Dual cable (serial + USB) for Nikon coolpixes

To assure that USB and serial segments are electrically isolated, we used an optocoupler (U1). The LED section of U1 is driven directly by DTR, so we should select a very sensitive optocoupler to prevent too much current flow from DTR (which is usually very limited, implementation-dependent; around 3 mA max. if we don't want to push the serial port to its limits). D1 protects the LED when DTR is negative. The NPN transistor section of U1 drives a PNP transistor which in turn operates a electromechanical relay. This relay has three associated contacts: two normally open and one normally closed. This is a weird configuration for a commercial relay, so you will have to use two relays working in parallel instead. When the relay is idle, USB-detect and USB-GND are disconnected, while Serial-detect and Serial-GND are connected to the computer serial port ground, so the dual cable is in serial mode. When the relay is activated, USB-detect is connected to +5V and USB-GND is connected to the computer USB port ground, so the cable is in USB mode.
Dual cable (serial + USB) for Nikon coolpixes - schematic

This circuit behaves the same as the previous one, but it is completely different. Now the relay is powered by a PNP transistor (Q1, for example, a BC556) which is on when DTR is negative or not connected. If DTR is positive or the USB plug is not connected (and hence there is no +5V power in the circuit), Q1 and the relay are off. With R1 set to 10K and R2 to 22K, DTR current is small. D1 can be a 1N4148. Using a multimeter, we measured the resistance between the USB and serial port grounds in my computer, and it turns out to be extremely low (under 0.1 ohm), so we assumed that they are connected together inside the computer, as one can expect because they are likely to share the computer power supply. The resistance between USB-GND and Serial-GND on the camera is also very low. If we consider all these grounds as the same electrical node (as it is deduced from these tests), the relay-based circuit can be simplified to the one depicted in figure 3.

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