TI-82 Link Protocol Guide v1.0 - Screenshots


Screenshots

The TI-82's screenshot ability is "silent," which means that the calculator does not need to be put in a special mode. Screenshots can be taken any time the calculator is ready to accept a keypress.

The protocol used for getting a screenshot is shown below. For information on packet formats, click here.

Step Direction Packet

1

Computer

===>

Calculator

SCR

2

<===

ACK

3

<===

DATA - Format described below.

4

===>

ACK

The data packet contains a 768-byte monochrome bitmap (1 bit per pixel). Each bit has a value of 1 if the corresponding pixel is dark or a value of 0 if the corresponding pixel is light.

The pixel order is left-to-right, then top-to-bottom (See figure below).
Byte 0 defines the first 8 pixels in the left side of the top row of the screen. Bytes 1-11 define the rest of the first row, bytes 12-23 define the second row, etc.

Example:
PC: 02 6D 00 00 Screenshot request
TI: 82 56 00 00 Acknowledgement of request
  82 15 00 03 <768 bytes plus 2 byte checksum> Screen data
PC: 02 56 00 00 Acknowledgement of data

The following figure demonstrates byte ordering of the pixels if the bitmap is in a 768-element array data[]:

[pixel grid diagram]

Note that this format is different from the standard Windows bitmap format, which specifies a value of 1 if the pixel is light and bottom-to-top row order.


Alternately, the TI-82 may send the screenshot by executing the PrintScreen command. In that case, this protocol is used:

Step Direction Packet

1

Computer

<===

Calculator

VAR with Type ID=0Eh

2

===>

ACK

3

===>

CTS

4

<===

ACK

5

<===

DATA (format described above).

6

===>

ACK
The data format is the same as in the above transfer.

Example:
TI: 82 06 0B 00 00 03 0E 00 00 00 00 00 00 00 00 11 00 Variable Header: Screenshot
PC: 02 56 00 00 Acknowledgement of header
  02 09 00 00 Clear to send header
TI: 82 56 00 00 Acknowledgement of CTS
  82 15 00 04 <768 bytes plus 2 byte checksum> Screen data
PC: 02 56 00 00 Acknowledgement of data

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