TI-85 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
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; 
}


Variable Headers

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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