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May even be some form of the laptop battery terminal voltage..
Depends how good it is at dealing with control signals and latencies. Just the same problem as trying to do a parallel port programmer from USB to Parallel port converter.
By the sound of things it needs MORE than 6V, could be 7V, 7.5V or 9V as well as 12V.
I would just try to use a bench power supply current limited with the unit stand alone, to determine when the unit starts to give reasonable RS232 levels out. I assume that as the unit is a serailport device working with RS232 ports it is actually trying to drive something like 0 to 9V levels and no -ve voltages.
The problem may also be that 6V is enough but the laptop does not provide any signals sitting at -ve voltage levels and uses 0/6V signalling. Not having a negative signal to create a negative rail may be sufficient to get this to foul up.
If you must do this, use a breakout cable/box on the WORKING system and scope ALL the signals as no doubt some signal may well be sitting at a negative level on working system but not on failing laptop.
Agreed.
Or even work out if he needs a DC/DC of some form to create a negative rail as well as possibly a positive rail.
Yes but they can be such fun these challenges.
Like the simple one I did for SWMBO, who had a solar powered water feature and 12V AC garden lights. Adding small box with relay enabled me to create a pseudo solar panel from the 12V AC feed so water feature worked when the garden lights were on.