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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