TI-73 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
07h	Computer sending TI-73 data
74h	TI-73

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 fixed-length, NUL padded 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
68h	Check ready (RDY)	None
6Dh	Silent - Request Screenshot (SCR)	None
92h	End of Transmission (EOT)	None
A2h	Silent - Request Variable (REQ)	A fixed-length, NUL padded variable header
C9h	Silent - Request to Send Variable (RTS)	A fixed-length, NUL padded 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. Not used for ID list.

Note: These bytes only make up the "data" section of the packet.
Note2:
- for lists (L1..L6), the first token is a 5D byte followed by another byte: 01 for L1 to 06 for L6.

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	Real List
02h	Y-Variable
05h	Program
06h	Assembly Program ?
07h	Picture
0Bh	Window Settings (See note below)
0Fh	Window Settings (See note below)
11h	Table Setup (See note below)
13h	Backup
19h	Directory (See note below) - only used when requesting a directory
23h	FLASH Operating System
24h	FLASH Application
26h	ID list
27h	Get Certificate

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 (flags area), in bytes
2	1 byte	Type ID Byte (13h in this case)
3	2 bytes	Size of second backup section (data area), in bytes
5	2 bytes	Size of third backup section (symbol area), in bytes
7	2 bytes	Start User Area Address

FLASH Header Format

If the type ID byte is in the range 22...28h, the variable header takes the following 10-byte format :

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