TI-85 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
05h	Computer sending TI-85 data
85h	TI-85

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 standard 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 NUL padded (fixed-length) variable header
C9h	Silent - Request to Send Variable (RTS)	A SPC padded (fixed-length) 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.
Standard and padded variable headers have the following format:

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	1 byte	Size of variable name, in characters
4	n bytes	Variable name (not zero-terminated)

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

A padded variable header will always be 12 bytes long. The unused bytes at the end are padded with space characters (20h) or NULLs (0h). The standard and padded formats for variable headers are interchangeable.

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	Complex Number
02h	Real Vector
03h	Complex Vector
04h	Real List
05h	Complex List
06h	Real Matrix
07h	Complex Matrix
08h	Real Constant
09h	Complex Constant
0Ah	Equation
0Ch	String
0Dh	Function GDB
0Eh	Polar GDB
0Fh	Parametric GDB
10h	Differential Equation GDB
11h	Picture
12h	Program
14h	Screenshot sent by PrtScrn (See note below)
17h	Func: Function Window Settings (See note below)
18h	Pol: Polar Window Settings (See note below)
19h	Param: Parametric Window Settings (See note below)
1Ah	DifEq: Differential Equation Window Settings (See note below)
1Bh	ZRCL: Saved Window Settings (See note below)
1Dh	Backup (No meaningful name)
1Eh	Unknown (only used when requesting variables of unknown type)

Note - If the Type ID is in the range 14h - 1Bh, then the name length byte has a value of 0 and the name field consists of a single 0 byte.

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 second section

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