Rather than use the serial port on the docking Sled, which has some drawbacks - not least a lack of connected handshaking wires.
Why not use the 2 serial port pins on the Tuner connector that extends from the Docking sled?
[I'm assuming we are all using the docking sled for in home docking as we wish to avoid the wear and tear on the 12volt socket on the empeg itself]
Hugo has previously stated that the tuner has its own seperate Serial port going to the Tuner module [at 3.3volt level only not RS232 level]. [its /dev/ttyS1 I think - do a search for 'Tuner module pinouts' post I did some months ago.]
To exploit this then all we need is a small loop back link between TXD and RXD lines on this connector.
[or some other similar combination of links]
When the empeg powers up it sends something to the serial port on the Tuner connector [before the player runs so its safe to do so] - if its gets the *same* message back it knows its in 'home docked' mode.
The only wrinkle here would be to prevent this 'message' from confusing a real tuner module if it were attached.
If a real tuner module is attached you could read the unique ID set on the Tuner module by the rotary hex switch on the side and use that to decide the settings to use for this case.
I am sure there is a message Hugo (or someone else within the group) can tell us is safe to use that we should send to the Tuner module and/or to request the module id - if we get the same packet back when we send this message we know we are in 'Tuner connector loopback' mode and act accordingly.
If we get no response you assume DC/Car mode [no Tuner attached like now]
If we get a valid response we know either we are in car with a tuner module, or we are in home and use theTuner module Id - which we ask theTuner module for, to determine which of these two modes is correct.
[I have assumed we can specify in config.ini or somwhere what the Tuner module ID is for 'in home' - all others imply 'in car'].
Thats my suggestion