You do not state what the "device" is, it may be helpfull to know.
You state the there is "one wire" in the comms link between the computer and device, I guess that there is a second wire making a return path somewhere, one wire does not make a circuit.
I had never heared of SCI until I read your post. A quick look with Google infers that is an old Motorola name for a device which the rest of the world calls a UART.
However, what we need to find out is:
1/ does the computer interrogate the device and the wait for a reply,
or
2/ does the device just send data to the computer?
also what is the function of the "Test Tool", is it intended to monitor the comms between the device and the computer, or is it a simulator to test the computer in the absence of the device?
If both the device and the test tool are trying to transmit at the same time, it will not work.
JimB