TI-92 Link Protocol Guide - File formats

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File Formats

The TI-92 Graph Link software stores variables in several types of files. Single variables are stored in files that have an extension that corresponds with their type. Groups of variables are stored in files with the extension ".92g" and backups with the extension ".92b". However, all of these file types have a common format.

The different file types and contents that the TI-92 Graph Link software can read and write are shown below.

Extension | Description
--- | ---
.92a | TI-92 figure
.92b | TI-92 system backup
.92c | TI-92 data
.92d | TI-92 GDB (Graphic Data Base)
.92e | TI-92 expression
.92f | TI-92 function
.92g | Multiple TI-92 variables of varying types (group)
.92i | TI-92 picture (image)
.92l | TI-92 list
.92m | TI-92 matrix
.92p | TI-92 program
.92s | TI-92 string
.92t | TI-92 text
.92x | TI-92 macro

Early link programs for the TI-92 used an additional file type ".92@" because the TI-Graph Link 92 software had not been released for the PC. That format is described in this section as well.

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The TI-92 variable file format

The TI-92 variable file format has three parts: a header, a variable table and a data section.

The header appears at the beginning of the file and takes the following format:
Note - All 2-byte and 4-byte integers are stored little-endian Intel-style (least significant byte first).

Offset | Length | Description
--- | --- | ---
0 | 8 bytes | 8-character signature. The signature is always "`**TI92**`".
8 | 2 bytes | 2-byte further signature. These two bytes always contain {01h, 00h} = {1, 0}.
10 (Ah) | 8 bytes | Default folder name (zero terminated unless 8 characters long).
18 (12h) | 40 (28h) bytes | Comment. The comment is either zero-terminated or padded on the right with space characters.
58 (3Ah) | 2 bytes | Number of variable and folder entries in the variable table.

Note: If the first byte of the default folder name is 0, then the default folder is the current folder on the receiving unit.

The variable table comes immediately after the header and consists of a number of 16-byte entries, each taking the format described below. Folder entries share the same table. The variables described before the first folder entry are assumed to be in the default folder. Variables described after any given folder entry are placed in that folder.
If no folder entries are included, then variables are considered to be part of the folder named at file offset 0x0A. If this is null, then it is considered to be in the current directory when a link program sends it.

Each variable entry follows this format:

Offset | Length | Description
--- | --- | ---
0 | 4 bytes | Offset to the data for this variable from the beginning of the file.
4 | 8 bytes | Name of variable (zero terminated if not 8 characters long).
12 (Ch) | 1 byte | Type ID of the variable.
13 (Dh) | 1 byte | Attribute (0: none, 1: locked)
14 (Eh) | 2 bytes | Unused - each byte has a value of 0.

Folder entries follow this format:

Offset | Length | Description
--- | --- | ---
0 | 4 bytes | Offset to the data for the next variable from the beginning of the file.
4 | 8 bytes | Name of variable (zero terminated if not 8 characters long).
12 (Ch) | 1 byte | Type ID of the variable. For folders, this value is always 1Fh.
13 (Dh) | 1 byte | Unused - this byte has a value of 0.
14 (Eh) | 2 bytes | The number of variables in this folder.

Finally, the data section, which follows the variable table, has this format:

Offset | Length | Description
--- | --- | ---
0 | 4 bytes | The file size, in bytes.
4 | 2 bytes | A 2-byte signature. These two bytes always contain {A5h, 5Ah}.
6 | n bytes | Data section.

The data section consists of several variable data parts back-to-back, in order of their listing in the variable table. Each variable part has this format:

Offset | Length | Description
--- | --- | ---
0 | m bytes | The variable data. This field includes all bytes transferred in data packets. The first four bytes usually all have a value of 0.
m | 2 bytes | Checksum. This is the lower 16 bits of the sum of all bytes in the first four bytes and the variable data.

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A simplification of the TI-92 file format for a single variable

Files containing single variables are a simple yet common example of the TI-92 file format. In this special case, the file format can be simplified as follows:

