TI-73 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
--- | ---
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;
    }

###

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. 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 :

Offset | Length | Description
--- | --- | ---
0 | 2 bytes | Size of actual data, in bytes (LSB).
2 | 1 byte | Type ID Byte (see type ID's below)
3 | 2 bytes | Size of actual data, in bytes (MSB).
5 | 1 byte | Flag - Always has a value of 0h but bit 7 is set if archived.
6 | 2 bytes | FLASH offset
8 | 2 bytes | FLASH page

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