TI-82 Link Protocol Guide v1.0 - Packet formats

##

Packet Formats

Texas Instruments calculators always send data and responses in self-contained "packets."
Packets have the following format:
Note - all 2-byte integers are transmitted little-endian Intel-style (least significant byte first).

Offset | Length | Description
--- | --- | ---
0 | 1 byte | Machine ID byte
1 | 1 byte | Command ID byte
2 | 2 bytes | Length of data (see note below)
4 | n bytes | Data (if attached)
4+n | 2 bytes | Checksum (if data is attached)

Note - The "Length of data" element may not be equal to zero if the packet contains no data. In this case, the Command ID byte will indicate that the packet contains no data.

#### The Machine ID Byte

The Machine ID byte identifies the machine that is sending the packet. It can have one of the following values:

Value | Description
--- | ---
02h | Computer sending TI-82 data
82h | TI-82

#### The Command ID Byte

The Command ID byte identifies the request or response that the machine is sending. It can have one of the following values:

Value | Description | Data Included
--- | --- | ---
06h | Variable Header (VAR) | A variable header
09h | Clear to send (CTS) | None
15h | Data packet (DATA) | Screenshot/variable/backup data
36h | Skip/Exit (SKIP/EXIT) | A one-byte rejection code (see codes below)
56h | Acknowledge (ACK) | None
5Ah | Checksum Error (ERR) The previous packet must be sent again. | None
6Dh | Silent - Request Screenshot (SCR) | None
92h | End of Transmission (EOT) | None
A2h | Silent - Request Variable (REQ) | A variable header
C9h | Silent - Request to Send Variable (RTS) | A variable header

Rejection codes (used with Command ID 36h) can have one of the following values:

Value | Description
--- | ---
01h | EXIT - The entire transmission has been cancelled.
02h | SKIP - The current variable has been skipped.
03h | OUT OF MEMORY (silent transmissions only) - The receiving calculator is out of memory.

#### The Checksum

The checksum is a 16-bit value used to verify the integrity of the data in the packet. It only present if data is present.
The checksum is calculated by taking the lower 16 bits of the sum of the data bytes, as shown below:

    int calculateChecksum(unsigned char* data, unsigned short datalength) {
       unsigned short x, checksum;
       for(x=0; x<datalength; x++) {
          checksum+=data[x];  //overflow automatically limits to 16 bits
       }
       return checksum;
    }

###

Variable Headers

A Variable header contains information about one variable in the calculator.

Offset | Length | Description
--- | --- | ---
0 | 2 bytes | Size of actual variable data, in bytes
2 | 1 byte | Type ID Byte (see type ID's below)
3 | 8 bytes | Variable name, padded on the right with NULL characters (0h). This field is always tokenized.

Note: These bytes only make up the "data" section of the packet.

Note2:
- f or named lists (not L1..L0), the first token is a single 5D byte, followed by up to 7 alphanumeric tokens.
- for lists (L1..L0), the first token is a 5D byte followed by another byte: 00 for L1 to 09 for L0.
- for matrices ([A]..[E]), the first token is 5C followed by another byte: 00 for [A] to 04 for [E].
- for equations: there are 30 equations (Y1..Y0 in func mode, X1t..X6t and Y1t..Y6t in param mode, r1..r6 in polar mode and Un, Vn in seq mode). The varname is 5E10 for Y1 to 5E19 for Y0. The varname is 5E20 for X1t, 5E21 for Y1t to 5E2A for X6t, 5E2B for Y6t. The varname is 5E40 for r1 to 5E45 for r6. The varname is 5E80 for Un or 5E81 for Vn.
- for pictures (Pic1.. Pic6), the first token is 60 followed by another byte: 00 for Pic1 to 06 for Pic6.
- for gdbs (GDB1..GDB6), the first token is 61 followed by another byte: 00 for GDB1 to 09 for GDB6.

#### The Type ID Byte

The type ID byte specifies the type of variable that is being transmitted. It can have one of the following values:

Value | Description (click for variable format)
--- | ---
00h | Real Number
01h | List
02h | Matrix
03h | Y-Variable
05h | Program
06h | Edit-locked Program
07h | Picture
08h | GDB
0Bh | Window Settings "Window" (See note below)
0Ch | Saved Window Settings "RclWindow" (See note below)
0Dh | Table Setup "TblSet" (See note below)
0Eh | Screenshot sent with PrintScreen
0Fh | Backup (no meaningful name)

Note - If the Type ID is in the range 0Bh - 0Eh, then the contents of the name field of the header do not matter.

#### Backup Header Format

If the type ID byte specifies a backup, the variable header takes the following 9-byte format:

Offset | Length | Description
--- | --- | ---
0 | 2 bytes | Size of first backup section, in bytes
2 | 1 byte | Type ID Byte (1Dh in this case)
3 | 2 bytes | Size of second backup section, in bytes
5 | 2 bytes | Size of third backup section, in bytes
7 | 2 bytes | Memory address of the second data section ?

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