Offset | Length | Description
--- | --- | ---
0 | 8 bytes | 8-character signature. The signature is always "`**TI92**`".
8 | 2 bytes | 2-byte further signature. These two bytes always contain {01h, 00h} = {1, 0}.
10 (Ah) | 8 bytes | The parent folder of the variable (zero terminated if not 8 characters long) - See note below.
18 (12h) | 40 (28h) bytes | Comment. The comment is either zero-terminated or padded on the right with space characters.
58 (3Ah) | 6 bytes | These 6 bytes always contain {01h, 00h, 52h, 00h, 00h, 00h} = {1, 0, 82, 0, 0, 0}.
64 (40h) | 8 bytes | The name of the variable (zero terminated if not 8 characters long).
72 (48h) | 1 byte | The type ID for this variable.
73 (49h) | 3 bytes | These three bytes always have a value of 0.
76 (4Ch) | 4 bytes | The file size, in bytes.
80 (50h) | 2 bytes | A 2-byte signature. These two bytes always contain {A5h, 5Ah}.
82 (52h) | n bytes | The variable data. This field includes all bytes transferred in data packets. The first four bytes usually all have a value of 0.
82 (52h)+n | 2 bytes | Checksum. This is the lower 16 bits of the sum of all bytes in the variable data (@52h).

Note: If the first byte of the name of the parent folder is 0, then the variable is put into the current folder on the receiving unit.

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The TI-92 backup file format

The TI-92 backup file format is similar to the group file format, but there are some functional differences. Backup data cannot be stored in the same file as variable data, and some of the bytes in a backup file have special meanings. While the backup is sent in several sections, there is only one variable entry for the backup.

TI-92 backup files follow this format:

Offset | Length | Description
--- | --- | ---
0 | 8 bytes | 8-character signature. The signature is always "`**TI92**`".
8 | 2 bytes | 2-byte further signature. These two bytes always contain {01h, 00h} = {1, 0}.
10 (Ah) | 8 bytes | The first byte always has a value of 0; the other seven can have any value.
18 (12h) | 40 (28h) bytes | Comment. The comment is either zero-terminated or padded on the right with space characters.
58 (3Ah) | 2 bytes | These 2 bytes always contain {01h, 00h}.
60 (3Ch) | 4 bytes | These 4bytes always contain {52h, 00h, 00h, 00h} = {82, 0, 0, 0}.
64 (40h) | 8 bytes | A string which contains the ROM version of the calculator (zero-terminated). Examples: "1.11", "2.1"
72 (48h) | 1 byte | This byte always contains 1Dh.
73 (49h) | 3 bytes | These three bytes always have a value of 0.
76 (4Ch) | 4 bytes | The file size, in bytes.
80 (50h) | 2 bytes | A 2-byte further signature. These two bytes always contain {A5h, 5Ah}.
82 (52h) | n bytes | Raw data, made up of the 1-kilobyte blocks stored back-to-back in order of transmission. These blocks do not include the first four bytes of each of the data packets.
82 (52h)+n | 2 bytes | File checksum. This is the lower 16 bits of the sum of all bytes in the raw data (@52h).

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The early 92@ file format

Linking programs were available for the TI-92 before TI released the TI-92 Graph Link for the PC. The creators of Link92xp, one of these early TI-92 linking programs, designed this file format to store the information. While this format was never supported by TI and is now obsolete, there are still some 92@ files floating about the internet.

92@ files start with a header with this format:

Offset | Length | Description
--- | --- | ---
0 | 8 bytes | 8-character signature. The signature is always "`**TI92**`".
8 | 3 bytes | 3-byte further signature. These two bytes always contain {1Ah, 0Ch, 00h} = {26, 12, 0}.
11 (Bh) | 42 (2Ah) bytes | Comment. The comment is either zero-terminated or padded on the right with space characters.
53 (35h) | 2 bytes | Length, in bytes, of the data section and the checksum (n+2). This value should be 37h less than the file length.
55 (37h) | n bytes | Data section - made up of a number of entries, described below.
54 (36h)+n | 2 bytes | File checksum. This is the lower 16 bits of the sum of all bytes in the data section.

Each variable entry in the data section has this format:

Offset | Length | Description
--- | --- | ---
0 | 1 byte | Offset from the next byte to the data length field. This number should be 4+m, where m is defined below.
1 | 2 bytes | Offset from the beginning of the entry to the raw data. This number should be 3 more than the value at offset 0.
3 | 1 byte | Variable type ID.
4 | 1 byte | Length, in characters, of the variable directory/name field.
5 | m bytes | Variable directory/name. If a directory is specified, its name comes first, followed by a backslash (\) character, then the variable name. If no directory is specified, there is no backslash and the variable is put in the current directory of the calculator.
5+m | 2 bytes | Length, in bytes, of the variable data.
7+m | n bytes | The variable data, not including the first five bytes sent with the data packet. The first four bytes of the packet are unimportant, and the fifth byte of the packet can be calculated by subtracting 2 from the value at offset 5+m and taking the upper 8 bits.

Variable Formats | Table of Contents | Tokens and Characters

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