         Byte 1            Byte 2
                     +-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+
                     |7 6 5 4 3 2 1 0++7 6 5 4 3 2 1 0|
                     ----------------++----------------

                   Figure 1: Bit numbering within bytes

   When multi-byte integers are used, the value is transmitted LSB-
   first, followed by additional bytes in order of increasing
   significance.

                      Multi-byte integer transmission

                                 Time -->
                           +----+----+----+----+
                           | 78 | 56 | 34 | 12 |
                           ---------------------

                       Figure 2: Sending 0x12345678


3.  Protocol description

   The protocol is based on the concept of a 'conversation', with only
   one device transmitting at once.  Each conversation consists of a
   packet transferred from the sender followed by a response from the
   receiver, repeating until an implicit end to the conversation.