Figuring it out
RigExpert makes a handheld 13cm band antenna analyzer, the IT-24. It is
small, lightweight, and both measures and graphs SWR. However, until today,
the IT-24 required a Windows tool called LCD2CLIP, which is used to copy the
current analyzer frame to the windows clipboard. The image transfer is started
by pressing the button labeled with a rectangle on the analyzer.
I started by examining the USB drivers - inside the IT-24 is a USB-FTDI
interface. These show up under /dev/ttyUSB in Linux. The baud-rate is 115200.
Pressing the screencapture button sends a newline,carriage-return,"screencomp",
resolution,and then a compressed bytestream terminated with another newline and
Gathering data and initial guesses
To verify the link quality, I recorded a few captures of the main menu
using dd on the USB-emulated serial port device, and all captures of the main
menu were the same. Changing the menu selection changed the capture data.
The menu images were 4 to 5 pixels per byte. Captures of the graph page were 7
to 8 pixels per byte. This indicated that some form of compression was in use.
Next I captured an SWR-sweep graph of the same antenna twice. Because they were
different sweeps, the images were similar but not identical. Converting both
dumps to hexadecimal and comparing in meld showed that the differences in
binary dumps were similar to where the graphs would be different. Also
suspicious was the fact that after subtracting the header and footer, the
dump size was always a multiple of 3 bytes.
The next step was building a simple parser and gathering statistics.
For each three-byte packet, the first and second bytes had a limited number
of possible values. Additionally, when the first and second bytes are combined,
the values line up, so this appeared to be a 16-bit integer. The third byte of
all packets has the interesting property, when added together, of equalling the
total number of pixels in the image, which is known from the ASCII header. This
pretty much nails it down as
RLE encoded, 3 bytes per packet representing between 1 and 255 pixels.
- 2 bytes RGB565.
- 1 byte repeat-count.
Code and closing
I wrote a python tool
to scrape images from the IT-24
and posted it to github. A few revisions
back you can find an earlier parser written in 'C' that was just enough
to work out the rest of the format. Using the above program, it's perfectly
reasonable to record a few graphs per minute to a PC using the IT-24 from RigExpert.