I wrote BT747, and a kind user sent me a M241 so that I can 100% confirm that it works with this device.
To use it (besides correct installation):
- You need to connect using 38400 baud;
- You can set the device type to Holux M241 (top bar menu/device) to make sure that logs are always interpreted correctly - this is not required for device configuration (the device is automatically recognized).
Some things can not be changed permanently - ‘on boot’ the holux checks certain settings and changes them to one of the predefined ones - the log interval setting is one of them, the same goes for distance and speed log conditions, fix interval and other stuff.
The logged fields can be changed though and is maintained between on/offs.
The log format is slightly different from the other models: the position and height (altitude) is less precise.
By default: 4 bytes for the UTC time, 4 bytes for lat, 4 for lon, three for the height and finally a byte for the checksum - a total of 16 bytes per position. On the other models this is 4 + 8 + 8 + 4 + 2 = 26 bytes. The other fields are the same size.
To log a waypoint (and to gain 2 bytes to be logged with every position as on the other models), the M241 writes a specific string to the log indicating that the next position is a waypoint. This string is 16 bytes (the same size as a normal position record). So a waypoint takes 16 bytes + the position record - by default 32 bytes (2 positions, not 10-20!). So on other models you need 28 bytes per position if you want waypoints.
So for similar information and same memory, the holux logs 130000 positions and the other models 74000. The main tradeoff is in the precision of the position - the height not being an real precision issue.
Once you change the logged fields, the original software becomes incompatible and BT747 will be mandatory. GPSBabel has been using the doc I wrote and added support for MTK devices - the holux will be supported too and is reported to be supported in the ‘CVS’ version - it may be compatible with the changed format but I do not vouch for that.