Taranis+Spektrum trainer port capabilities and requirements

[dermo]

Using an old transmitter I converted it to a dumb buddy box with an arduino programmed to provide ppm out. It works fine when connected to the Taranis but not an old Spektrum DX6i. I am thinking that maybe the ppm signal needs to be inverted relative to the way I have it set up for the Taranis, i.e. initialisation pulse to be low for 300us then channel data to be high for the remainder.
Can anybody confirm what the requirements of the Spektrum trainer port is, i.e. what voltage and signal specs it requires? Could I have damaged it by connected 5V from the Arduino? The Taranis seemed fine with 5V but I don't know the specs for its port also.