TI-92 Link Protocol Guide - 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
09h	Computer sending TI-92 data
89h	TI-92 to Computer or TI92 to TI92

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
36h	Out of Memory (MEM)	Five bytes of unknown data
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.
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 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	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)

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
19h	Complete Directory (See note below) - only used when requesting a listing of all variables on the calculator.
1Dh	Backup (See Backup Header Format below)
1Fh	Folder - only used when listing a directory.

Note - If the Type ID is 19h or 1Ah, then the name length byte has a value of 0 and the name field is empty.

Backup Header Format

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

Offset	Length	Description
0	4 bytes	Length, in bytes, of the backup data without the extra four bytes at the beginning of each data packet.
3	1 byte	Type ID Byte (1Dh in this case).
4	1 bytes	Length, in characters, of the name field.
5	n bytes	Name.

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

If the header is a send-style header, then the data length is the length of the entire backup and the name field contains the ROM version string (e.g. "1.11" or "2.1").

If the header is a data-style header, then the data length is the length of the following 1-kilobyte section and the name field contains the ROM version string. Note that the data length field can be less than 400h if the following section is the final section.
 

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