TI-89 Link Protocol Guide - Packet formats

TI-89 Link Protocol Guide - 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
08h	Computer sending TI-89/92+/V200 data
88h	TI-92+/V200 to Computer or TI-92+/V200 to TI-92+/V200
98h	TI-89 (Titanium) to Computer or TI89 (Titanium) to TI89 (Titanium)

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 or padded variable header
09h	Clear to send (CTS)	None
15h	Data packet (DATA)	Screenshot/variable/backup data
2Dh	Silent - Request Versions (VER)	None
36h	Skip/Exit (SKIP/EXIT)	A five-byte rejection code (see codes below)
56h	Acknowledge (ACK)	None
5Ah	Checksum Error (ERR) The previous packet must be sent again.	None
68h	Check Ready (RDY)	None
6Dh	Silent - Request Screenshot (SCR)	None
78h	Continue (CONT)	None
87h	Direct command (CMD)	None - all data needed is in the packet header. There is no checksum.
88h	DeleteVariable (DEL)	A standard variable header
92h	End of Transmission (EOT)	None
A2h	Silent - Request Variable (REQ)	A standard or padded variable header
C9h	Silent - Request to Send Variable (RTS)	A standard or padded variable header

The SKP packet

It is made up of 5 bytes: 05 00 XX 00 00 where XX seems to be an error ID:
- 03: can't delete application
- 07: FLASH app not targeted for this model (01 00 07 00 81)
- 1E: can't overwrite variable
- 21: can't delete variable
- 24: malformed RTS packet
- 25: invalid varname

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	4 bytes	Size of actual variable data, in bytes, with or without the first four bytes of each data packet.
4	1 byte	Type ID Byte (see type ID's below)
5	1 byte	Size of variable name, in characters
6	n bytes	Variable name (not zero-terminated)
6+n	1 bytes	Extra byte for FLASH transfers

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

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	Expression
04h	List
06h	Matrix
0Ah	Data
0Bh	Text
0Ch	String
0Dh	GDB
0Eh	Figure
10h	Picture
12h	Program
13h	Function
14h	Macro
15h	Cause execution of assembly block (never encountered)
18h	Set/Get calculator date
1Ah	List the folder/apps table entries (global/local directory list request)
1Bh	with 1Ah, list content of a given folder (local dirlist)
1Ch	Other file
1Dh	Backup (See Backup Header Format below) (used only in sending mode)
1Fh	with 1Ah: list all main folders (global dirlist)
20h	Get a certificate
21h	Assembly program
22h	Get IDLIST
23h	Advanced Mathematic Software: the Operating System of the calculator (AMS)
24h	FLASH application (free apps, at least)
25h	Certificate

Note - If the Type ID is 1Ah, 1Bh or 22h, then the name length byte has a value of 0 and the name field is empty. Moreover, the upper byte of the length field (offset 0) is a byte used for filtering file types.

Backup Header Format (in sending only)

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

Offset	Length	Description
0	4 bytes	Always {00h, 00h, 00h, 00h}.
3	1 byte	Type ID Byte (1Dh in this case).
4	1 bytes	Length, in characters, of the name field. Always 04h.
5	n bytes	Name: "main".

If the header is a request-style header, then the data length field (offset 0) contains 4 and the name field (offset 5) contains "main".

A remark: the backup request is only used in sending mode. Indeed, the TI89 (and also the TI92+) does not have a true backup mode. A backup is simply a group of all variables.

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