TI-82 Link Protocol Guide v1.0 - 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;
}

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:
- for 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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